Editing Conversion of units (section)

=== Limitations ===
The factor–label method can convert only unit quantities for which the units are in a linear relationship intersecting at 0 ([[Level of measurement#Ratio scale|ratio scale]] in Stevens's typology). Most conversions fit this paradigm. An example for which it cannot be used is the conversion between the [[Celsius scale]] and the [[Kelvin scale]] (or the [[Fahrenheit scale]]). Between degrees Celsius and kelvins, there is a constant difference rather than a constant ratio, while between degrees Celsius and degrees Fahrenheit there is neither a constant difference nor a constant ratio. There is, however, an [[affine transform]] ({{tmath|1= x \mapsto ax+b }}, rather than a [[linear transform]] {{tmath|1= x \mapsto ax }}) between them.

For example, the freezing point of water is 0&nbsp;°C and 32&nbsp;°F, and a 5&nbsp;°C change is the same as a 9&nbsp;°F change. Thus, to convert from units of Fahrenheit to units of Celsius, one subtracts 32&nbsp;°F (the offset from the point of reference), divides by 9&nbsp;°F and multiplies by 5&nbsp;°C (scales by the ratio of units), and adds 0&nbsp;°C (the offset from the point of reference). Reversing this yields the formula for obtaining a quantity in units of Celsius from units of Fahrenheit; one could have started with the equivalence between 100&nbsp;°C and 212&nbsp;°F, which yields the same formula.

Hence, to convert the numerical quantity value of a temperature ''T''[F] in degrees Fahrenheit to a numerical quantity value ''T''[C] in degrees Celsius, this formula may be used:
: ''T''[C] = (''T''[F] − 32) × 5/9.

To convert ''T''[C] in degrees Celsius to ''T''[F] in degrees Fahrenheit, this formula may be used:
: ''T''[F] = (''T''[C] × 9/5) + 32.