Editing Horsepower

{{short description|Unit of power with different values}}
{{Use dmy dates|date=December 2024}}
{{Other uses}}
{{Infobox unit
| bgcolor       = 
| name          = Horsepower
| image         = Imperial Horsepower.svg
| caption       = One ''imperial horsepower'' lifts {{convert|550|lb|lk=in}} by {{convert|1|foot|cm|lk=in}} in 1&nbsp;[[second]].
| standard      = 
| quantity      = [[Power (physics)|power]]
| symbol        = hp
| symbol2       = 
| namedafter    = 
| extralabel    = 
| extradata     = 
| units1        = 
| inunits1      = <!--...-->
| units6        = 
| inunits6      = 
| units_imp1    = 
| inunits_imp1  = <!--...-->
| units_imp6    = 
| inunits_imp6  = 
| units_us1     = 
| inunits_us1   = <!--...-->
| units_us6     = 
| inunits_us6   = 
}}

'''Horsepower''' ('''hp''') is a [[unit of measurement]] of [[Power (physics)|power]], or the rate at which [[Work (physics)|work]] is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the '''imperial horsepower''' as in "hp" or "bhp" which is about {{convert|1|hp|W|1|lk=on|disp=out|abbr=off}}, and the '''metric horsepower''' as in "cv" or "PS" which is approximately {{convert|1|PS|W|1|disp=out|abbr=off}}. The electric horsepower "hpE" is exactly {{convert|1|hp-electric|W|0|disp=out|abbr=off}}, while the boiler horsepower is 9809.5 or 9811 watts, depending on the exact year.{{clarify|reason=What year did it change?|date=January 2025}}

The term was adopted in the late 18th century by [[Scottish people|Scottish]] engineer [[James Watt]] to compare the output of [[steam engine]]s with the power of [[draft horse]]s. It was later expanded to include the output power of other power-generating machinery such as [[piston engines]],  [[turbine]]s, and [[electric motor]]s.<ref>{{cite encyclopedia |title=Horsepower |encyclopedia=Encyclopædia Britannica Online |url=https://www.britannica.com/EBchecked/topic/272384/horsepower |access-date=2012-06-24}}</ref><ref>{{cite encyclopedia |title=International System of Units (SI) |encyclopedia=Encyclopædia Britannica Online |url=https://www.britannica.com/EBchecked/topic/291305/International-System-of-Units-SI |access-date=2012-06-24}}</ref> The definition of the unit varied among geographical regions. Most countries now use the [[International System of Units|SI]] unit [[watt]] for measurement of power. With the implementation of the EU Directive [[Units of Measure Directive#Directive 80/181/EEC|80/181/EEC]] on 1 January 2010,<ref>{{Cite journal |url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:114:0010:0013:EN:PDF |title=Directive 2009/3/EC of the European Parliament and of the Council of 11 March 2009 |journal=Official Journal of the European Union |date=7 May 2009}}</ref> the use of horsepower in the EU is permitted only as a supplementary unit.

==History==
[[File:6 horse team East Lampeter TWP LanCo PA 1.jpg|thumb|A team of six horses mowing hay in [[East Lampeter Township, Pennsylvania]], U.S.]]

The development of the [[steam engine]] provided a reason to compare the output of horses with that of the engines that could replace them. In 1702, [[Thomas Savery]] wrote in ''The Miner's Friend'':<ref>{{Cite web |url=http://www.history.rochester.edu/steam/savery/ |publisher=University of Rochester History Department |archive-url=https://web.archive.org/web/20090511121051/http://www.history.rochester.edu/steam/savery |archive-date=May 11, 2009 |title=The Miner's Friend |access-date=July 21, 2011 }}</ref>

:So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work...

The idea was later used by [[James Watt]] to help market the [[Watt steam engine]], an improved [[Newcomen steam engine]]. He had previously agreed to take royalties of one-third of the savings in coal from the older Newcomen steam engines.<ref>{{Cite web |url= http://www.pballew.net/arithm17.html#hp |title= Math Words — horsepower |publisher= pballew.net |access-date= 2007-08-11 |archive-date= 2018-09-20 |archive-url= https://web.archive.org/web/20180920001001/http://www.pballew.net/arithm17.html#hp |url-status= usurped }}</ref> This royalty scheme did not work with customers who did not have existing steam engines but used horses instead.

Watt determined that a horse could turn a [[Horse mill|mill wheel]] 144 times in an hour (or 2.4 times a minute).<ref name=Engineers121>Hart-Davis, Adam (2012). ''Engineers''. Dorling Kindersley. p. 121.</ref> The wheel was {{convert|12|ft}} in radius; therefore, the horse travelled {{nowrap|2.4 × 2π × 12}} feet in one minute. Watt judged that the horse could pull with a [[force]] of {{convert|180|lbf}}.<ref>{{cite book |title=James Watt: Craftsman and Engineer |first=H. W. |last=Dickenson |publisher=Cambridge University Press |year=2010 |isbn=9781108012232 |page=145 |quote=... based his calculations on data supplied to him to the effect that a mill horse walks, in a path of 24 ft. diameter, {{sfrac|2|1|2}} turns in a minute. Watt assumed that the mill horse exerted a pull of 180 lb.—we do not know where he got this figure—and found that it exerts 32,400 lb. per minute. By the following year he has rounded off the figure to 33,000, doubtless for ease in calculation.}}</ref> So:

:<math qid=Q120634922> P = \frac{W}{t} = \frac{Fd}{t} = \frac{180~\text{lbf} \times 2.4 \times 2\,\pi \times 12~\text{ft}}{1~\text{min}} = 32{,}572~\frac{\text{ft} \cdot \text{lbf}}{\text{min}}.</math>

''Engineering in History'' recounts that [[John Smeaton]] initially estimated that a horse could produce {{convert|22,916|ftlb}} per minute.<ref name="kirby-p171"/> [[John Theophilus Desaguliers|John Desaguliers]] had previously suggested {{convert|44,000|ftlb|0}} per minute, and [[Thomas Tredgold]] suggested {{convert|27,500|ftlb|0}} per minute. "Watt found by experiment in 1782 that a '[[Draft horse|brewery horse]]' could produce {{convert|32,400|ftlb|disp=sqbr|0}} per minute."<ref name="kirby-p171"/> James Watt and [[Matthew Boulton]] standardized that figure at {{convert|33,000|ftlb|0}} per minute the next year.<ref name="kirby-p171">{{Cite book |url=https://books.google.com/books?id=MXNtDQAAQBAJ&pg=PA171 |title=Engineering in History |first=Richard Shelton |last=Kirby |publisher=Dover Publications |page=171 |date=August 1, 1990 |isbn=0-486-26412-2 |access-date=June 13, 2018 }}</ref>

