Britain officially went metric decades ago, but daily life never fully followed. Road signs are in miles, beer is sold in pints, people weigh themselves in stone, and cars are rated in miles per gallon, while food is labelled in grams, fuel is sold by the litre and science and engineering work almost entirely in metric. Most of the time the mix is harmless. In a few places it causes real mistakes, and those are worth knowing.
The gallon problem
A UK gallon is about 4.546 litres. A US gallon is about 3.785 litres, roughly 17% smaller. That matters because many car reviews, websites and apps quote US figures. A car advertised at 40 miles per US gallon would show about 48 miles per UK gallon. If you compare a UK figure with a US one without converting, the American car looks much less efficient than it really is. The volume converter handles both gallons separately.
Fuel economy runs backwards in Europe
Britain measures fuel economy in miles per gallon, where higher is better. Most of Europe uses litres per 100 kilometres, where lower is better. Because one is the inverse of the other, the conversion isn't a simple multiplication. A car doing 50 mpg in UK gallons uses about 5.6 litres per 100 km, and one doing 25 mpg uses about 11.3. Halving the mpg doubles the fuel used, which is easier to see in litres per 100 km.
Stones, pounds and kilograms
Bathroom scales, gyms and GP surgeries don't always use the same units. One stone is 14 pounds, about 6.35 kg. Medicine doses, especially for children, are usually based on weight in kilograms, so converting carefully matters there more than anywhere. The weight converter converts between all three.
Pressure on your tyres
Tyre pressures appear in psi on most UK forecourt gauges, but in bar on many car manuals and European pumps. One bar is about 14.5 psi, so a recommended 2.3 bar is about 33 psi. Mixing the two up can leave tyres badly under- or over-inflated. The pressure converter gives both.
Converting reliably
Three habits prevent most conversion errors. Check which version of a unit you have, especially gallons, tons and ounces, which all have UK and US forms. Convert the underlying measurement, not a derived figure: for fuel economy, convert gallons to litres and miles to kilometres rather than guessing a factor. And keep a sensible number of decimal places. Rounding too early, then multiplying, is how small conversion errors become large ones. Every converter on this page uses exact definitions and lets you choose the precision.