Free Unit Converters

Conversion without the guesswork

A unit converter is the simplest kind of tool there is, which is why there are thousands of them and why so many are subtly wrong. They round the conversion factor. They confuse the US gallon with the imperial one. They present a result to fifteen decimal places, most of which are noise. They wrap two input boxes in enough advertising that you lose track of which one you typed in.

These converters do the boring thing properly. Type a number into either side and the other updates immediately. Change a unit and the result follows. Every page shows the factor it applied — 1 kg = 2.20462262 lb — directly beneath the result, so you can sanity-check the answer without leaving the page or trusting us.

Where the numbers come from

Most unit relationships people need day to day are defined exactly rather than measured. The international yard and pound agreement of 1959 fixed the pound at exactly 0.45359237 kilograms and the inch at exactly 25.4 millimetres. The litre is exactly one cubic decimetre. These are definitions, not approximations, and there is no reason for a converter to use a rounded version of them.

We store the exact factor for each unit and do the arithmetic in full double precision, rounding only when the answer is displayed. In practice that means you can chain conversions — kilograms to pounds to ounces and back — and land on the number you started with, which is not true of tools that round at each step.

Temperature is the exception worth flagging: Celsius, Fahrenheit and Kelvin differ by an offset as well as a scale, so they are handled by their own formulas rather than a single multiplier. That tool shows the formula it used for the same reason the others show the factor.

US, UK and metric

A great deal of confusion comes from unit names that mean different things in different places. A US gallon is about 3.785 litres; an imperial gallon is about 4.546. A US ton is 2,000 pounds; a UK long ton is 2,240. A US fluid ounce and a UK fluid ounce differ by roughly four percent — small enough to look like a rounding error and large enough to ruin a recipe or a shipping calculation.

Rather than choosing a default and hoping, every unit list here names the system explicitly. If you pick the wrong one you will see it in the dropdown, and the factor line underneath the result gives you a second chance to catch it.

Built for the quick check

Most visits to a converter last under thirty seconds: you have one number, you need it in another unit, and then you are gone. The pages are built around that. The inputs are focused and ready when the page opens, results update as you type with no submit button, and the two most common units for the conversion are preselected so a large share of visits need no dropdown at all.

More converters on the way

Length, temperature, volume, area, speed, data storage and cooking measures are all queued, along with the sub-pages that answer a specific question directly — how many pounds is 70 kilograms, how many millilitres in a US cup — for the conversions people look up most.

Every one of them will follow the same three rules as this page: exact factors where an exact definition exists, the factor shown alongside the result, and all the arithmetic done on your own device. If a converter you need is missing, tell us and it will go near the top of the list.