A common legend states that the unit was created when one of Watt's first customers, a brewer, specifically demanded an engine that would match a horse, and chose the strongest horse he had and driving it to the limit. In that legend, Watt accepted the challenge and built a machine that was actually even stronger than the figure achieved by the brewer, and the output of that machine became the horsepower.<ref>{{cite magazine |magazine=[[Popular Mechanics]] |url={{Google books|id=_d0DAAAAMBAJ|page=394|plain-url=yes}} |title=Motorcycle equipped with wireless |date=September 1912 |page=394}}</ref>

In 1993, R. D. Stevenson and R. J. Wassersug published correspondence in ''[[Nature (journal)|Nature]]'' summarizing measurements and calculations of peak and sustained work rates of a horse.<ref name="nature">{{Cite journal |last1=Stevenson |first1=R. D. |last2=Wassersug |first2=R. J. |year=1993 |title=Horsepower from a horse |journal=[[Nature (journal)|Nature]] |volume=364 |issue=6434 |page=195 |doi=10.1038/364195a0|pmid=8321316 |bibcode=1993Natur.364..195S |s2cid=23314938 |doi-access=free }}</ref> Citing measurements made at the 1925 [[Iowa State Fair]], they reported that the peak power over a few seconds has been measured to be as high as {{convert|14.88|hp|kW|abbr=on}}<ref>{{cite journal|last1=Collins|first1=E. V.|last2=Caine|first2=A. B.|year=1926|title=Testing Draft Horses|url=https://lib.dr.iastate.edu/bulletin/vol20/iss240/1|url-status=live|journal=Iowa Agricultural Experiment Station Bulletin|volume=240|pages=193–223|archive-url=https://web.archive.org/web/20200607104620/https://lib.dr.iastate.edu/bulletin/vol20/iss240/1/|archive-date=2020-06-07|access-date=2021-09-06}}</ref> and also observed that for sustained activity, a work rate of about {{convert|1|hp|kW|abbr=on}} per horse is consistent with agricultural advice from both the 19th and 20th centuries and also consistent with a work rate of about four times the [[basal rate]] expended by other vertebrates for sustained activity.<ref name="nature" />

When considering [[Human power|human-powered equipment]], a healthy human can produce about {{convert|1.2|hp|kW|abbr=on}} briefly (see [[orders of magnitude (power)|orders of magnitude]]) and sustain about {{convert|0.1|hp|kW|abbr=on}} indefinitely; trained athletes can manage up to about {{convert|2.5|hp|kW|abbr=on}} briefly<ref>Eugene A. Avallone et al., (ed), ''Marks' Standard Handbook for Mechanical Engineers 11th Edition '', Mc-Graw Hill, New York 2007, {{ISBN|0-07-142867-4}}, page 9-4.</ref>
and {{convert|0.35|hp|kW|abbr=on}} for a period of several hours.<ref>{{cite journal |last1=Ebert |first1=T. R. |title=Power output during a professional men's road-cycling tour |journal=International Journal of Sports Physiology and Performance |volume=1 |issue=4 |date=Dec 2006 |pages=324–325 |pmid=19124890|doi=10.1123/ijspp.1.4.324 |s2cid=13301088 }}</ref> The Jamaican sprinter [[Usain Bolt]] produced a maximum of {{convert|3.5|hp|kW|abbr=on}} 0.89 seconds into his 9.58 second {{convert|100|m|yd|1|adj=on}} sprint world record in 2009.<ref>{{cite web |title=Scientists Model "Extraordinary" Performance of Bolt |url=http://www.iop.org/news/13/jul/page_60709.html |publisher=Institute of Physics |date=26 July 2013 |archive-url=https://web.archive.org/web/20160309194600/http://www.iop.org/news/13/jul/page_60709.html |archive-date=9 March 2016 |access-date=15 December 2023 }}</ref>{{Failed verification|date=December 2023}}

In 2023 a group of engineers modified a [[dynamometer]] to be able to measure how much power a horse can produce. This horse was measured to {{convert|5.7|hp|kW|abbr=on}}.<ref>{{Cite AV media |publisher=Donut |title=How Much Horsepower is a Horse? |date=24 November 2023 |url=https://www.youtube.com/watch?v=7qxTKtlvaVE&ab_channel=Donut |access-date=30 November 2023 |via=YouTube }}</ref>

==Calculating power==
When [[torque]] {{mvar|T}} is in [[pound-foot (torque)|pound-foot]] units, [[rotational speed]] {{mvar|N}} is in [[Revolutions per minute|rpm]], the resulting power in horsepower is

: <math>\{P\}_\mathrm{hp} = \frac{\{T\}_\mathrm{ft {\cdot} lbf} \{N\}_\mathrm{rpm}}{5252}.</math><ref>{{cite book |first1=Terrell |last1=Croft |first2=Wilford |last2=Summers |title=American Electrician's Handbook |edition=11th |publisher=McGraw Hill |year=1987 |isbn=0-07-013932-6 |pages=7–175}}</ref>

The constant 5252 is the [[Rounding|rounded]] value of (33,000&nbsp;ft⋅lbf/min)/(2π&nbsp;rad/rev).

