Type a number once and every unit in the category fills in. Nothing is hidden behind a second dropdown, because the answer you want is usually the one you did not think to ask for.

The numbers here are the defining ones

inch          0.0254 metres exactly, since 1959
pound         0.45359237 kilograms exactly
nautical mile 1852 metres exactly
atmosphere    101,325 pascals exactly

These are not measurements. They are definitions, agreed
so that the two systems line up on paper.

Most of the imperial units have been defined in metric terms since the International Yard and Pound Agreement of 1959. An inch is not approximately 2.54 centimetres, it is exactly that, and every foot, yard, mile and acre falls out of the same definition.

Where a value genuinely is approximate, it says so on the unit. Mach depends on air temperature, an average month is not any real month, and a millimetre of mercury is a conventional figure, not a measurement of actual mercury.

A pint is not a pint

US pint        473.176473 ml
imperial pint  568.26125 ml

A British pint is about a fifth bigger. Same word,
different drink, and the same is true of the gallon
and the fluid ounce.

The United States kept the pre-1824 English measures and Britain redefined its own, so every liquid unit split in two. American and imperial fluid ounces, pints, quarts and gallons are all different sizes and all still called the same thing.

Recipes are the usual casualty. An American cup is 237 ml, a metric cup is 250, and a British recipe from before the 1970s might mean 284. All three are in the list.

Kilobytes, and the missing space on your disk

A 1 TB disk

sold as       1,000,000,000,000 bytes
shown as      931 GB by Windows

Nothing is missing. The shop counted in thousands and
the operating system counted in 1024s.

Kilo means a thousand everywhere except computing, where it has also meant 1,024 since the 1960s because that is a round number in binary. The IEC invented kibi, mebi and gibi in 1998 to separate them, and almost nobody outside Linux uses those names.

So a terabyte disk really does hold a trillion bytes, and Windows really does report 931 GB, and both are telling the truth about different units. The gap grows with size: it is 2.4% at kilobytes and 10% at terabytes.

Fuel economy runs backwards

30 mpg (US)   = 7.84 l/100km
40 mpg (US)   = 5.88 l/100km

Doubling the miles per gallon halves the litres. The
two run in opposite directions, so the conversion is a
division, not a multiplication.

Miles per gallon measures distance over fuel, and litres per 100 km measures fuel over distance. One goes up as the other goes down, so converting means dividing instead of multiplying, and doubling one halves the other.

It also means the improvement from 15 to 20 mpg saves more fuel than the improvement from 40 to 50, which is the opposite of what the numbers look like. Litres per 100 km makes that obvious and miles per gallon hides it.

American and imperial mpg differ too, because the gallons do. The same car reads about 20% better in imperial, which is worth knowing before comparing a British review to an American one.

Temperature has an offset

Every other unit here is a straight multiplication, so half a metre is half a yard's worth of the same thing. Temperature scales start in different places, so converting means scaling and then shifting.

The consequence: a temperature difference converts differently from a temperature. A rise of 10°C is a rise of 18°F, not 50°F. This tool converts temperatures, so subtract first if what you have is a difference.

Precision

Everything converts through one base unit per category, at full double precision, and is rounded only when it is displayed. Ten significant figures are shown, which is well inside what a double holds exactly, and anything enormous or microscopic switches to exponent notation instead of printing nineteen digits.

Clicking a unit name makes it the one you are converting from, carrying the number across, so you can walk through a chain without retyping.