Quick answer: The UK uses stone for body weight through habit and tradition, not law. A stone is 14 pounds, or 6.35 kg, a unit tied to the medieval wool trade and handed down through generations. It cannot be used to sell goods except alongside a metric figure, but bathroom scales and horse racing keep it alive.
Ask someone in Britain or Ireland what they weigh and the answer will often come back as “eleven and a half stone” rather than a figure in kilograms. Hardly anywhere else in the world uses the unit, and it vanished from shop counters decades ago. This explainer looks at where the stone came from, why it is exactly 14 pounds, what the law says about it today and why it has proved so hard to shift. It sits within our wider guide to the UK measurement system and where imperial units still apply.
How much is a stone in pounds and kilograms?
One stone equals 14 pounds, or 6.35029318 kilograms exactly. The kilogram figure follows from the legal definition of the pound as exactly 0.45359237 kg, multiplied by 14. In everyday use, 6.35 kg is accurate to within a gram per stone, so it is the figure most people use.
Body weights in the UK are normally written as stones and leftover pounds, such as 10 st 7 lb. To convert, turn the whole thing into pounds first (10 × 14 + 7 = 147 lb) and then multiply by 0.45359237, which gives 66.68 kg. The pounds to kilograms converter does the second step for you.
| Stone | Pounds | Kilograms |
|---|---|---|
| 8 st | 112 lb | 50.8 kg |
| 9 st | 126 lb | 57.2 kg |
| 10 st | 140 lb | 63.5 kg |
| 11 st | 154 lb | 69.9 kg |
| 12 st | 168 lb | 76.2 kg |
| 13 st | 182 lb | 82.6 kg |
| 14 st | 196 lb | 88.9 kg |
| 15 st | 210 lb | 95.3 kg |
| 16 st | 224 lb | 101.6 kg |
| 18 st | 252 lb | 114.3 kg |
| 20 st | 280 lb | 127.0 kg |
Where did the stone come from?
The stone began, quite literally, as a stone: a convenient rock used as a local standard weight. For centuries there was no single stone. Its value depended on what was being weighed and where, and reference works such as Encyclopaedia Britannica record trade stones ranging from about 4 to 32 pounds. A “glass stone” of 5 pounds is one example of a variant that lasted a long time.
Wool changed that. Raw wool was England’s most valuable export in the Middle Ages, and the Crown taxed it heavily, so the wool trade needed consistent weights. By the late fourteenth century royal statutes had fixed the stone of wool at 14 pounds, with a sack of wool set at 26 stones (364 lb). The 14 lb stone gradually became the default, and the Weights and Measures Act 1835 recognised it for general trade, although local variants lingered for decades afterwards.
Why is a stone 14 pounds?
Fourteen pounds was the wool-trade stone, and the rest of the imperial weight system was built around it. Once the 14 lb stone became standard, larger units were defined as whole numbers of stones, which made mental arithmetic easier for merchants who halved and doubled quantities rather than working in tens.
- A quarter is 28 lb, which is 2 stone (12.7 kg).
- A hundredweight is 112 lb, which is 8 stone (50.8 kg). It is called a hundredweight even though it is not 100 lb.
- A ton (the UK long ton) is 2,240 lb, which is 20 hundredweight or 160 stone (1,016 kg).
These relationships are still written into UK law. The Weights and Measures Act 1985 defines the stone as 14 pounds, the quarter as 28 pounds, the hundredweight as 112 pounds and the ton as 2,240 pounds. The US never adopted the stone and uses a short hundredweight of 100 lb, which is one reason American weights are quoted in pounds only.
Is it legal to use stone in the UK?
Yes for personal use, but not as the main unit for selling anything. The Weights and Measures Act 1985 lists the stone in Part VI of Schedule 1, alongside units such as the hundredweight, the acre and the mile, which “may not be used for trade except as supplementary indications”. A supplementary indication is an imperial figure shown next to the metric one and no more prominent than it.
In practice that means a farm shop cannot price potatoes per stone on its own, and weighing equipment used for trade must work in metric units. Weighing yourself at home, at a gym or at a weight-loss group is not trade, so nothing stops anyone using stones and pounds there. The same split explains why the UK still uses miles on its road signs: the law restricts units used to buy and sell, not units used in everyday life.
Why does the stone survive for body weight?
Because body weight was never something people bought or sold, metrication rules never forced it to change. Packaged food went metric in 1995 and loose food in 2000, but bathroom scales, family conversations and magazine articles carried on in stones and pounds. Several other factors help keep it going:
- Convenient numbers. An adult weight in stone is usually a two-digit figure such as 11 or 12, which feels easier to remember and compare than 70 or 76 kg.
- Scales that show both. Many bathroom scales sold in the UK can switch between stones, pounds and kilograms, so there is no pressure to choose.
- Health information in both systems. NHS BMI guidance, for example, gives worked figures in both forms, such as “70kg (around 11 stone)”.
- Sport. In British horse racing, the weight a horse carries is still given in stones and pounds on every racecard, and boxing commentary often describes fighters the same way.
Clinical settings are different. Hospitals, GP surgeries and pharmacies work in kilograms, partly because many medicine doses are calculated per kilogram of body weight. If a nurse records your weight in kg, it is worth knowing the conversion: 70 kg is about 11 st (more precisely 11 st 0.3 lb).
Do other countries use the stone?
Ireland is the only other country where the stone is still in common everyday use for body weight. The United States, which otherwise sticks to imperial-style units, never used it: Americans give their weight in pounds alone, so 11 st becomes 154 lb. Most former British territories now use kilograms.
How do you convert kilograms back to stones and pounds?
Divide the kilograms by 0.45359237 to get pounds, then divide by 14: the whole number is stones and the remainder is pounds. For 80 kg, 80 ÷ 0.45359237 = 176.4 lb. Fourteen goes into 176 twelve times (168), leaving 8.4 lb, so 80 kg is 12 st 8.4 lb.
The kilograms to pounds converter handles the first step, and the main weight converter covers grams, ounces, tonnes and other units.
Frequently asked questions
How many kilograms is 1 stone?
One stone is 6.35029318 kg exactly, which is usually rounded to 6.35 kg. It is also exactly 14 pounds.
Can shops sell things by the stone?
Only as a supplementary indication. Under the Weights and Measures Act 1985 the stone may be shown alongside a metric quantity, which must be the more prominent of the two, but goods cannot be sold by the stone alone.
Why is a hundredweight 112 pounds?
Because it is defined as 8 stone, and 8 × 14 = 112. The name survives from older systems in which a hundredweight really was about 100 lb. The US hundredweight is 100 lb.
Do doctors in the UK use stone?
Clinical records normally use kilograms, because many medicine doses are calculated per kilogram. Staff will often convert to stones and pounds when talking to patients, and NHS guidance gives examples in both.
Which countries use stone for weight?
The stone is used informally for body weight in the UK and Ireland. The United States uses pounds only, and most other countries use kilograms.
Sources
- Weights and Measures Act 1985, Schedule 1 (definitions of units), legislation.gov.uk
- Weights and Measures Act 1985, section 8 (units lawful for trade), legislation.gov.uk
- Stone, unit of weight, Encyclopaedia Britannica
- Calculate your body mass index (BMI) for adults, NHS
- Choice on units of measurement: guidance on markings and sales, GOV.UK