Weight Converter

How the conversion works

Body weight is written three different ways in English-speaking countries: kilograms almost everywhere, pounds alone in the United States, and stone and pounds in Britain and Ireland. Stone behaves exactly like feet do for height — a larger unit holding a fixed number of smaller ones — so the reliable approach is the same: collapse the weight into pounds first, convert, and only then split it back out.

A weight of 12 stone 8 pounds collapses to (12 × 14) + 8 = 176 pounds. Multiplying by 0.45359237 gives 79.8 kg. Going the other way, 80 kg divided by 0.45359237 is 176.4 pounds — 12 whole stone with 8.4 pounds remaining, so 80 kg is 12 stone 8.4 pounds.

The pound is defined as exactly 0.45359237 kilograms, fixed by the international yard and pound agreement of 1959 — the same agreement that set the inch at 2.54 centimetres. It is not a measured approximation, so any imprecision you see comes from the number of decimal places displayed, or from your scales, never from the conversion factor.

Kilograms to stone and pounds chart

Common body weights from 40 kg to 130 kg, in five-kilogram steps.

Frequently asked questions

How many pounds are in a stone?

Fourteen, and it has been fixed at fourteen since the Weights and Measures Act of 1835 standardised the stone for trade. One stone is therefore 6.35029318 kg exactly. Dividing a weight in kilograms by 6.35 gives it in stone alone.

Is 12.5 stone the same as 12 stone 5 pounds?

No, and it is the most common mistake in weight conversion. Decimal stone counts tenths of a stone, not pounds. Half a stone is seven pounds, so 12.5 stone means 12 stone 7 pounds, which is 79.4 kg. 12 stone 5 pounds is 78.5 kg — a difference of two pounds.

Why does converting 79.8 kg back give 12 st 7.9 lb rather than 12 st 8 lb?

12 stone 8 pounds is 79.832 kg, not 79.8 kg. Once the kilogram figure has been rounded for display, converting it back lands slightly short. Nothing is wrong with the arithmetic — the rounding happened when the number was shown, not when it was converted.

Why does Max show more digits for pounds than for kilograms?

Because the pound is defined as exactly 0.45359237 kg, so stone and pounds into kilograms terminates: 12 st 8 lb is exactly 79.83225712 kg, eight decimals and no more. The reverse divides by that figure and never terminates, so it fills every digit Max allows. Max stops at fifteen significant figures on purpose — beyond that a browser is printing artefacts of how it stores numbers, not a more accurate answer, which is why no setting here is labelled exact.

Do British and American pounds weigh the same?

Yes. Both use the international avoirdupois pound, fixed at exactly 0.45359237 kg by the same 1959 agreement that defined the inch. The difference is only in how weights are spoken: Britain uses stone and pounds for body weight, the United States uses pounds alone.

Why do scales in kilograms and pounds sometimes disagree slightly?

Because most scales round to the nearest 0.1 kg or 0.2 lb before displaying, and the two rounding steps do not line up. A reading of 80.0 kg could be anything from 176.2 to 176.6 pounds depending on where the scale rounded. The conversion factor itself is exact.

Related tools

Converting your height too? The height converter handles feet and inches to centimetres, and works the same way. To put a weight in context against a height, the BMI calculator uses both. The full list of tools is on the homepage.