A wool-trade weight that outlived the wool trade
The stone is a survivor. It began life in medieval England as a rough-and-ready trading weight, literally a stone kept at a market as the physical reference against which sacks of wool, wheat, or wares were checked. There was no single "stone" for centuries — different trades and different towns used stones of different sizes, so a stone of wool, a stone of cheese, and a stone of meat were not necessarily the same weight as each other. That local variation was normal for pre-industrial weights generally; what makes the stone unusual is that, alone among historic English units of that kind, it is still in everyday use eight centuries later — for one purpose only: the weight of a human body.
1835: Parliament fixes the number
The variation ended, on paper at least, with the Weights and Measures Act 1835, which standardised the imperial system and fixed the stone at exactly 14 pounds. That is a legal definition, not a physical measurement, which is why stone-to-pound conversions carry no rounding error: 1 stone is 14 lb by statute, full stop. The 1835 Act sat within a broader nineteenth-century project of standardising units across a rapidly industrialising economy — the same project that fixed the yard and the imperial pound as the legal reference standards for trade throughout the UK.
1985: metric trade, imperial bodies
The next turning point came a century and a half later. The Weights and Measures Act 1985 consolidated UK weights and measures law and set the framework for the metrication of trade that followed through the rest of the twentieth century — goods sold loose by weight moved to metric units, and imperial-only pricing in shops was progressively phased out. Body weight was never part of that trade regulation to begin with: nobody buys or sells a person by the kilogram, so there was no legal lever forcing a change in how individuals describe their own weight. Stone simply carried on being what people said out loud, on bathroom scales, at the GP, and in casual conversation, even as kilograms took over supermarket shelves and petrol pumps switched from gallons to litres around it.
That split explains a pattern many people notice without naming it: the UK is metric where a business is doing the measuring (fuel, packaged food, medicine dosing) and imperial where an individual is describing themselves informally (stone for body weight, miles for a journey, pints for a pint). Body weight sits squarely in the second category, and NHS records typically show both — kilograms for clinical calculations like BMI or drug dosing, stone and pounds alongside it because that is the figure a patient recognises.
Converting a stone-and-pound reading to kilograms
Take a discharge letter or a scale reading of 11 st 0.3236 lb that needs converting for a form that only accepts kilograms — a European clinic, an airline's baggage tool, or an international fitness app. First convert everything to pounds: 11 × 14 + 0.3236 = 154.3236 lb. Then multiply by 0.45359237: 154.3236 × 0.45359237 ≈ 70 kg. So 11 st 0.3236 lb is 70 kg. As a standalone reference figure, a single stone on its own converts to 6.3503 kg — useful for a rough mental estimate: multiply a stone figure by roughly 6.35 to get kilograms.
Other countries went different ways
The stone was never adopted outside the British Isles and a handful of former colonies, so the comparison is really between three habits. The United States never used the stone at all — American body weight is given in pounds alone, without the stone's grouping into 14s, which is why a US weight like "220 lb" looks unfamiliar to a British reader used to hearing it split into stone and pounds. Continental Europe uses kilograms exclusively and always has, as part of a full metric system adopted far earlier and more completely than Britain's. Ireland sits in between historically — it inherited the same stone-based habits as the UK under shared administration before 1922, but as an EU member state it has followed EU metrication policy more fully in the decades since independence, and kilograms are now standard in Irish medical and official contexts, with stone increasingly a generational or informal usage rather than the default. Britain, having negotiated permanent exemptions from full EU metrication for exactly this kind of everyday measure, is the outlier that kept stone as the normal way to state a person's weight out loud.
Frequently asked
Is stone still legal to use in the UK?
Yes, for describing body weight informally there is no restriction at all — the metrication rules that apply to trade (selling goods by weight) never covered how a person describes themselves. Bathroom scales sold in the UK routinely display stone and pounds as a selectable unit precisely because it remains the expected format.
Why do UK doctors sometimes only show kilograms?
Clinical calculations — drug dosing, BMI, growth charts — are standardised in kilograms internationally, so that is what clinical software calculates in natively. Many NHS systems then convert and display the stone-and-pound figure alongside it, since that is what patients recognise, but the underlying calculation is done in kilograms.
How many kilograms is a stone, exactly?
One stone is 14 lb, and each pound is 0.45359237 kg by the 1959 international pound agreement, which multiplies out to 6.3503 kg. You can run any stone-and-pound figure through our weight converter, and read how every ToolHare tool is checked on the how we build and verify our tools page.