Editing Metric system (section)

==Everyday notions==
The basic units of the metric system have always represented commonplace quantities or relationships in nature; even with modern refinements of definition and methodology. In cases where laboratory precision may not be required or available, or where approximations are good enough, the commonplace notions may suffice.

===Time===
The second is readily determined from the Earth's rotation period. Unlike other units, time multiples are not decimal. A second is {{sfrac|1|60}} of a minute, which is {{sfrac|1|60}} of an hour, which is {{sfrac|1|24}} of a day, so a second is {{sfrac|1|{{val|86400}}}} of a day.

===Length===
The length of the [[equator]] is close to {{val|40000000|u=metres}} (more precisely {{val|40075014.2|u=metres}}).<ref>{{cite web |last1=Science |first1=Tim Sharp 2017-09-15T15:47:00Z |last2=Astronomy |title=How Big Is Earth? |url=https://www.space.com/17638-how-big-is-earth.html |access-date=22 October 2019 |website=Space.com}}</ref> In fact, the dimensions of our planet were used by the French Academy in the original definition of the metre.<ref>{{cite web |title=Metre {{!}} measurement |url=https://www.britannica.com/science/metre-measurement |access-date=22 October 2019 |website=Encyclopedia Britannica}}</ref> A dining tabletop is typically about 0.75 metres high.<ref>{{cite web |title=Standard Table Sizes |url=https://www.bassettfurniture.com/blog/standard-table-sizes.aspx |access-date=22 October 2019 |website=Bassett Furniture}}</ref> A very tall human is about 2 metres tall.<ref>{{cite web |date=9 December 2018 |title=The Average Height of NBA Players – From Point Guards to Centers |url=https://www.thehoopsgeek.com/average-nba-height/ |access-date=22 October 2019 |website=The Hoops Geek}}</ref>

===Mass===
A [[1 euro coin|1-euro coin]] weighs 7.5&nbsp;g;<ref>{{cite web |title=RUBINGHSCIENCE.ORG / Using Euro coins as weights |url=http://www.rubinghscience.org/surv/euroweights1.html |access-date=22 October 2019 |website=www.rubinghscience.org}}</ref> a [[Sacagawea dollar|Sacagawea US 1-dollar coin]] weighs 8.1&nbsp;g;<ref>{{cite web |date=20 September 2016 |title=Coin Specifications {{!}} U.S. Mint |url=https://www.usmint.gov/learn/coin-and-medal-programs/coin-specifications |access-date=22 October 2019 |website=www.usmint.gov}}</ref> a [[Fifty pence (British coin)|UK 50-pence coin]] weighs 8.0&nbsp;g.<ref>{{cite web |title=Fifty Pence Coin |url=https://www.royalmint.com/discover/uk-coins/coin-design-and-specifications/fifty-pence-coin/ |access-date=22 October 2019 |website=www.royalmint.com}}</ref>

===Temperature===
In every day use, Celsius is more commonly used than Kelvin, however a temperature difference of one Kelvin is the same as one degree Celsius and that is defined as {{sfrac|1|100}} of the temperature differential between the freezing and boiling points of water at [[sea level]]. A temperature in Kelvin is the temperature in Celsius plus about 273. [[Human body temperature]] is about 37&nbsp;°C or 310&nbsp;K.

===Length, mass, volume relationship===
The mass of a litre of cold water is 1 kilogram. 1 millilitre of water occupies 1 cubic centimetre and weighs 1 gram.

===Candela and Watt relationship===
Candela is about the luminous intensity of a moderately bright candle, or 1 candle power. A 60&nbsp;[[Watt]] tungsten-filament [[incandescent light bulb]] has a luminous intensity of about 800 lumens<ref>{{cite web|title=Lumens and the Lighting Facts Label|url=https://www.energy.gov/energysaver/save-electricity-and-fuel/lighting-choices-save-you-money/lumens-and-lighting-facts|access-date=11 June 2020|website=Energy.gov|language=en}}</ref> which is radiated equally in all directions (i.e. 4{{pi}} steradians), thus is equal to {{math|''I''<sub>v</sub> {{=}} {{sfrac|800 lm|4{{pi}} sr}} ≈ 64 cd}}.

===Watt, Volt and Ampere relationship===
A 60&nbsp;W incandescent light bulb consumes 0.5&nbsp;A at 120&nbsp;V (US mains voltage). A 60&nbsp;W bulb rated at 230&nbsp;V (European mains voltage) consumes 0.26&nbsp;A at this voltage. This is evident from the formula {{math|''P'' {{=}} ''I'' ''V''}}.

===Mole and mass relationship===
A mole of a substance has a mass that is its [[molecular mass]] expressed in units of grams. The mass of a mole of carbon is 12.0&nbsp;g, and the mass of a mole of table salt is 58.4&nbsp;g.

Since all gases have the same volume per mole at a given temperature and pressure far from their points of liquefaction and solidification (see [[Perfect gas]]), and air is about {{sfrac|1|5}} oxygen (molecular mass 32) and {{sfrac|4|5}} nitrogen (molecular mass 28), the density of any near-perfect gas relative to air can be obtained to a good approximation by dividing its molecular mass by 29 (because {{nowrap|{{sfrac|4|5}} × 28 + {{sfrac|1|5}} × 32 {{=}} 28.8 ≈ 29}}). For example, [[carbon monoxide]] (molecular mass 28) has almost the same density as air.