How to use the unit converter
- Pick a category — length, weight, temperature, volume or speed.
- Type a number and choose the units to convert from and to.
- The answer updates instantly. Hit the swap button to flip the direction.
Metric and imperial units
Most of the world uses the metric system — metres, kilograms, litres and Celsius — which is built on powers of ten. The imperial system — feet, pounds, gallons and Fahrenheit — is still common in the United States and for some everyday measures in the UK. Converting between them is one of the most frequent everyday maths tasks, whether you’re following a recipe, reading a weather forecast, or buying something from abroad.
Common conversion factors
| Conversion | Factor |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot | 0.3048 m |
| 1 mile | 1.6093 km |
| 1 pound | 0.4536 kg |
| 1 kilogram | 2.2046 lb |
| 1 gallon (US) | 3.785 L |
| 0°C | 32°F |
The five categories, explained
Each category groups units that measure the same physical quantity, so you can only convert within a category (you can’t turn kilograms into kilometres). Here’s what each one covers:
- Length — from millimetres and centimetres up to metres, kilometres, inches, feet, yards and miles. The most-used everyday conversions, like cm to inches and km to miles, live here.
- Weight (mass) — milligrams, grams, kilograms, ounces, pounds and stone. Handy for cooking, parcels and body weight; see kg to lbs.
- Temperature — Celsius, Fahrenheit and Kelvin. Unlike the others, temperature scales have an offset as well as a factor, which is why °C to °F uses “× 1.8 + 32” rather than a single multiplier.
- Volume — millilitres, litres, and US/imperial cups, pints and gallons. Note that US and imperial gallons differ, so check which one a recipe means.
- Speed — metres per second, km/h, mph and knots, for driving, running and weather.
Why converting units matters
Getting a conversion wrong isn’t always harmless. The 1999 loss of NASA’s Mars Climate Orbiter was traced to one team using imperial units and another using metric. On an everyday level, a mixed-up conversion can ruin a recipe, order the wrong-size part, or misjudge a driving distance abroad. Using exact, agreed factors — and a tool that shows its working — keeps you on the safe side.
Frequently asked questions
Length (mm to miles), weight (mg to stone), temperature (Celsius, Fahrenheit, Kelvin), volume (millilitres to gallons) and speed (m/s, km/h, mph, knots).
Yes. We use the exact international definitions — for example one inch is exactly 2.54 cm and one pound is exactly 0.45359237 kg — and round only the displayed result.
Yes — see the popular conversions above for dedicated cm to inches, kg to lbs, °C to °F pages and more, each with a quick-reference table.