Unit converter

Type a number and say what it is. Every other unit it could be appears at once, so there is no second dropdown to hunt through for the one you wanted.

Most of these are exact, and that is not obvious

An inch is exactly 25.4 millimetres. Not approximately: in 1958 a conference of the English-speaking nations agreed to unify their standards of length and mass and define them in metric terms, and the resulting value took effect on 1 July 1959. Converting inches to millimetres therefore involves no rounding at all. The same is true of the pound, which is exactly 0.45359237 kilograms, and of the nautical mile, which is exactly 1,852 metres.

This means a figure like 1,609.344 metres in a mile is not an approximation that might be refined later. It is the definition, worked out. Where a conversion genuinely is approximate the reading says so.

Which gallon, and which ton?

A US gallon is 231 cubic inches, or about 3.785 litres. An imperial gallon is exactly 4.54609 litres. They are both called a gallon and the imperial one is about 20% larger, which is enough to ruin a recipe, misprice a tank of fuel, and make two perfectly correct fuel-economy figures look like a disagreement.

Worse, there are three tons. A short ton is 2,000 pounds, a long ton is 2,240 pounds, and a tonne is 1,000 kilograms. They are within about 10% of each other, which is exactly the range in which a wrong one looks plausible. This converter never says just “gallon” or just “ton”; it makes you pick, and it tells you which you picked.

Temperature is not a ratio, and that breaks most converters

Every other conversion on this page is a multiplication: double the miles and you double the kilometres. Temperature is not like that, because its scales do not start in the same place. Zero degrees Celsius is 32 degrees Fahrenheit, not zero, so a converter built only on multiplication gets every temperature wrong by 32 and is right only at a single point.

The one place the two scales agree is −40, which is the same temperature in both. It is a good check on any converter you are handed: if it cannot get −40 right, it is multiplying when it should be shifting.

Why there is no second dropdown

A converter normally asks you for three things: a category, a unit to convert from, and a unit to convert to. The category is redundant — saying “miles” already says you mean length — and the third is a list you have to search for the answer you wanted.

So this asks for two and prints the lot. The unit you most likely wanted is the headline; everything else in the same family is underneath it.

What this does not do

Cooking measures are a minefield this stays out of. A cup is not the same in America, Britain, Australia and Japan, and a “tablespoon” ranges from 15 to 20 millilitres depending on where the recipe was written. The US cup here is the 240 ml one used on nutrition labels; if your recipe predates that, it may mean something else.

Nor does it convert weight to volume. A litre of water weighs a kilogram and a litre of honey does not, so flour in cups to flour in grams is a question about flour, not about units.

Where to go next

Conversions that need their own page, because the rules are not arithmetic:

• tyre size reader — millimetres, inches and a percentage, all in one code
• percentage calculator — when the conversion is a proportion

Where this comes from

The conversion factors here are definitions rather than measurements, so they are not taken from anybody's table — they are what the units mean. The date is a fact about history rather than arithmetic, and it was checked rather than remembered: this page first said “the agreement of 1959”, which is wrong by a year.