Why does the UK still weigh people in stone?

Ask someone in the UK how much they weigh and you'll probably hear an answer like "11 stone 4". Most of the world uses kilograms, so why does the stone hang on?

Where the stone comes from

The stone is one of the oldest units of weight in Britain. As the name suggests, people once used actual stones as weights at markets. For centuries, a "stone" meant different amounts depending on what was being weighed and where. Wool, meat and other goods each had their own stone.

Why a stone is 14 pounds

The 14-pound stone was already the most common one, and in the nineteenth century British law settled it as the standard. Since then, a stone has been exactly 14 pounds. When the pound was defined as exactly 0.45359237 kg in the twentieth century, the stone became exactly 6.35029318 kg.

Metric for trade, stone for people

From the 1960s onwards, Britain gradually moved to the metric system. Shops, schools and science switched to kilograms, and the stone stopped being used for trade. Body weight was different. It was never sold or taxed, so there was no rule forcing a change. People kept describing their own weight in stone and pounds, and still do.

Who uses what today

• Everyday life: most adults in the UK and Ireland know their weight in stone and pounds.
• Health services: NHS clinics and hospitals usually record weight in kilograms, which is why so many people need to convert.
• Scales and apps: most bathroom scales and slimming apps sold in the UK offer stone as an option.
• Other countries: the US uses pounds rather than stone, and most other countries use kilograms.

That mix of systems is exactly why a quick kg to stone calculator comes in handy.