Stones, pounds and kilograms, and how they convert
How to convert stones and pounds to kilograms and back, why a stone is 14 pounds, who still uses it, a chart of body weights from 7 to 20 stone, and how to avoid the usual rounding slips.
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In short
- One stone is 14 pounds, and one pound is exactly 0.45359237 kg, so one stone is 6.35029318 kg; ten stone is 63.5 kg and twelve stone is 76.2 kg.
- To convert stones and pounds to kg, change everything to pounds first and multiply by 0.45359237: 11 stone 4 pounds is 158 pounds, which is 71.67 kg.
- To convert kg to stones, divide by 0.45359237 to get pounds, then by 14: 70 kg is 154.3 pounds, or 11 stone 0.3 pounds.
- The pounds part of a weight in stones is never 14 or more; 13.96 pounds left over rounds up to the next whole stone.
- The UK and Ireland use stones for body weight, the US uses pounds, and the rest of the world uses kilograms.
Body weight is one of the few measurements that has not settled on one unit. In the United Kingdom and Ireland people say they weigh "eleven stone four". In the United States it is pounds alone, such as 158. In India and most of the rest of the world it is kilograms, 71.7. A person reading a British health article, a travel form or a fitness plan often needs to move between the three. This page gives the conversions, explains why a stone is the size it is, and shows how to avoid the rounding slips that creep in.
The definitions
Everything rests on two exact numbers. The international pound is defined as 0.45359237 kilograms, exactly, by agreement between the English-speaking countries in 1959. And a stone, in the UK and Ireland, is 14 pounds. Multiplying gives the third:
1 stone = 14 × 0.45359237 = 6.35029318 kg
Because both inputs are exact by definition, a conversion is exact too; only the rounding on the screen makes it look approximate.
The kilogram figure for a stone is awkward to remember, but the following ladder helps: ten stone is 63.5 kg, eleven is 69.9, twelve is 76.2 and thirteen is 82.6. Each extra stone adds about 6.35 kg.
Stones and pounds to kilograms
Change the whole thing to pounds first, then multiply by 0.45359237.
Take 11 stone 4 pounds. In pounds that is 11 × 14 + 4 = 158. Then 158 × 0.45359237 = 71.67 kg.
Take 9 stone exactly. That is 9 × 14 = 126 pounds, and 126 × 0.45359237 = 57.15 kg.
The mistake to avoid is treating the number after "stone" as a decimal. "11 stone 4" does not mean 11.4 stone. The 4 is pounds, out of 14, so 11 stone 4 pounds is 11 + 4/14 = 11.29 stone, not 11.4. People who write 11.4 stone are off by about 1.6 pounds.
Kilograms to stones and pounds
Go the other way: divide by 0.45359237 to get pounds, then divide by 14. The whole number is the stones, and what is left over is the pounds.
70 kg is 70 ÷ 0.45359237 = 154.3 pounds. Dividing by 14 gives 11 with a remainder of 0.3 pounds, so 70 kg is 11 stone 0.3 pounds, close to 11 stone exactly.
The conversion for some round weights in kilograms:
| Kilograms | Pounds | Stones and pounds |
|---|---|---|
| 50 kg | 110.2 | 7 st 12.2 lb |
| 60 kg | 132.3 | 9 st 6.3 lb |
| 70 kg | 154.3 | 11 st 0.3 lb |
| 80 kg | 176.4 | 12 st 8.4 lb |
| 90 kg | 198.4 | 14 st 2.4 lb |
| 100 kg | 220.5 | 15 st 10.5 lb |
A chart in stones
Going the other way, from a weight in stones, this is a quick reference:
| Stones | Pounds | Kilograms |
|---|---|---|
| 7 st | 98 lb | 44.5 kg |
| 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 |
Pounds alone
American sources give weight in pounds only. To get kilograms from pounds, multiply by 0.45359237: 100 pounds is 45.36 kg, 150 pounds is 68.04 kg and 200 pounds is 90.72 kg. To get stones from pounds, divide by 14: 154 pounds is exactly 11 stone. The calculator has a mode for pounds to kilograms for that reason.
A fast mental estimate: kilograms are roughly pounds divided by 2.2. So 154 pounds is about 70 kg. It is accurate to within a percent, and good enough for a quick check, but use the exact factor when the number matters, such as a dose or a baggage limit.
Who uses stones, and why
The stone is an old unit of mass that was once used for weighing goods such as wool, and different regions had different stones. The 14 pound stone is the one that survives, and it survives in everyday speech in the United Kingdom and Ireland, where people give their own weight in stones and pounds and a baby's in pounds and ounces. Official records, doctors and hospitals in the UK mostly use kilograms, so a person is often weighed in kilograms and then converts in conversation.
If you are filling in a form that asks for kilograms and you only know your weight in stones, convert it first and round to the precision the form asks for, usually the nearest whole kilogram or one decimal.
Rounding without drifting
Two small traps are worth knowing:
- Carrying the pounds. A weight of 167.96 pounds, rounded to a tenth, is 168.0 pounds, which is exactly 12 stone. It must be shown as "12 st 0 lb", not "11 st 14.0 lb". The calculator carries the pounds into the next stone for this reason.
- Rounding too early. If you round the pounds to a whole number, then convert, a weight like 70 kg shows as "11 st 0 lb", which loses the 0.3 pounds. Keep one decimal of a pound until the end.
Round trips are also worth a sanity check: convert a weight to kilograms and back, and you should land where you started, within the display's rounding.
Checking your own working
A conversion is easy to get wrong by a pound or two, so build in a check. Take 12 stone 6 pounds. In pounds that is 12 × 14 + 6 = 174, and 174 × 0.45359237 = 78.93 kg. Check it against the chart: 12 stone is 76.2 kg and 13 stone is 82.6 kg, and 78.93 falls between them, a little under half of the way up, which fits 6 pounds out of 14. If the answer had come out at 90 or 60, you would know a step went wrong.
The most frequent errors are small and avoidable:
- Multiplying the stones by 6.35 and forgetting the pounds. 11 stone 4 pounds is not 11 × 6.35 = 69.85 kg; the 4 pounds add 1.81 kg more.
- Using 2.2 as if it were exact. The kilogram is 2.2046 pounds, so 2.2 is accurate only to about a tenth of a percent. That is fine for estimates and not for medication or travel limits.
- Dividing by 14 before converting pounds. The 14 belongs in the stones step, not in the kilogram factor.
- Confusing the US and UK units in conversation. An American "stone" is not a common unit at all, so a US reader who sees "11 stone" will usually need the pounds figure, 154, or the kilograms.
When the weight matters for health, such as a drug dose or a clinical record, use the kilogram figure from a scale that displays it rather than converting from stones.
Weight and health measures
Body weight is only meaningful against height, and the standard measures use kilograms and metres. The body mass index is weight in kilograms divided by height in metres squared, and so someone who knows their weight in stones needs the conversion before using a BMI calculator. The ideal weight calculator takes the weight in kilograms or pounds, and a height given in feet and inches can be converted with the height converter.
What this calculator assumes
- One pound is exactly 0.45359237 kg, and one stone is exactly 14 pounds.
- The "stone" here is the British and Irish stone. Other historical stones, of different sizes, are not used.
- Stones and pounds are shown with the pounds to one decimal place, carried into the next stone when the remainder rounds to 14.
- The weight must be above zero, and no upper limit is imposed beyond what the input accepts.