When torque {{mvar|T}} is in inch-pounds,

: <math>\{P\}_\mathrm{hp} = \frac{\{T\}_\mathrm{in {\cdot} lbf} \{N\}_\mathrm{rpm}}{63{,}025}.</math>

The constant 63,025 is the approximation of

: <math>33{,}000~\frac{\text{ft} {\cdot} \text{lbf}}{\text{min}} \times \frac{12~\frac{\text{in}}{\text{ft}}}{2\pi~\text{rad}} \approx 63{,}025 \frac{\text{in} {\cdot} \text{lbf}}{\text{min}}.</math>

==Definitions==

===Imperial horsepower===
Assuming the third [[General Conference on Weights and Measures|CGPM]] (1901, CR 70) definition of [[standard gravity]], {{nowrap|1=''g''<sub>n</sub> = 9.80665 m/s<sup>2</sup>}}, is used to define the pound-force as well as the kilogram force, and the [[international avoirdupois pound]] (1959), one imperial horsepower is:

:{|
|-
|1&nbsp;hp
|≡ 33,000&nbsp;ft·lbf/min
| colspan="2" |by definition
|-
|
|= 550&nbsp;ft⋅lbf/s
|since
|1 min = 60 s
|-
|
|= 550 × 0.3048 × 0.45359237 m⋅[[kilogram-force|kgf]]/s
|since
|1&nbsp;ft ≡ 0.3048 m and 1&nbsp;lb ≡ 0.45359237&nbsp;kg
|-
|
|= 76.0402249068&nbsp;kg<sub>f</sub>⋅m/s
|
|
|-
|
|= 76.0402249068 × 9.80665&nbsp;kg⋅m<sup>2</sup>/s<sup>3</sup>
|since
|''g'' = 9.80665&nbsp;m/s<sup>2</sup>
|-
|
|= 745.69987158227022 W ≈ 745.700 W
|since
|1 W ≡ 1 [[joule|J]]/s = 1 [[Newton (unit)|N]]⋅m/s = 1 (kg⋅m/s<sup>2</sup>)⋅(m/s)
|}

Or given that 1&nbsp;hp = 550&nbsp;ft⋅lbf/s, 1&nbsp;ft = 0.3048&nbsp;m, 1&nbsp;lbf&nbsp;≈ 4.448&nbsp;N, 1&nbsp;J = 1&nbsp;N⋅m, 1&nbsp;W = 1&nbsp;J/s: 1&nbsp;hp ≈ 745.7&nbsp;W

==={{anchor|Metric horsepower|Metric|PS|cv|hk|pk|ks|ch}} Metric horsepower (PS, KM, cv, hk, pk, k, ks, ch)===
[[File:Horsepower plain.svg|thumb|right|One ''metric horsepower'' is needed to lift 75&nbsp;[[kilogram]]s by 1&nbsp;[[metre]] in 1&nbsp;[[second]].]]

The various units used to indicate this definition (''PS'', '' KM'', ''cv'', ''hk'', ''pk'', ''k'', ''ks'' and ''ch'') all translate to ''horse power'' in English. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question.<ref>{{cite web | url = https://ateupwithmotor.com/terms-technology-definitions/gross-versus-net-horsepower/ | archiveurl = https://web.archive.org/web/20221202230849/https://ateupwithmotor.com/terms-technology-definitions/gross-versus-net-horsepower/ | archivedate = 2022-12-02 | title = Understanding Gross Versus Net Horsepower Ratings | work = Ate Up With Motor | date = 2008-04-15 | first = Aaron | last = Severson }}</ref>

[[Deutsches Institut für Normung|DIN]] 66036 defines one metric horsepower (Pferdestärke, or PS) as the power to raise a mass of 75 kilograms against the Earth's gravitational force over a distance of one metre in one second:<ref>{{cite web |url=http://www.ptb.de/cms/fileadmin/internet/Themenrundgaenge/hueterin_der_einheiten/einheiten_d.pdf |title=Die gesetzlichen Einheiten in Deutschland |language=de |page=6 |trans-title=List of units of measure in Germany |publisher=Physikalisch-Technische Bundesanstalt (PTB) |access-date=13 November 2012 }}</ref> {{nowrap|75&nbsp;kg × 9.80665&nbsp;m/s<sup>2</sup> × 1&nbsp;m / 1&nbsp;s}} = 75&nbsp;[[Kilogram-force|{{abbr|kgf|Kilogram-force}}]]⋅m/s = 1&nbsp;PS. This is equivalent to 735.49875&nbsp;W, or 98.6% of an imperial horsepower. In 1972, the PS was replaced by the [[kilowatt]] as the official power-measuring unit in EEC directives.<ref>{{cite web
|url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=DD:I:1971_III:31971L0354:EN:PDF | archive-url=https://web.archive.org/web/20210306071848/https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A31971L0354 | archive-date=2021-03-06
|title=Council Directive 71/354/EEC: On the approximation of the laws of the Member States relating to units of measurement |publisher=The Council of the European Communities
|date=18 October 1971 }}</ref>

Other names for the metric horsepower are the Italian {{lang|it|cavallo vapore (cv)}}, Dutch {{lang|nl|paardenkracht (pk)}}, the French {{lang|fr|cheval-vapeur (ch)}}, the Spanish {{lang|es|caballo de vapor}} and Portuguese {{lang|pt|cavalo-vapor (cv)}}, the Russian {{lang|ru|лошадиная сила (л.&nbsp;с.)}}, the Swedish {{lang|sv|hästkraft (hk)}}, the Finnish {{lang|fi|hevosvoima (hv)}}, the Estonian {{lang|et|hobujõud (hj)}}, the Norwegian and Danish {{lang|da|hestekraft (hk)}}, the Hungarian {{lang|hu|lóerő (LE)}}, the Czech {{lang|cs|koňská síla}} and Slovak {{lang|sk|konská sila (k}} or {{lang|sk|ks}}), the Serbo-Croatian {{lang|sh|konjska snaga (KS)}}, the Bulgarian {{lang|bg|конска сила}}, the Macedonian {{lang|mk|коњска сила (KC)}}, the Polish {{lang|pl|koń mechaniczny (KM)}} ({{literal translation|mechanical horse}}), Slovenian {{lang|sl|konjska moč (KM)}}, the Ukrainian {{lang|uk|кінська сила (к.&nbsp;с.)}}, the Romanian {{lang|ro|cal-putere (CP)}}, and the German {{lang|de|Pferdestärke (PS)}}.

In the 19th century, [[French Revolution|revolutionary-era France]] had its own unit used to replace the ''cheval vapeur'' (horsepower); based on a 100 [[kilogram-force|kgf]]⋅m/s standard, it was called the [[poncelet]] and was abbreviated ''p''.

