Article — Kg to Stones Converter
Kilograms to stones: the UK body-weight conversion
One stone equals exactly 6.35029318 kilograms — the value of 14 international avoirdupois pounds fixed by the International Yard and Pound Agreement of 1959. To convert kilograms to stones, divide by 6.35029. So 70 kg = 11.02 stones (typically reported as 11 stone 0 lb), 80 kg = 12.60 stones (12 st 8 lb), and 100 kg = 15.75 stones (15 st 10 lb). The reverse multiplies kg by the same factor.
The stone is a UK and Irish unit. The British officially adopted metric in 1985, but the stone kept its hold on body weight: 60–75% of UK adults still describe their weight in stones, NHS scales show both, and gym instructors talk in stones plus pounds. The math below covers the conversion, the BMI implications, and the format conventions UK users actually use.
What is a stone (unit)?
A stone is a unit of mass equal to 14 international pounds, or 6.35029318 kilograms. The name comes from the medieval practice of using a literal stone as a market weight standard. Different regions used different stones — a wool stone in Britain was 14 pounds, but local "stones" varied from 5 to 40 pounds across Europe. The Statute of Weights of 1495 fixed the English wool stone at 14 pounds, and the value held.
The Weights and Measures Act of 1985 formally withdrew the stone from UK trade. Shops no longer sell potatoes by the stone, and licenses for stone-graded scales lapsed in the 1990s. But the law explicitly preserved the stone for measuring body weight — a carve-out that reflects how deeply the unit is embedded in British culture. NHS scales still display stones; UK government health surveys quote both kg and stones.
The stone was once used to weigh almost any commodity in Britain. A 1389 record from London prices wool at "11 pence the stone". The 14-pound wool stone became the standard, but butchers used a 8-pound stone for meat, and a 12.5-pound stone for cheese in some counties. The Weights and Measures Act of 1825 abolished all stones except the 14-pound one — the same value still used for body weight today.
The kilograms to stones formula
The conversion uses one factor: 6.35029318 kg per stone. Divide kg by the factor to get stones, or multiply stones by the factor to get kg. The number is exact because both the kilogram and the international pound are defined values. The kg is fixed by the Planck constant (SI redefinition of 2019); the pound is fixed at 0.45359237 kg by treaty. Multiply by 14 and you get the stone.
1 stone = 14 lb 1 lb = 0.45359 kg1 stone = 6.35029 kg 1 kg = 0.1574 stFor mental math, round 6.35 to 6.5 and divide kg by 6.5. 80 kg ÷ 6.5 ≈ 12.3 st (true: 12.60). The 6.5 round-up overshoots by about 2.3%. Better mnemonic: divide by 6.35, which most people can manage if they multiply 6.35 × the nearest round stone value. 12 × 6.35 = 76.2, so 76 kg is just under 12 stone.
Common adult body weights in stones
The middle of the UK adult weight range sits between 60 and 85 kg, or 9 to 13 stone. Below the table covers the full distribution, with the stones-and-pounds breakdown.
- 50 kg = 7.87 st = 7 st 12 lb (small adult woman)
- 60 kg = 9.45 st = 9 st 6 lb (average UK adult woman)
- 65 kg = 10.24 st = 10 st 3 lb
- 70 kg = 11.02 st = 11 st 0 lb (UK median adult)
- 75 kg = 11.81 st = 11 st 11 lb
- 80 kg = 12.60 st = 12 st 8 lb (typical UK adult man)
- 85 kg = 13.39 st = 13 st 5 lb (slightly above UK male average)
- 90 kg = 14.17 st = 14 st 2 lb
- 100 kg = 15.75 st = 15 st 10 lb (obese class I for most heights)
- 120 kg = 18.90 st = 18 st 12 lb (obese class II–III)
The Office for National Statistics 2021 Health Survey for England reported average adult weights of 84.0 kg (men) and 70.7 kg (women) — both well above 1990s figures of 78 kg and 67 kg respectively. In stones, that is 13 st 3 lb for men and 11 st 2 lb for women. The trend is upward across the entire age distribution.
