Metric ↔ Imperial: Exact Conversions and How to Use Them

The exact factors between metric and imperial units for length, weight, volume and temperature — with worked examples and why US and UK measures differ.

Last updated: September 2026

Converting between metric and imperial units trips people up because the two systems come from different histories. The good news: the factors between them are fixed by international definition — they are exact, not rough approximations — so once you know the right number, every conversion is just one multiplication.

Length

These four cover almost everything you meet day to day:

Worked example: a 6-foot person is 6 × 0.3048 = 1.8288 m, about 1.83 m. A 26.2-mile marathon is 26.2 × 1.609344 = 42.16 km.

Weight (mass)

Worked example: 150 lb is 150 × 0.45359237 = 68.0 kg. In the UK, 11 stone is 11 × 6.35029318 = 69.9 kg.

Volume — mind the gallon

This is where people get caught: a US gallon and a UK (imperial) gallon are not the same size.

A UK gallon is about 20% larger than a US gallon, which is why a UK "miles per gallon" figure looks better than the US one for the same car. If a recipe or fuel figure does not say which gallon, check the country it came from.

Temperature

Temperature is different because the scales do not share a zero, so you convert with a formula rather than a single factor:

Worked example: a 180 °C oven is 180 × 9/5 + 32 = 356 °F. Body temperature 98.6 °F is (98.6 − 32) × 5/9 = 37 °C.

All factors are exact by international definition. Source: NIST Special Publication 811 (Guide for the Use of the International System of Units).

Do it instantly

The unit converter applies all of these for length, weight, temperature and volume, with the results to six decimals — handy for cooking, travel, DIY and schoolwork.