===Tax horsepower===
{{Main|Tax horsepower}}

Tax or fiscal horsepower is a non-linear rating of a motor vehicle for tax purposes.<ref>{{cite web |url=http://www.numericana.com/answer/units.htm |title=Measurements, Units of Measurement, Weights and Measures |website=numericana.com |access-date=2011-07-18 }}</ref> Tax horsepower ratings were originally more or less directly related to the size of the engine; but as of 2000, many countries changed over to systems based on {{CO2}} emissions, so are not directly comparable to older ratings.{{Citation needed|date=December 2023}} The [[Citroën 2CV]] is named for its French fiscal horsepower rating, "deux chevaux" (2CV).{{Citation needed|date=December 2023}}

===Electrical horsepower===
Nameplates on electrical motors show their available shaft power output, not the electrical power input. This power output is ordinarily stated in watts or kilowatts. In the United States, the power output is stated in horsepower. Wattage is calculated by multiplying voltage by amperage and power factor.<ref>{{cite book |first=H. Wayne |last=Beatty |title=Handbook of Electric Power Calculations |edition=3rd |publisher=McGraw Hill |year=2001 |isbn=0-07-136298-3 |pages=6–14}}</ref>

===Hydraulic horsepower===
Hydraulic horsepower can represent the power available within [[hydraulic machinery]], power through the down-hole nozzle of a [[drilling rig]],<ref name="Schlumberger, Hydraulic horsepower" >{{cite web |title=Hydraulic Horsepower |work=Oilfield Glossary |publisher=Schlumberger |url=http://www.glossary.oilfield.slb.com/Terms/h/hydraulic_horsepower.aspx }}</ref> or can be used to estimate the mechanical power needed to generate a known hydraulic flow rate.

It may be calculated as<ref name="Schlumberger, Hydraulic horsepower" />
: <math>\text{hydraulic power} = \frac{\text{pressure} \times \text{volumetric flow rate}}{1714},</math>
where pressure is in psi, and flow rate is in [[US gallon]]s per minute.

Drilling rigs are powered mechanically by rotating the drill pipe from above. Hydraulic power is still needed though, as 1 500 to 5 000 W are required to push [[drilling mud|mud]] through the drill bit to clear waste rock. Additional hydraulic power may also be used to drive a down-hole mud motor to power [[directional drilling]].<ref name="Schlumberger, Hydraulic horsepower" />

When using SI units, the equation becomes coherent and there is no dividing constant.

: <math>\text{hydraulic power} = \text{pressure} \times \text{volumetric flow rate}</math>
where pressure is in pascals (Pa), and flow rate is in [[cubic metre]]s per second (m<sup>3</sup>).

===Boiler horsepower===
Boiler horsepower is a [[steam boiler|boiler]]'s capacity to deliver [[steam]] to a [[steam engine]] and is not the same unit of power as the 550&nbsp;ft lb/s definition. One boiler horsepower is equal to the thermal energy rate required to evaporate {{convert|34.5|lb}} of fresh water at {{convert|212|°F}} in one hour. In the early days of steam use, the boiler horsepower was roughly comparable to the horsepower of engines fed by the boiler.<ref>{{citation | first=Robert | last=McCain Johnston | title=Elements of Applied Thermodynamics | publisher=Naval Institute Press | date=1992 | isbn=1557502269 | page=503 }}</ref>

The term "boiler horsepower" was originally developed at the [[Philadelphia Centennial Exhibition]] in 1876, where the best steam engines of that period were tested. The average steam consumption of those engines (per output horsepower) was determined to be the evaporation of {{convert|30|lb}} of water per hour, based on feed water at {{convert|100|°F}}, and saturated steam generated at {{cvt|70|psi}}. This original definition is equivalent to a boiler heat output of {{cvt|33,485|Btu/h|kW}}.{{cn|date=January 2025}} A few years later in 1884, the [[American Society of Mechanical Engineers|ASME]] re-defined the boiler horsepower as the thermal output equal to the evaporation of 34.5 pounds per hour of water "from and at" {{convert|212|F}}. This considerably simplified boiler testing, and provided more accurate comparisons of the boilers at that time. This revised definition is equivalent to a boiler heat output of {{cvt|33,469|Btu/h|kW}}.{{cn|date=January 2025}} Present industrial practice is to define "boiler horsepower" as a boiler thermal output equal to {{cvt|33,475|Btu/h|kW}},{{cn|date=January 2025}} which is very close to the original and revised definitions.

Boiler horsepower is still used to measure boiler output in industrial boiler engineering in the US. Boiler horsepower is abbreviated BHP, which is also used in many places to symbolize brake horsepower.

===Drawbar power===
{{See also|Power at rail}}

Drawbar power (dbp) is the power a [[railway]] [[locomotive]] has available to haul a [[train]] or an agricultural tractor to pull an implement. This is a measured figure rather than a calculated one. A special [[railway car]] called a [[dynamometer car]] coupled behind the locomotive keeps a continuous record of the [[drawbar (haulage)|drawbar]] pull exerted, and the speed. From these, the power generated can be calculated. To determine the maximum power available, a controllable load is required; it is normally a second locomotive with its brakes applied, in addition to a static load.

If the drawbar force ({{mvar|F}}) is measured in pounds-force (lbf) and speed ({{mvar|v}}) is measured in miles per hour (mph), then the drawbar power ({{mvar|P}}) in horsepower (hp) is

<math display=block>\{P\}_\mathrm{hp} = \frac{\{F\}_\mathrm{lbf} \{v\}_\mathrm{mph}}{375}.</math>

Example: How much power is needed to pull a drawbar load of 2,025 pounds-force at 5 miles per hour?

<math display=block>\{P\}_\mathrm{hp} = \frac{2025 \times 5}{375} = 27.</math>

The constant 375 is because 1&nbsp;hp = 375&nbsp;lbf⋅mph. If other units are used, the constant is different. When using coherent [[SI]] units (watts, newtons, and metres per second), no constant is needed, and the formula becomes {{math|1=''P'' = ''Fv''}}.

This formula may also be used to calculate the power of a jet engine, using the speed of the jet and the thrust required to maintain that speed.

Example: how much power is generated with a thrust of 4000 pounds at 400 miles per hour?

<math display=block>\{P\}_\mathrm{hp} = \frac{4000 \times 400}{375} = 4266.7.</math>

===RAC horsepower (taxable horsepower)===
{{See also|Tax horsepower}}

This measure was instituted by the [[Royal Automobile Club]] and was used to denote the power of early 20th-century British cars. Many cars took their names from this figure (hence the Austin Seven and Riley Nine), while others had names such as "40/50 hp", which indicated the RAC figure followed by the true measured power.

