Conversions are only as good as their factors. An inch is defined as exactly 25.4 millimetres and a pound as exactly 0.45359237 kilograms; a converter that uses rounded values will be slightly wrong in ways that only show up once the numbers get large. The unit converter here uses the exact definitions published by the US National Institute of Standards and Technology and the international SI brochure, lists the factor it used for every result, and formats the answer to a sensible number of significant digits instead of printing floating-point noise.
Same name, different unit
Several everyday units share a name but not a size. A US gallon is about 3.785 litres, an imperial gallon about 4.546; a US fluid ounce and an imperial fluid ounce differ by about four percent; a "ton" can be short, long or metric. The converter names each variant explicitly so you never have to guess which one you got. Traditional Chinese units are included as they are officially defined today — one jin is exactly half a kilogram, one mu is 2,000/3 square metres — because they are still what markets and land records use.
Temperature is not a multiplication
Most conversions multiply by a factor, but Celsius and Fahrenheit have different zero points, so the converter adds and multiplies in the right order. The page also explains the quick mental approximations — and where they break down — because in a kitchen or on a trip you usually need a rough answer quickly more than an exact one slowly.
Numbers in other notations
- Unit converter: length, mass, temperature, area, volume and speed, with a full comparison table for any value.
- Number base converter: binary, octal, decimal, hexadecimal and any base up to 36, with arbitrarily large integers, two's complement and IEEE 754 views.
- Roman numerals: to and from Roman numerals from 1 to 3,999, with strict validation of the subtractive rules.
Every converter keeps the current values in the address bar, so you can bookmark or share a specific conversion and it opens exactly as you left it.
Which unit is it?
The multiplication is the easy part of a conversion; knowing which unit you are looking at is harder. A cup is 236.6 mL in an American recipe and 250 mL in an Australian one, an ounce can be a weight or a volume, and an imperial gallon is about 20% bigger than a US gallon, so the same car shows a higher mpg figure in the UK. The guide to units in recipes and travel works through the kitchen, oven temperatures, speed limits, fuel economy and luggage allowances, and points out where each conversion usually goes wrong. It also covers percentages you meet abroad — tips, prices that include VAT, and stacked discounts, where 20% off followed by 10% off is 28% off, not 30%.
Precision and rounding
The conversion factors used here are the exact legal definitions — an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg — so results are as precise as the numbers you type in. Results are shown to up to ten significant figures, far more than any real measurement supports; round them to match the precision of what you measured, not the precision of the display.