What this does

This converts between the units people actually need day to day — centimetres to inches, kilos to pounds, Celsius to Fahrenheit, litres to gallons, square metres to square feet — across ten categories, and it updates as you type rather than making you press a button. Open the panel at the bottom and you get your figure in every unit in the category at once, which is faster than running the same conversion five times.

The conversion factors are the exact defined ones wherever an exact definition exists. An inch is precisely 0.0254 metres and a pound is precisely 0.45359237 kilograms by international agreement, so converting a value across and back gives you the number you started with instead of something a hair off.

How to use it

Pick a category — length, weight, temperature, volume, area, speed, time, data, energy or pressure.
Type the amount and choose the two units. The answer appears immediately.
Use the swap button to flip the direction, or copy the result with one click.

The amount box takes fractions as well as decimals, so 1 1/2 and3/4 work as typed. That saves converting a recipe into decimals in your head before you can convert it into millilitres.

The conversions people look up most

1 inch = 2.54 centimetres, and 1 centimetre = 0.3937008 inches
1 foot = 30.48 centimetres, and 1 metre = 3.280839895 feet
1 mile = 1.609344 kilometres, and 1 kilometre = 0.6213711922 miles
1 kilogram = 2.204622622 pounds, and 1 pound = 0.45359237 kilograms
1 stone = 6.35029318 kilograms (14 pounds)
1 US gallon = 3.785411784 litres, and 1 imperial gallon = 4.54609 litres
1 US cup = 236.5882365 millilitres
1 square metre = 10.76391042 square feet, and 1 acre = 4046.8564224 square metres

Converting temperature

Temperature is the odd one out. Every other unit here is a straight multiplication, because zero means the same thing on both scales — nought metres and nought feet are the same nothing. Celsius and Fahrenheit don't share a zero, so a temperature conversion has to shift as well as scale: °F = °C × 9/5 + 32, and back the other way°C = (°F − 32) × 5/9. That's also why you can convert a distance of 10 metres into feet but you can't convert a temperature difference of 10 °C into Fahrenheit the same way — a 10-degree rise in Celsius is an 18-degree rise in Fahrenheit, not 50.

US or imperial?

They are not the same, and volume is where it bites. A US gallon is 3.785 litres while an imperial gallon is 4.546 — about 20% bigger — so US and imperial pints, quarts and fluid ounces all differ too. Both are listed separately here. Weight is friendlier: a pound and an ounce are the same on both sides of the Atlantic. A ton isn't, which is why the US ton (2,000 lb) and the imperial ton (2,240 lb) each have their own entry, alongside the metric tonne of 1,000 kg.

Kilobytes, kibibytes and the missing gigabytes

Storage has two competing definitions. Drive manufacturers use the decimal one, where a gigabyte is 1,000,000,000 bytes. Operating systems have historically used binary units, where the same quantity is measured in chunks of 1,024 — properly called a gibibyte. That's the whole reason a "1 TB" drive shows up as about 931 GB: nothing is missing, the two systems are just counting differently. Both are in the Data category so you can see the gap for yourself.

Is it private?

Yes. Every conversion is arithmetic done by JavaScript inside this page — nothing you type is sent anywhere, and there is no server involved at any point. Once the page has loaded you can go offline and it will carry on working.