Reading "12 stone 4" — stones-and-pounds format
UK body weights are usually said as "X stone Y", where X is whole stones and Y is leftover pounds (0–13). "Twelve stone four" means 12 stone + 4 pounds. The math: 12 × 6.35029 + 4 × 0.45359 = 76.20 + 1.81 = 78.02 kg. In decimal stones, that is 12.286 stones.
To convert a decimal stones figure to stones-and-pounds, separate the integer and fractional parts. The integer is the stone count. Multiply the fractional part by 14 to get the pound count. 11.43 stones → 11 stone, 0.43 × 14 = 6.02 pounds → 11 stone 6 lb. The pound count is always 0 to 13; if it rounds to 14, carry over to the next stone.
The biggest mistake in stone-and-pounds arithmetic is treating the pound part as a decimal of a stone. "Eleven stone five" means 11 stone + 5 pounds = 11.357 stones. Not 11.5 stones. The pound part runs 0 to 13, not 0 to 9. Mistaking the format converts 11 st 5 lb (72.21 kg) to 73.03 kg — a 1% error that compounds across multiple weights.
Stones versus kilograms in the UK today
Modern UK life uses both. NHS GP scales and bathroom scales typically show kg and stone. Hospitals record patient weight in kg on charts (international convention), but staff routinely translate to stones when talking to patients. Weighing-in for sports is mixed — boxing uses pounds (within stones), athletics uses kg, weightlifting is entirely metric.
The Office for National Statistics found in a 2018 survey that 67% of UK respondents preferred stones for stating their own weight, 28% preferred kg, and 5% had no preference. Younger respondents (18–24) were more likely to use kg (38%), reflecting metric-only schooling since the 1990s. Older adults (65+) overwhelmingly preferred stones (84%).
Ireland uses stones identically. Australia and New Zealand abandoned the stone in the metric switchovers of the 1970s. Canada uses pounds (occasionally) but never stones. The unit is essentially UK-and-Ireland today, with no other widespread market.
BMI using stones
BMI (Body Mass Index) is calculated globally in metric: kg divided by height in meters squared. To use stones, convert to kg first. A 12-stone person at 1.75 m: kg = 12 × 6.35029 = 76.20; BMI = 76.20 / (1.75)² = 76.20 / 3.0625 = 24.9. That is right at the top of the "healthy" range (18.5–24.9 per WHO).
A quick BMI estimate from stones and feet: BMI ≈ (stones × 70) / (height in inches)². So 12 stone at 5'9" (69 in): (12 × 70) / 69² = 840 / 4761 = 17.65... close, but off from the true 24.9 because the formula needs different scaling. Stick with the kg/m² form: convert stones to kg, height to meters, then square.
BMI bands map to common stone weights at typical heights. At 1.70 m (5 ft 7 in), 10 stone is BMI 22 (healthy), 12 stone is 26.4 (overweight), 14 stone is 30.8 (obese class I). At 1.83 m (6 ft 0 in), the same numbers all drop one band — 12 stone at 6 ft is BMI 22.8, still healthy. Height moves BMI more than the weight does at this scale.
Common stone-conversion mistakes
The format error — treating "X stone Y" as a decimal stone X.Y — is the most common. "Thirteen stone seven" is not 13.7 stones; it is 13.5 stones (13 + 7/14). The pound part divides by 14 to get the decimal stone, not by 10. Writing 13.7 instead of 13.5 turns 85.75 kg into 86.99 kg, a small but persistent error.
A second error is using 6.5 instead of 6.35029 as the conversion factor. The rounded value overshoots by 2.3% — a small individual error but enough to misclassify someone near a BMI threshold. For clinical purposes, always use the exact factor. The mental shortcut should be "divide by six and a third" rather than "divide by six and a half".
A third pitfall is forgetting that some Commonwealth countries (Australia, New Zealand) historically used 8-stone hundredweights but never the stone-for-personal-weight convention. Body weight in those countries goes straight from kg to pounds, skipping the stone entirely. Reading a 1960s Australian medical chart in stones is a sign the document is British, not local.