How unit conversion works

Most unit conversions are simple multiplication by a fixed factor, because the units are defined in relation to each other. One kilometre is defined as 1,000 metres, and one mile is defined as 1,609.344 metres, so one kilometre is always 0.621371 miles. The same logic applies to kilograms and pounds, centimetres and inches, and litres and gallons. There is no opinion or estimate in the factor itself — only rounding in how the answer is displayed.

Temperature is the exception. Celsius and Fahrenheit do not share the same zero point, so conversion needs both multiplication and addition: Fahrenheit equals Celsius multiplied by 9/5, plus 32. That is why 0°C is 32°F, not 0°F, and why −40 is the same in both scales.

Gallons are the trap. The US gallon and the imperial gallon are different volumes. This converter uses US gallons and says so on the page, because most online recipes, car forums, and imported manuals that Australians read use US gallons. If a British recipe or an older Australian manual means imperial gallons, use 0.219969 imperial gallons per litre instead, and do not mix the two in one calculation.