Unit Converter

Switch between length, weight, temperature, volume and area units instantly.

How to use

  1. Pick a measurement category like length, weight, or temperature.
  2. Select the unit you are converting from.
  3. Enter the value to convert.
  4. Pick one or more target units and see the conversions live.
  5. Use the swap button to flip source and target instantly.

Frequently asked questions

Which categories are supported?

Length, area, volume, weight, temperature, speed, time, pressure, energy, power, and data storage. Each category includes both metric and imperial units plus common scientific ones.

Are the conversion factors official?

Yes. Conversions use the SI-defined or NIST-published factors. For some imperial units there is both a US and a UK definition (gallons, tons); the tool clearly labels which is which.

How precise are the results?

Results are shown to 6 significant figures by default and can be expanded. JavaScript double precision gives roughly 15-17 significant digits of headroom.

Why does Celsius to Fahrenheit need more than multiplication?

Temperature scales have different zero points, so the conversion needs both a multiplication and an addition: F = C × 9/5 + 32. Most other unit conversions are pure scaling.

Exact definitions, not approximations

Since 1959 the inch has been defined as exactly 2.54 cm, the pound as exactly 453.59237 g, and the international mile as 1,609.344 m. These are definitions, not measurements, so conversions between metric and imperial can be exact to as many decimals as you like. Where fuzziness creeps in is units that share a name but not a size: a US gallon is 3.785 L while an imperial gallon is 4.546 L, a US pint is 473 ml against the British 568 ml, and a "cup" in recipes ranges from 236 ml (US) to 250 ml (metric) depending on the cookbook's origin.

Conversions people actually need

  • Driving: 100 km/h is 62 mph; a quick rule is mph = km/h x 0.6.
  • Cooking: 1 US cup of flour is about 120-125 g, but 1 cup of sugar is about 200 g. Volume-to-weight depends on the ingredient, which is why serious baking recipes use grams.
  • Body stats: 1 stone = 6.35 kg; 5 ft 10 in = 177.8 cm.
  • Fuel economy inverts between systems: higher mpg is better, lower L/100km is better. 7.8 L/100km is about 30 mpg (US).

Precision discipline

Carry full precision through intermediate steps and round only the final answer. Converting 12.7 cm to 5 in and then back gives you 12.7 again only if you never rounded mid-chain. For engineering or medical doses, the rule is stricter: convert once, directly, from the source unit to the target unit, never through a casual intermediate like feet or cups.

Approximations safe to do in your head

  • Kilometres to miles: multiply by 0.6 (or take half and add a fifth). Miles to km: add 60%.
  • Kilograms to pounds: double it and add 10%. So 70 kg is about 154 lb.
  • Centimetres to inches: divide by 2.5 for a rough cut; the exact factor is 2.54.
  • Litres to US gallons: divide by 4 and shave a little (exact: 3.785).
  • Metres to feet: multiply by 3.3.

These get you within a few percent, which is fine for conversation and packing. The converter exists for the other cases... medicine, materials, money... where a few percent is the whole problem.

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