Taxable horsepower does not reflect developed horsepower; rather, it is a calculated figure based on the engine's bore size, number of cylinders, and a (now archaic) presumption of engine efficiency. As new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations, which used the rating for [[tax horsepower|tax purposes]]. The United Kingdom was not the only country that used the RAC rating; many states in Australia used RAC hp to determine taxation.<ref>{{citation | url=https://books.google.com/books?id=JOg2WxEye1QC&pg=PA409 | title=Official Year Book of the Commonwealth of Australia | volume=44 | date=1958 | page=409 | first=S.R. | last=Carver | publisher=Commonwealth Bureau of Census and Statistics | location=Canberra }}</ref><ref>{{citation | url=https://books.google.com/books?id=wkP3icv3ilcC&pg=RA12-PA39 | title=Motor-Vehicle Taxation and Regulations in Foreign Countries | work=The Balance of International Payments of the United States in 1922- | first=C.E. | last=Haynes | publisher=Bureau of Foreign and Domestic Commerce | date=1923 | volume=Trade Information Bulletin no. 463 | pages=39–42 }}</ref> The RAC formula was sometimes applied in British colonies as well, such as [[Kenya Colony|Kenya (British East Africa)]].<ref>{{harvp|Haynes|1923|p=[https://books.google.com/books?id=wkP3icv3ilcC&pg=RA12-PA43 43]}}</ref>

: <math>\text{RAC h.p.} = \frac{D \times D \times n}{2.5}</math>

where

: ''D'' is the diameter (or [[Bore (engine)|bore]]) of the cylinder in inches,
: ''n'' is the number of cylinders.<ref>{{Cite web |url=http://www.designchambers.com/wolfhound/wolfhoundRACHP.htm |title=The RAC HP (horsepower) Rating - Was there any technical basis? |first=Richard |last=Hodgson |website=WolfHound.org.uk |access-date=2007-08-11 }}</ref>

Since taxable horsepower was computed based on bore and number of cylinders, not based on actual displacement, it gave rise to engines with "undersquare" dimensions (bore smaller than stroke), which tended to impose an artificially low limit on [[rotational speed]], hampering the potential power output and efficiency of the engine.

The situation persisted for several generations of four- and six-cylinder British engines: For example, [[Jaguar Cars|Jaguar's]] 3.4-litre XK engine of the 1950s had six cylinders with a bore of {{convert|83|mm|in|abbr=on|sigfig=3}} and a stroke of {{convert|106|mm|in|abbr=on|sigfig=3}},<ref>{{Cite web |first=Dan |last=Mooney |url=http://www.classicjaguar.com/xkengine.html |title=The XK engine by Roger Bywater |website=ClassicJaguar.com |access-date=2010-03-13 |url-status=dead |archive-url=https://web.archive.org/web/20100223052130/http://www.classicjaguar.com/xkengine.html |archive-date=2010-02-23 }}</ref> where most American automakers had long since moved to oversquare (large bore, short stroke) [[V8 engine]]s. See, for example, the early [[Chrysler Hemi engine#First generation: FirePower|Chrysler Hemi engine]].

==Measurement==

The power of an engine may be measured or estimated at several points in the transmission of the power from its generation to its application. A number of names are used for the power developed at various stages in this process, but none is a clear indicator of either the measurement system or definition used.

In general:
:[[#Nominal horsepower|nominal]] horsepower is derived from the size of the engine and the piston speed and is only accurate at a steam pressure of {{convert|48|kPa|psi|0|abbr=on}};<ref name=Brown>{{citation|last1= Brown |first1 = David K | title = Before the ironclad|publisher= Conway|date=1990|page=188|isbn=0851775322}}</ref>
:[[#Indicated horsepower|indicated]] or gross horsepower is the theoretical capability of the engine [PLAN/ 33000];
:[[#Brake horsepower|brake]]/net/crankshaft horsepower (power delivered directly to and measured at the engine's crankshaft) equals
::indicated horsepower minus frictional losses within the engine (bearing drag, rod and crankshaft windage losses, oil film drag, etc.);
:[[#Shaft horsepower|shaft]] horsepower (power delivered to and measured at the output shaft of the transmission, when present in the system) equals
::crankshaft horsepower minus frictional losses in the transmission (bearings, gears, oil drag, windage, etc.);
:effective or true (thp), commonly referred to as wheel horsepower (whp), equals
::shaft horsepower minus frictional losses in the universal joint/s, differential, wheel bearings, tire and chain, (if present).

All the above assumes that no power inflation factors have been applied to any of the readings.

Engine designers use expressions other than horsepower to denote objective targets or performance, such as brake mean effective pressure (BMEP). This is a coefficient of theoretical brake horsepower and cylinder pressures during combustion.

==={{anchor|nhp}}{{anchor|Nominal horsepower}} Nominal horsepower===
Nominal horsepower (nhp) is an early 19th-century [[rule of thumb]] used to estimate the power of steam engines.<ref name="Brown" /> It assumed a steam pressure of {{convert|7|psi|kPa|0|abbr=on}}.<ref name=WhitePage520/>

Nominal horsepower = 7 × area of piston in square inches × equivalent piston speed in feet per minute/33,000.

For paddle ships, the Admiralty rule was that the piston speed in feet per minute was taken as 129.7 × (stroke)<sup>1/3.38</sup>.<ref name="Brown" /><ref name=WhitePage520/><!-- White says 3.38, whereas Brown says 3.35. Brown's table also has an error and says the piston speed is feet per second, when it is feet per minute as pointed out by User:Fredrosse 20:25, 22 January 2009.--> For screw steamers, the intended piston speed was used.<ref name=WhitePage520/>

The stroke (or length of stroke) was the distance moved by the piston measured in feet.

