Translators and localisers often get confused when converting American units to SI units (see Notes). Temperature conversions are not as simple as they look. Fahrenheit and Centigrade conversion different depending on whether the temperature is absolute or relative. Most people are familiar with temperatures such as these:
Freezing point of water: 0 ºC and 32 ºF
Boiling point of water: 100 ºC and 212 ºF
Temperature at which the two curves cross: -40 ºC = -40 ºF
You can get from ºC to ºF by multiplying by 9/5 and adding 32 (or by adding 40, multiplying by 9/5 and subtracting 40).
You get from ºF to ºC by subracting 32 and multiplying by 5/9 (or by adding 40, multiplying by 5/9 and subtracting 40).
But in relative terms, 5 degrees C equals 9 degrees 5. Now our table looks like this:
0 degrees C = 0 degrees F
100 degrees C = 180 degrees F
-40 degrees C = -72 degrees F
Note that the º symbol should be used ONLY for absolute temperatures, and “degrees” (or its translation) is used for temperature differences. So by using the units correctly, we can say that 100 ºC = 212 ºF but 100 degrees C = 180 degrees F.
- “American”, not “English”, since every other English-speaking country in the world has adopted SI (Système International) units, and because not all the old English units are the same as in America.
- Not “Imperial”, because this only refers to some of the units; an Imperial Gallon is 4.542 litres, 20% more than a US Gallon, which is 3.785 litres, which is on reason why US cars look like they use much more fuel than British ones!
- Not “metric”, since all American units are already defined in terms of metric units. For example, the official definition of an inch has for many years been 2.54 millimeters.