Type a number into any field above and every other unit updates at once. There is no “from” and “to” here — each box is both, so you can work in whichever direction you happen to be thinking in.
Switch between Length and Weight with the toggle. The units you reach for most are shown straight away; the rest sit behind More units.
Key terms
- Metric units — millimetres, centimetres, metres and kilometres for length; milligrams, grams, kilograms and tonnes for weight. Each step is a factor of ten or a thousand, which is why converting inside the metric system is just a moved decimal point.
- Imperial and US customary units — inches, feet, yards and miles; ounces, pounds and stone. The steps between them are historical rather than decimal: twelve inches to a foot, three feet to a yard, sixteen ounces to a pound.
- Base unit — the unit everything is converted through internally. This calculator uses the metre for length and the gram for weight.
How it works
Every conversion goes through one base unit. Whatever you type is multiplied by its own factor to reach metres or grams, and that base value is then divided by the factor of every other unit. Two steps — and it is the same two steps regardless of which box you typed in, which is exactly why every field can be an input.
Conversion
value in B = (value in A × factor of A) ÷ factor of B
Where a factor is how many metres, or grams, one of that unit is worth.
The two factors everything else is built from are defined exactly, by international agreement in 1959: one inch is precisely 0.0254 metres, and one pound is precisely 0.45359237 kilograms. Because those are definitions rather than measurements, conversions that look like they should be round numbers really are round numbers — a mile is exactly 1.609344 km, and a stone is exactly 14 pounds. This calculator derives every other factor from those two, so nothing accumulates a rounding error along the way.
Common conversions
| From | To | Exact? |
|---|---|---|
| 1 inch | 2.54 cm | Yes, by definition |
| 1 foot | 0.3048 m | Yes, by definition |
| 1 mile | 1.609344 km | Yes, by definition |
| 1 pound | 0.45359237 kg | Yes, by definition |
| 1 stone | 14 lb | Yes, by definition |
| 1 kilogram | ≈ 2.2046 lb | No, a repeating decimal |
How to use this calculator
- Choose Length or Weight.
- Type into whichever unit you already know. Everything else fills in as you type.
- Open More units if the one you need is not on show.
- To convert the other way, just type into the other box — there is nothing to swap.
Frequently asked questions
Why does every box change when I type in one?
Because a conversion has no natural direction. The field you are typing in is treated as the source and everything else is derived from it, so you never have to set up which way round you want to work.
Is a US pound the same as a UK pound?
For weight, yes. Both countries use the international avoirdupois pound of exactly 0.45359237 kg, agreed in 1959. Volume is where the two systems diverge — a US pint and a UK pint are genuinely different sizes.
Why is 1 kg not exactly 2.2 lb?
Because the pound is defined in terms of the kilogram, not the other way around, and 1 ÷ 0.45359237 is not a round number. It comes to about 2.2046226, which is why “2.2” is a good mental shortcut but not an exact answer.
Is a tonne the same as a ton?
No, and the difference is worth watching. A tonne (metric) is 1,000 kg. A US short ton is 2,000 lb, about 907 kg, and a UK long ton is 2,240 lb, about 1,016 kg. This calculator uses the metric tonne.
What is a stone?
A unit of 14 pounds, still widely used for body weight in the UK and Ireland. It appears here because people who think in stone rarely want to convert through pounds first.
