Unit Converter

How to use the unit converter

1. Choose a category: length, weight, temperature, volume, area, speed, data storage or time.
2. Pick the unit you have and the unit you want.
3. Type the value. The answer updates as you type, and the swap button reverses the direction.

Where the numbers come from

Most everyday units are defined exactly in terms of metric ones, so conversion is plain multiplication with no approximation in the factor itself. Since an international agreement of 1959, an inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg; every other length and weight in the foot-pound system follows from those two.

Units that share a name but not a size

• Gallons, pints and fluid ounces. The volume units here are the US ones. The UK imperial gallon is 4.546 litres, about 20% larger than the US gallon of 3.785 litres, so check which one a recipe or a fuel figure means.
• Ounces. An ounce of weight (28.35 g) and a fluid ounce of volume (29.57 ml in the US) are different things, and gold is weighed in troy ounces of 31.1 g.
• Tons. The tonne here is the metric one, 1,000 kg. The US short ton is 907 kg and the UK long ton is 1,016 kg.
• Miles. A nautical mile, used at sea and in the air, is 1.852 km, longer than the road mile of 1.609 km.
• Kilobytes. Data sizes here use 1,024 bytes to the kilobyte, as operating systems traditionally report them. Drive makers count 1,000, which is why a “1 TB” disk shows as about 931 GB.

Temperature is not a multiplication

Celsius, Fahrenheit and Kelvin have different zero points, so converting needs an offset as well as a scale: °F = °C × 9/5 + 32 and K = °C + 273.15. That is also why “twice as hot” means nothing in Celsius or Fahrenheit — 20 °C is not double 10 °C in any physical sense.

Months and years

For time, a month is taken as 30 days and a year as 365 days. That is fine for rough conversions; for the exact gap between two dates, use the Age Calculator.

How many digits to keep

Results are shown to more digits than most measurements deserve. A height measured to the nearest centimetre converts to the nearest half inch at best, so round the answer to match the precision of what you started with.