For the nominal horsepower to equal the actual power it would be necessary for the mean steam pressure in the cylinder during the stroke to be {{convert|7|psi|kPa|0|abbr=on}} and for the piston speed to be that generated by the assumed relationship for paddle ships.<ref name="Brown" />

The French Navy used the same definition of nominal horse power as the Royal Navy.<ref name="Brown" />

{| class="wikitable sortable collapsible"
!colspan=5|Comparison of nominal and indicated horse power
|-
! | Ship
! | Indicated horse power (ihp)
! | Nominal horse power (nhp)
! | Ratio of ihp to nhp
! | Source
|-valign="top"
|| [[HMS Dee (1832)|''Dee'']]
| align = right | 272
| align = right | 200
| align = right | 1.36
||<ref name="Brown" />
|-valign="top"
|| [[HMS Locust (1840)|''Locust'']]
| align = right | 157
| align = right | 100
| align = right | 1.57
||<ref name="Brown" />
|-valign="top"
|| [[HMS Rhadamanthus (1832)|''Rhadamanthus'']]
| align = right | 400
| align = right | 220
| align = right | 1.82
||<ref name="Brown" />
|-valign="top"
|| [[HMS Albacore (1856)|''Albacore'']]
| align = right | 109
| align = right | 60
| align = right | 1.82
||<ref name=WhitePage520>{{citation |title=A Manual of Naval Architecture |first1=William Henry |last1=White |author-link=William Henry White |edition=2nd |date=1882 |publisher=John Murray|page=520 |url=https://archive.org/details/amanualnavalarc04whitgoog/page/n542/mode/1up}}</ref>
|-valign="top"
|| [[HMS Porcupine (1844)|''Porcupine'']]
| align = right | 285
| align = right | 132
| align = right | 2.16
||<ref name="Brown" />
|-valign="top"
|| [[HMS Harpy (1845)|''Harpy'']]
| align = right | 520
| align = right | 200
| align = right | 2.60
||<ref name="Brown" />
|-valign="top"
|| [[HMS Spitfire (1845)|''Spitfire'']]
| align = right | 380
| align = right | 140
| align = right | 2.70
||<ref name="Brown" />
|-valign="top"
|| [[HMS Spiteful (1842)|''Spiteful'']]
| align = right | 796
| align = right | 280
| align = right | 2.85
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Jackal (1844)|''Jackal'']]
| align = right | 455
| align = right | 150
| align = right | 3.03
||<ref name="Brown" />
|-valign="top"
|| [[HMS Supply (1854)|''Supply'']]
| align = right | 265
| align = right | 80
| align = right | 3.31
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Simoom (1849)|''Simoom'']]
| align = right | 1,576
| align = right | 400
| align = right | 3.94
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Hector (1862)|''Hector'']]
| align = right | 3,256
| align = right | 800
| align = right | 4.07
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Agincourt (1865)|''Agincourt'']]
| align = right | 6,867
| align = right | 1,350
| align = right | 5.08
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Bellerophon (1865)|''Bellerophon'']]
| align = right | 6,521
| align = right | 1,000
| align = right | 6.52
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Monarch (1868)|''Monarch'']]
| align = right | 7,842
| align = right | 1,100
| align = right | 7.13
||<ref name=WhitePage520/>
|-valign="top"
|| [[HMS Penelope (1867)|''Penelope'']]
| align = right | 4,703
| align = right | 600
| align = right | 7.84
||<ref name=WhitePage520/>
|-
|}

==={{anchor|Indicated horsepower|ihp}} Indicated horsepower===
Indicated horsepower (ihp) is the theoretical power of a reciprocating engine if it is completely frictionless in converting the expanding gas energy (piston pressure × displacement) in the cylinders. It is calculated from the pressures developed in the cylinders, measured by a device called an ''[[indicator diagram|engine indicator]]'' – hence indicated horsepower. As the piston advances throughout its stroke, the pressure against the piston generally decreases, and the indicator device usually generates a graph of pressure vs stroke within the working cylinder. From this graph the amount of work performed during the piston stroke may be calculated.

Indicated horsepower was a better measure of engine power than nominal horsepower (nhp) because it took account of steam pressure. But unlike later measures such as shaft horsepower (shp) and brake horsepower (bhp), it did not take into account power losses due to the machinery internal frictional losses, such as a piston sliding within the cylinder, plus bearing friction, transmission and gear box friction, etc.

==={{anchor|bhp|Brake_horsepower_(bhp)}} Brake horsepower===
<!-- "Brake horsepower" redirects here. -->
'''Brake horsepower''' ('''bhp''') is the power measured using a brake type (load) dynamometer at a specified location, such as the crankshaft, output shaft of the transmission, rear axle or rear wheels.<ref>{{cite web |url=https://sciencestruck.com/what-is-bhp-brake-horsepower |title=What is Brake Horsepower (BHP)? |website=ScienceStruck.com |date=June 2009 |access-date=26 September 2022 }}</ref>

In Europe, the [[#Deutsches Institut für Normung 70020 (DIN 70020)|DIN 70020]] standard tests the engine fitted with all ancillaries and the exhaust system as used in the car. The older American standard ([[#SAE gross power|SAE gross horsepower]], referred to as ''bhp'') used an engine without [[alternator]], water pump, and other auxiliary components such as power steering pump, muffled exhaust system, etc., so the figures were higher than the European figures for the same engine. The newer American standard (referred to as [[#SAE net power|SAE net horsepower]]) tests an engine with all the auxiliary components (see "Engine power test standards" below).{{Citation needed|date=December 2023}}

''Brake'' refers to the device which is used to provide an equal braking force, load to balance, or equal an engine's output force and hold it at a desired rotational speed. During testing, the output torque and rotational speed are measured to determine the brake horsepower. Horsepower was originally measured and calculated by use of the "indicator diagram" (a James Watt invention of the late 18th century), and later by means of a [[Prony brake]] connected to the engine's output shaft. Modern [[dynamometer]]s use any of several braking methods to measure the engine's brake horsepower, the actual output of the engine itself, before losses to the drivetrain.{{Citation needed|date=December 2023}}

===Shaft horsepower===
Shaft horsepower (shp) is the power delivered to a propeller or turbine shaft.<ref>[https://web.archive.org/web/20161220105927/https://en.oxforddictionaries.com/definition/shaft_horsepower Oxford Dictionary. Retrieved 2016-12-06.] [http://www.dictionary.com/browse/shaft-horsepower Dictionary.com Unabridged, Random House Inc. Retrieved 2016-12-06.]</ref> Shaft horsepower is a common rating for turboshaft and turboprop engines, industrial turbines, and some marine applications.

