What Is a Stone in Weight? The UK's Unusual Unit Explained

By ConvertStuffNow  ·  5 min read

If you've ever looked at a British health article that talks about someone losing "two stone" and felt mildly confused, you're not alone. The stone is a unit of weight that's still in everyday use in the United Kingdom and Ireland, largely for one specific purpose: measuring people's body weight.

It's an oddity by any modern standard — a pre-metric holdover that never quite got replaced, even as the UK switched to grams and kilograms for almost everything else. This guide explains what a stone is, where it came from, how to convert it and when you're actually likely to encounter it.

The basic definition

One stone equals 14 pounds. One stone equals 6.35029 kilograms. That's it.

The reason it's 14 pounds rather than a round number is purely historical. The stone was one of many traditional English units that varied by commodity and region — a stone of wool might be 14 pounds in one part of the country and a different amount elsewhere. The Weights and Measures Act of 1835 standardised the stone at 14 pounds for all uses, which is why it stuck at that number.

Where the stone came from

Units of weight called "stones" existed in Britain for centuries before the metric system arrived. The name was literal — actual stones were used as counterweights on scales in markets and trading posts. The problem was that different trades used different stones. A stone of glass was 5 pounds. A stone of hemp was 32 pounds. A stone of wool was 14 pounds.

The 1835 Act settled on 14 pounds as the official stone, partly because of the prominence of the wool trade (where the 14-pound stone was most widely used) and partly because it creates a convenient relationship: there are exactly two stones in 28 pounds, and 14 is easily divisible into quarters and halves for market trading.

Why is the stone still used?

The UK officially adopted the metric system for most purposes during the 1960s and 1970s. Grams and kilograms became the legal standard for food labelling and trade. But personal body weight was one area where public habit resisted the change. British people who grew up knowing their weight in stone and pounds simply didn't switch — and that cultural memory has been passed down.

Today, body weight is probably the only common everyday context in which stones appear. A British GP might record your weight in kilograms (the medical standard), but the patient might still think of themselves as "twelve and a half stone." British tabloids reporting celebrity weight changes invariably use stone. The NHS provides weight information in both systems.

Outside the UK and Ireland, the stone is rarely used. Australians, New Zealanders and Canadians use kilograms for body weight. Americans use pounds.

How to convert between stone, pounds and kilograms

The maths is straightforward once you remember the key equivalences:

British people typically express their weight as "stone and pounds" — for example, "12 stone 4" means 12 stone plus 4 additional pounds, which is 172 lb or approximately 78 kg.

To convert stone and pounds to kilograms: multiply the stone by 6.35029, then add the number of extra pounds multiplied by 0.453592. So "12 stone 4" = (12 × 6.35029) + (4 × 0.453592) = 76.20 + 1.81 = 78.02 kg.

Quick mental shortcut

To convert stone to kilograms roughly in your head: multiply by 6 and add a bit more (6.35 per stone). A quicker approximation: multiply by 6.5. So 10 stone × 6.5 = 65 kg. The exact answer is 63.5 kg — close enough for a quick sense of scale.

To convert kilograms to stone roughly: divide by 6.35. Or, for a fast estimate, divide by 6.5. So 80 kg ÷ 6.5 ≈ 12.3 stone, which is close to the exact 12.6 stone.

Stone in the context of BMI

Body mass index (BMI) calculations always use kilograms and metres — even in the UK. If you know your weight in stone, you'll need to convert to kilograms before calculating BMI. The formula is weight (kg) ÷ height (m)².

Most BMI calculators accept stone as an input and do the conversion internally. But if you're using a formula directly, convert to kilograms first.

Is the stone going to disappear?

Probably not soon. Despite the UK's official metric status, the stone endures because it's embedded in cultural identity in a way that units of volume and length aren't. People have emotional relationships with their weight in a way they don't with, say, their height in feet or the distance to the next town.

Several attempts to phase out the stone in the UK have stalled due to public resistance. It's likely to remain in common use for body weight for the foreseeable future — which makes knowing how to convert it a genuinely practical skill.

Weight conversions use exact SI definitions. BMI and health metrics should be interpreted in consultation with a qualified health professional.