The Complete Guide to Metric and Imperial Conversions

Metric and imperial units coexist awkwardly in daily life, especially if you deal with people or recipes or products from different countries. Here's a practical guide to the conversions that come up most, with the quick mental shortcuts that get you close without a calculator.

Length

One inch is exactly 2.54 centimetres. One foot is 30.48 cm, and one metre is about 3.28 feet. For a quick head estimate, a metre is a bit over a yard, and a kilometre is roughly 0.62 miles — so to convert kilometres to miles, multiply by about 0.6 (10 km ≈ 6 miles). Going the other way, a mile is about 1.6 km.

Weight

One kilogram is about 2.2 pounds, so to convert kilos to pounds, double the number and add a bit (10 kg ≈ 22 lb). One pound is roughly 0.45 kg. This is exactly the conversion behind our BMI Calculator, which lets you enter weight in either pounds or kilograms.

Temperature

This is the awkward one, because the scales don't share a zero. To go from Celsius to Fahrenheit, multiply by 9/5 and add 32. To go the other way, subtract 32 and multiply by 5/9. A useful anchor: 0°C is 32°F (freezing), 100°C is 212°F (boiling), and a comfortable room at 20°C is 68°F. For a rough mental estimate of Celsius to Fahrenheit, double it and add 30 — not exact, but close enough to know whether to bring a coat.

Volume

One litre is about 1.06 US quarts, or roughly 0.26 US gallons — so a gallon is a little under 4 litres. A US cup is about 240 millilitres. Note that US and imperial (UK) gallons differ: a UK gallon is about 4.55 litres, larger than the US gallon's 3.79 litres. When following a recipe, check which country it's from.

Why precision sometimes matters

Mental shortcuts are perfect for everyday estimates — knowing whether 30°C is warm, or whether 5 km is walkable. But for anything where accuracy counts (medication doses, engineering, finance), use exact factors, not approximations. The rounding that's harmless when packing for a trip can be costly elsewhere. When you need exactness, our calculators use precise conversion constants throughout.