Equivalent shaft horsepower (eshp) is sometimes used to rate [[turboprop]] engines. It includes the equivalent power derived from residual jet thrust from the turbine exhaust.<ref>{{cite web|url=http://aviation_dictionary.enacademic.com/2550/equivalent_shaft_horsepower|title=equivalent shaft horsepower|website=aviation_dictionary.enacademic.com|access-date=2018-01-17|archive-date=2018-01-17|archive-url=https://web.archive.org/web/20180117190551/http://aviation_dictionary.enacademic.com/2550/equivalent_shaft_horsepower|url-status=dead}}</ref> {{convert|2.5|lbf}} of residual jet thrust is estimated to be produced from one unit of horsepower.<ref name="AF1961">{{cite book |title=Aircraft performance: Reciprocating and turboprop engine aircraft |url={{GBurl|e16zH8NumIQC|pg=SA7-PA36}} |author=[[Department of the Air Force]] |date=November 30, 1961 |pages=7–36}}</ref>

=={{anchor|netgross|standards}} Engine power test standards==
There exist a number of different standards determining how the power and torque of an automobile engine is measured and corrected. Correction factors are used to adjust power and torque measurements to standard atmospheric conditions, to provide a more accurate comparison between engines as they are affected by the pressure, humidity, and temperature of ambient air.<ref>Heywood, J.B. "Internal Combustion Engine Fundamentals", {{ISBN|0-07-100499-8}}, page 54</ref> Some standards are described below.

==={{anchor|SAE}} Society of Automotive Engineers/SAE International===

====Early "SAE horsepower"====
{{for|the SAE horsepower formula|#RAC horsepower (taxable horsepower)}}
In the early twentieth century, a so-called "SAE horsepower" was sometimes quoted for U.S. automobiles. This long predates the [[Society of Automotive Engineers|Society of Automotive Engineers (SAE)]] horsepower measurement standards and was another name for the industry standard [[ALAM]] or [[National Automobile Chamber of Commerce|NACC]] horsepower figure and the same as the British RAC horsepower also used for tax purposes. [[Alliance of Automobile Manufacturers|Alliance for Automotive Innovation]] is the current successor of ALAM and NACC.

===={{anchor|SAE gross}} SAE gross power====
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower, ''[[#Brake horsepower|bhp]]'', which was a version of brake horsepower called SAE gross horsepower because it was measured according to Society of Automotive Engineers (SAE) standards (J245 and J1995) that call for a stock test engine without accessories (such as dynamo/alternator, radiator fan, water pump),<ref name="Lucchesi" /> and sometimes fitted with long tube test [[Exhaust manifold|headers]] in lieu of the [[Original equipment manufacturer|OEM]] exhaust manifolds. This contrasts with both SAE net power and [[#Deutsches Institut für Normung 70020 (DIN 70020)|DIN 70020]] standards, which account for engine accessories (but not transmission losses). The atmospheric correction standards for barometric pressure, humidity and temperature for SAE gross power testing were relatively idealistic.

===={{anchor|SAE net}} SAE net power====
In the United States, the term ''[[#Brake horsepower|bhp]]'' fell into disuse in 1971–1972, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, similar to the [[#Deutsches Institut für Normung 70020 (DIN 70020)|DIN 70020]] standard, SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.

====SAE certified power====
In 2005, the SAE introduced "SAE Certified Power" with SAE J2723.<ref>{{cite web |url=http://www.sae.org/certifiedpower/ |title=Certified Power - SAE J1349 Certified Power SAE International |publisher=Sae.org |access-date=2011-07-18 |archive-url=https://web.archive.org/web/20110728010616/http://www.sae.org/certifiedpower/ |archive-date=2011-07-28 |url-status=dead }}</ref> To attain certification the test must follow the SAE standard in question, take place in an [[ISO 9000]]/9002 certified facility and be witnessed by an SAE approved third party.

A few manufacturers such as Honda and Toyota switched to the new ratings immediately.<ref name=JPlungis>Jeff Plungis, ''Asians Oversell Horsepower'', Detroit News</ref> The rating for [[Toyota Motor Corporation|Toyota's]] [[Toyota Camry|Camry]] 3.0&nbsp;L ''[[Toyota MZ engine#1MZ-FE|1MZ-FE]]'' V6 fell from {{convert|210|to|190|hp|kW|abbr=on}}.<ref name=JPlungis/> The company's Lexus ES 330 and Camry SE V6 (3.3&nbsp;L V6) were previously rated at {{convert|225|hp|kW|abbr=on}} but the ES 330 dropped to {{convert|218|hp|kW|abbr=on}} while the Camry declined to {{convert|210|hp|kW|abbr=on}}. The first engine certified under the new program was the 7.0&nbsp;L [[GM LS engine#7.0 L|LS7]] used in the 2006 [[Chevrolet Corvette]] Z06. Certified power rose slightly from {{convert|500|to|505|hp|kW|sigfig=3|abbr=on}}.

While Toyota and Honda are retesting their entire vehicle lineups, other automakers generally are retesting only those with updated powertrains.<ref name=JPlungis/> For example, the 2006 Ford Five Hundred is rated at {{convert|203|hp}}, the same as that of 2005 model. However, the 2006 rating does not reflect the new SAE testing procedure, as Ford did not opt to incur the extra expense of retesting its existing engines.<ref name=JPlungis/> Over time, most automakers are expected to comply with the new guidelines.

SAE tightened its horsepower rules to eliminate the opportunity for engine manufacturers to manipulate factors affecting performance such as how much oil was in the crankcase, engine control system calibration, and whether an engine was tested with high octane fuel. In some cases, such can add up to a change in horsepower ratings.

==={{anchor|DIN|70020}}''Deutsches Institut für Normung'' 70020 (DIN 70020)===
DIN 70020 is a German [[DIN]] standard for measuring road vehicle horsepower. DIN hp is measured at the engine's output shaft as a form of [[#Metric horsepower|metric horsepower]] rather than [[#Mechanical horsepower|mechanical horsepower]]. Similar to [[#SAE gross power|SAE net power]] rating, and unlike [[#SAE gross power|SAE gross power]], DIN testing measures the engine as installed in the vehicle, with cooling system, charging system and stock exhaust system all connected. DIN&nbsp;hp is often abbreviated as "[[#Metric horsepower|PS]]", derived from the German word '''''Pferdestärke''''' (literally, "horse strength").

