Unit conversion reference

Temperature is the odd one out

Every other conversion here is a single multiplication, because both scales start at zero. Temperature does not: Celsius and Fahrenheit have different zero points, so you need °F = °C × 9/5 + 32, and the offset matters as much as the ratio.

The rough mental version — double it and add 30 — is within a couple of degrees across normal weather. 20°C gives 70 against a true 68°F.

Factors worth memorising

1 kg is 2.2 lbs, 1 inch is 2.54 cm exactly, 1 mile is 1.61 km, 1 litre is 0.26 US gallons. Those four cover most everyday needs and each is accurate enough to check whether a converted figure is plausible.

The 2.54 is exact by definition — the inch was redefined in terms of the metre in 1959. The others are rounded.

US and imperial gallons are different

A US gallon is 3.785 litres; an imperial gallon is 4.546. That is a 20% difference and it silently corrupts fuel-economy comparisons between US and UK figures. Everything in this table uses US units where the two diverge.

The same split affects pints and fluid ounces, which is why a recipe converted from the wrong side of the Atlantic can be noticeably off.

Weight and mass, and why nobody minds

Strictly, kilograms measure mass and pounds measure force, so "converting" between them assumes standard Earth gravity. In practice everyone means the everyday sense and the conversion is fine. It only matters if you are doing physics, at which point you already know.

Common questions