How to convert units
- Pick the kind of quantity: length, weight, temperature, area, volume or speed.
- Type a value. Thousands separators are fine — 1,250 and 1 250 both work — and so is a European decimal comma such as 2,5.
- Choose the unit you have and the unit you want. The ⇄ button swaps them.
- Read the result, the conversion factor underneath it, and the table showing the same value in every unit of that kind.
The page address updates as you go, so a bookmark or a shared link opens the same conversion. For example, the link ending in ?d=temperature&from=c&to=f&v=37 opens a conversion of 37 °C.
Where the numbers come from
Since the international yard and pound agreement of 1959, the customary units used in the US and UK have been defined exactly in terms of metric ones. An inch is exactly 25.4 millimetres, a foot 0.3048 metres, a mile 1,609.344 metres and a pound 0.45359237 kilograms. These are definitions, not measurements, so a converter can be exact. This one stores every unit as an exact multiple of a base unit — the metre, kilogram, square metre, litre or metre per second — and converts through it. The factors follow NIST Special Publication 811 and NIST Handbook 44.
US and imperial are different
Volume is where most conversion mistakes happen. The US gallon is defined as 231 cubic inches, about 3.785 litres; the imperial gallon used in the UK is 4.54609 litres, about 20 percent more. Pints, quarts and fluid ounces differ too, and in opposite directions: a US pint is smaller than an imperial pint, but a US fluid ounce is slightly larger than an imperial one, because the imperial pint holds 20 fluid ounces rather than 16. The converter lists each variant separately. Tons are similar: a US short ton is 2,000 pounds, a UK long ton 2,240 pounds, and a metric tonne 1,000 kilograms.
Temperature
Celsius, Fahrenheit and kelvin do not share a zero point, so temperature conversion adds or subtracts as well as multiplies. The formula is shown under the result. Kelvin starts at absolute zero, so negative kelvin values are rejected, and so is anything below −273.15 °C or −459.67 °F.
Traditional Chinese units
Chinese markets and land records still use the shi (market) system, whose units were tied to metric ones in 1929: one jin is exactly 0.5 kg, one chi 1/3 m, one li 500 m, and one mu 2,000/3 m² (about 666.67 m²), so a hectare is exactly 15 mu. Since the 1959 reform one jin has had 10 liang rather than the traditional 16, so one liang is 50 g.
Display precision
Computers store numbers in binary, so a sum like 0.1 + 0.2 comes out as 0.30000000000000004. Results here are rounded to ten significant digits for display, which is far more precision than any practical measurement and hides that noise. Very large or very small results switch to scientific notation.
Examples
| Conversion | Result | Note |
|---|---|---|
| 1 mile → metres | 1,609.344 m | exact by definition |
| 37 °C → °F | 98.6 °F | normal body temperature |
| −40 °C → °F | −40 °F | the one temperature where both scales agree |
| 1 lb → kg | 0.45359237 kg | exact by definition |
| 1 US gallon → litres | 3.785411784 L | |
| 1 imperial gallon → litres | 4.54609 L | 20% more than a US gallon |
| 1 US fl oz → mL | 29.57352956 mL | vs 28.4130625 mL imperial |
| 100 km/h → mph | 62.13711922 mph | |
| 1 acre → m² | 4,046.856422 m² | |
| 1 hectare → mu | 15 mu |
A quick sense check for recipes from the US: one cup is about 237 mL, one tablespoon about 14.8 mL and one teaspoon about 4.9 mL. Australian tablespoons are 20 mL, so a recipe from Australia will not match the US tablespoon here.
Limits
- Volume and weight are not interchangeable. Converting "a cup of flour" to grams depends on the ingredient's density, which this tool does not know. Only water weighs almost exactly 1 kg per litre.
- Regional variants. Australian tablespoons (20 mL), metric cups (250 mL) and historical units are not included; the listed US and imperial units are the legally defined ones.
- Precision beyond ten digits is hidden for readability. The underlying calculation carries about fifteen significant digits.
- Currency and fuel economy conversions are not unit conversions in this sense and are not provided: exchange rates change daily, and litres per 100 km is the reciprocal of miles per gallon rather than a multiple.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54. The inch has been defined as 25.4 millimetres since 1959.
Is a UK gallon the same as a US gallon?
No. An imperial (UK) gallon is 4.54609 litres and a US gallon 3.785411784 litres, so a UK gallon is about 20 percent larger.
How do I convert Celsius to Fahrenheit in my head?
Double it and add 30 for a rough answer; for an exact one, multiply by 9/5 and add 32. The rough method is within a few degrees between about 0 °C and 30 °C.
What is a jin in kilograms?
One jin (市斤) is exactly 0.5 kg, so 1 kg is 2 jin. One liang is 50 g.
Can I share a conversion?
Yes. The address bar always contains the current quantity, units and value, so copying the link shares exactly what you see.