How this instrument works
A stone is fourteen avoirdupois pounds, no more and no less, and since 1959 an avoirdupois pound has been 0.45359237 kg by agreement among six national standards bodies. Multiply those two numbers and a stone becomes 6.35029318 kg precisely. Nothing on this page was measured; it was all defined, then divided out.
That tidiness is recent. Medieval English markets ran a separate stone for almost every commodity — fourteen pounds for wool, twelve for wax, eight for beef at some butchers' scales, five for glass — so traders argued endlessly over which one applied to a given sack. Parliament settled on fourteen pounds in an 1835 weights and measures act, then Britain struck the unit off its list of measures legal for selling goods in 1985. Popular use survives regardless: body weight across Britain plus Ireland is still spoken this way, as are jockey or handicap weights at British and Irish racecourses.
Its partner travelled in reverse. From 1889 until 2019 the kilogram was simply whatever one platinum-iridium cylinder inside a vault at Sèvres happened to weigh. On 20 May 2019 that cylinder was retired, and mass was tied instead to one fixed numerical value of Planck's constant, 6.62607015 × 10⁻³⁴ J s. Kilograms now rest on physics while stone rests on an old treaty about pounds, yet no error creeps into this bridge, because that treaty defines the pound in kilograms rather than the reverse.
- Type your figure into the Kilograms (kg) field; 70 kg sits there as a starting point.
- Read the Stone (st) line below it — that value refreshes on every keystroke.
- For the familiar stones-and-pounds form, keep the whole number of stone and multiply what follows the decimal point by 14 to get pounds.
- Working backwards from stone? Multiply your stone figure by 6.35029318 to land on kilograms, or divide by 0.157473044418.
Worked example — 70 kg on a clinic scale
A surgery scale in Cork reads 70 kg. Type 70 into Kilograms (kg) and Stone (st) answers 11.0231131092. That is a complete decimal answer, and it is also where most people stumble, because 11.02 does not mean eleven stone two pounds.
Handle that tail properly: 0.0231131092 × 14 = 0.324 pounds, so 70 kg is eleven stone plus roughly a third of a pound — near enough eleven st dead. Cross-check through pounds if you want reassurance: 70 ÷ 0.45359237 = 154.3236 lb, which divided by 14 returns 11.0231 st once more.
Questions
Is one stone exactly 6.35029318 kilograms?
Yes. A stone is defined as 14 avoirdupois pounds, and the 1959 international yard and pound agreement fixed that pound at 0.45359237 kg with no uncertainty attached. Fourteen times 0.45359237 gives 6.35029318 kg, exact to its final digit. Running that backwards produces 0.1574730444177697…, a decimal that never ends, so this sheet carries twelve places — plenty to reproduce any bathroom or clinical scale reading.
Why does my result read 11.02 stone rather than 11 stone 2 pounds?
Because a decimal remainder counts fourteenths of a stone, not pounds. Multiply whatever sits after the decimal point by 14: 0.02 st works out at 0.28 lb, whereas an actual two pounds would appear as 0.142857 st. Misreading that tail as though it were the spoken stones-plus-pounds form is far the commonest error with this unit, capable of throwing a bodyweight figure out by nearly two pounds.
Do Americans weigh themselves in stone?
No. Stone never caught on in the United States, where scales, medical charts and driving licences all give body weight in pounds alone — an American would quote 70 kg as 154 lb. Everyday use survives in Britain and Ireland, plus British and Irish racing. Canada, Australia and New Zealand let the unit lapse during metrication, so anyone under fifty there will usually answer in kilograms.
Can shops still sell goods by the stone?
Not in Britain. Legislation phased stone out of trade during the twentieth century, and by 1985 goods could no longer be sold by it; butchers and greengrocers price in kilograms. Conversation and bathroom scales are untouched by any of that. Hospitals will note a patient's stated weight in stone, then convert to kilograms, since drug dosing and anaesthesia are calculated metric without exception.
How heavy is a stone in practical terms?
About 6.35 kg. Any standard 14 lb bowling ball weighs exactly one stone, which makes it a handy mental yardstick. Eight of them add up to one British long hundredweight, 50.80234544 kg, while the 25 kg builder's bag of cement sold by UK merchants lands at 3.94 st. Carrying such anchors in your head saves you from trusting some converted figure that has come out an order of magnitude wrong.
Can I round 0.157473044418 to something shorter?
Usually, provided you know what precision costs. Substituting 0.1575 moves a 70 kg reading by about 0.002 stone — under half an ounce, invisible on any bathroom scale — and using 6.35 kg per stone loses a similar amount. Body weight tolerates either shortcut happily. Freight, laboratory work or anything traceable deserves the full factor, and the safest habit anywhere is to compute in kilograms throughout and convert only once, at the end.