Stones and Pounds

What a stone actually is

A stone is fourteen pounds. Because the pound is fixed by international agreement at exactly 0.45359237 kilograms, the stone is fixed too: 14 × 0.45359237 = 6.35029318 kg. There is no rounding in the definition, only in what people print on a scale, which is why a stone is usually described as 6.35 kg.

For adult body weight the stone gives numbers small enough to remember: 10 st is 140 lb or 63.50 kg, and 14 st is 196 lb or 88.90 kg. A half stone is 7 lb, which is why weights are often quoted to the nearest half stone.

Stones are used for body weight in everyday speech, on bathroom scales and in slimming and sports contexts in the UK and Ireland. The US has no equivalent unit: body weight there is pounds, and bulk materials are in short tons. The wider comparison of US and imperial units is covered in US and imperial units.

Why the decimal stone causes mistakes

The single most common error is reading a decimal stone as stone-and-pounds. It is not one. 11.5 st is eleven and a half stones: 11 × 14 = 154 lb, plus half of 14, which is 7 lb, giving 11 st 7 lb. Reading it as 11 st 5 lb would give 159 lb, a difference of 2 lb, and it is wrong.

The decimal part of a stone is always worth seven pounds, whatever the number. 10.5 st is 10 st 7 lb. 9.5 st is 9 st 7 lb. In kilograms, 11.5 st is 11.5 × 6.35029318 = 73.03 kg.

Kilograms to stones and pounds, by hand

Divide the kilograms by 6.35029318 to get stones. The whole number is the stones; the decimal part is a fraction of a stone, and multiplying that fraction by 14 gives the pounds.

1. 70 ÷ 6.35029318 = 11.0231 st.
2. Whole stones: 11.
3. Pounds: 0.0231 × 14 = 0.32 lb.
4. Read as 11 st 0.3 lb.

The same method on 15 kg: 15 ÷ 6.35029318 = 2.3621 st, so 2 st with 0.3621 × 14 = 5.07 lb left over, which is 2 st 5.1 lb. For a straight stones-only figure, kilograms to stones gives the decimal form, and kilograms to pounds gives the total in pounds (70 kg is 154.3 lb).

Stones and pounds back to kilograms

Add the pounds to fourteen times the stones, then multiply by 0.45359237. For 12 st 4 lb: 12 × 14 = 168, plus 4 is 172 lb, and 172 × 0.45359237 = 78.01788764 kg, so 78.02 kg. A useful cross-check is that each stone is worth 6.35 kg and each pound 0.4536 kg, so 12 st 4 lb is roughly 76.2 + 1.8 = 78.0 kg.

The stones to kilograms converter takes a decimal number of stones, so enter 12 st 4 lb as 12 + 4 ÷ 14 = 12.2857 st, or convert the total of 172 lb with pounds to kilograms. In the other direction, kilograms to stones shows the result in stones and pounds as well as decimal stones. The weight converter carries stones alongside kilograms, pounds, ounces and both tons, which is useful when a weight arrives with an ounce in it.

Whole stones in kilograms

These are the values that turn up on scales and in clinic records, where the pounds part is usually dropped.

Why a result never reads 11 st 14 lb

Splitting a weight into whole stones and pounds has a trap in it. If the pounds figure rounds up to 14, the result is really a whole stone more, and printing 11 st 14 lb is nonsense. UnitUtils rounds the pounds remainder first, then carries any excess into the stone count, so a weight of 11 st 13.96 lb is shown as 12 st 0.0 lb rather than 11 st 14.0 lb. The method is described in how mixed units are calculated.

Decimal results use six significant figures, so 75 kg reads as 165.347 lb rather than a long tail of decimals, and whole numbers keep their own length so 1,234,567.891 does not collapse into 1,235,000. That rounding behaviour is set out in the precision notes.

Where kilograms win

Even where people think in stones, kilograms are the unit of record in UK and Irish healthcare, on laboratory results and on most hospital scales. That is why a person who says "eleven and a half stone" may need 73.03 kg on a form. The stone is a spoken and displayed unit; the kilogram is the shared numeric one, and converting between them accurately is the whole of the task.