===CUNA===
A test standard by [[Italy|Italian]] CUNA (''Commissione Tecnica per l'Unificazione nell'Automobile'', Technical Commission for Automobile Unification), a federated entity of [[standards organisation]] [[Ente Nazionale Italiano di Unificazione|UNI]], was formerly used in Italy.
CUNA prescribed that the engine be tested with all accessories necessary to its running fitted (such as the water pump), while all others – such as alternator/dynamo, radiator fan, and exhaust manifold – could be omitted.<ref name="Lucchesi">{{cite book |url=https://books.google.com/books?id=FVF6CERm7poC&pg=PA550 |first=Domenico |last=Lucchesi |title=Corso di tecnica automobilistica, vol. 1<sup>o</sup>—Il motore |page=550 |publisher=[[Hoepli Editore|Ulrico Hoepli Editore S.p.A.]] |edition=6th |year=2004 |language=it |isbn=88-203-1493-2}}</ref> All calibration and accessories had to be as on production engines.<ref name="Lucchesi" />

==={{anchor|ECE R24}}Economic Commission for Europe R24===
ECE R24 is a [[UNECE Regulations|UN standard]] for the approval of compression ignition engine emissions, installation and measurement of engine power.<ref>{{cite web|url=http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r024r2e.pdf|title=Text of the 1958 Agreement, ECE Regulation 24, Revision 2, Annex 10|website=www.unece.org}}</ref> It is similar to DIN 70020 standard, but with different requirements for connecting an engine's fan during testing causing it to absorb less power from the engine.<ref>{{cite web |first=Jim |last=Breen |url=http://www.farmersjournal.ie/2003/0322/farmmanagement/machinery/tractor.htm |archive-url=https://archive.today/20030406025121/http://www.farmersjournal.ie/2003/0322/farmmanagement/machinery/tractor.htm |url-status=dead |archive-date=2003-04-06 |title= Tractor and machine comparison: what's the 'true' measure |publisher=Irish Farmers Journal |date=2003-03-22 }}</ref>

==={{anchor|ECE R85}}Economic Commission for Europe R85===
ECE R85 is a [[UNECE Regulations|UN standard]] for the approval of internal combustion engines with regard to the measurement of the net power.<ref>{{cite web|url=http://live.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r085e.pdf |title=ECE Regulation 85 |access-date=2011-07-18}}</ref>

===80/1269/EEC===
[[Directive 80/1269/EEC|80/1269/EEC]] of 16 December 1980 is a European Union standard for road vehicle engine power.

==={{anchor|ISO}}International Organization for Standardization===
The [[International Organization for Standardization]] (ISO) publishes several standards for measuring engine horsepower.
* [[ISO 14396]] specifies the additional and method requirement for determining the power of reciprocating internal combustion engines when presented for an [[ISO 8178]] exhaust emission test. It applies to reciprocating internal combustion engines for land, rail and marine use excluding engines of motor vehicles primarily designed for road use.<ref>{{cite web|url=http://www.iso.org/iso/catalogue_detail.htm?csnumber=32909 |title=ISO 14396:2002 - Reciprocating internal combustion engines - Determination and method for the measurement of engine power - Additional requirements for exhaust emission tests in accordance with ISO 8178 |publisher=Iso.org |date=2007-09-30 |access-date=2011-07-18}}</ref>
* [[ISO 1585]] is an engine net power test code intended for road vehicles.<ref>{{cite web|url=http://www.iso.org/iso/catalogue_detail.htm?csnumber=6179 |title=ISO 1585:1992 - Road vehicles - Engine test code - Net power |publisher=Iso.org |date=1999-11-15 |access-date=2011-07-18}}</ref>
* [[ISO 2534]] is an engine gross power test code intended for road vehicles.<ref>{{cite web|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=7476 |title=ISO 2534:1998 - Road vehicles - Engine test code - Gross power |publisher=Iso.org |date=2009-03-31 |access-date=2011-07-18}}</ref>
* [[ISO 4164]] is an engine net power test code intended for mopeds.<ref>{{cite web|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=9932 |title=ISO 4164:1978 - Road vehicles - Mopeds - Engine test code - Net power |publisher=Iso.org |date=2009-10-07 |access-date=2011-07-18}}</ref>
* [[ISO 4106]] is an engine net power test code intended for motorcycles.<ref>{{cite web|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=31406 |title=ISO 4106:2004 - Motorcycles - Engine test code - Net power |publisher=Iso.org |date=2009-06-26 |access-date=2011-07-18}}</ref>
* [[ISO 9249]] is an engine net power test code intended for earth moving machines.<ref>{{cite web|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=38008 |title=ISO 9249:2007 - Earth-moving machinery - Engine test code - Net power |publisher=Iso.org |date=2011-03-17 |access-date=2011-07-18}}</ref>

==={{anchor|Japan|JIS}}Japanese Industrial Standard D 1001===
JIS D 1001 is a Japanese net, and gross, engine power test code for [[automobile]]s or [[truck]]s having a spark ignition, [[diesel engine]], or fuel injection engine.<ref>{{cite web |url=http://www.webstore.jsa.or.jp/webstore/Com/FlowControl.jsp?lang=en&bunsyoId=JIS+D+1001%3A1993&dantaiCd=JIS&status=1&pageNo=0 |title=JSA Web Store - JIS D 1001:1993 Road vehicles - Engine power test code |publisher=Webstore.jsa.or.jp |access-date=2011-07-18 |archive-url=https://web.archive.org/web/20110722121539/http://www.webstore.jsa.or.jp/webstore/Com/FlowControl.jsp?lang=en&bunsyoId=JIS+D+1001:1993&dantaiCd=JIS&status=1&pageNo=0 |archive-date=2011-07-22 |url-status=dead }}</ref>

==See also==
* [[Brake-specific fuel consumption]] – how much fuel an engine consumes per unit energy output
* [[Dynamometer#How dynamometers are used for engine testing|Dynamometer engine testing]]
* [[European units of measurement directives]]
* [[Horsepower-hour]]
* [[Mean effective pressure]]
* [[Torque]]

==References==
{{Reflist|30em}}

==External links==
{{EB1911 poster|Horse-power}}
* {{cite web |url=https://www.voltagelab.com/how-much-horsepower-does-a-horse-have/ |title=How Much Horsepower Does a Horse Have? |author=Md Nazmul Islam |work=Voltage Lab |date=2024-06-10}}
* {{cite AV media |url=https://www.youtube.com/watch?v=7qxTKtlvaVE |title=How Much Horsepower is a Horse? |via=YouTube |author=Donut |date=2023-11-24}}
* {{cite web |url=http://auto.howstuffworks.com/horsepower.htm |title=How Horsepower Works |website=How Stuff Works |first1=Marshall |last1=Brain |first2=Talon |last2=Homer |date=2024-05-23}}

{{Imperial units}}
{{United States Customary Units}}

[[Category:Imperial units]]
[[Category:Units of power]]
[[Category:Customary units of measurement in the United States]]
[[Category:James Watt]]