Stone to Kilograms Converter

The stone is a unit of mass equal to fourteen pounds, or about 6.35029 kilograms, and it survives in everyday use in the United Kingdom and Ireland almost exclusively for describing body weight. Someone will give their weight as eleven and a half stone rather than 161 pounds or 73 kilograms, even while buying food in grams and fuel in litres — a good illustration of how partial metrication leaves different units embedded in different parts of life.

Converting is a straightforward multiplication by 6.35029, but the notation causes more trouble than the arithmetic. British weights are usually expressed as stone and pounds together, as in 11 stone 4, and the pounds portion runs from 0 to 13 before rolling over into another stone. Reading 11 stone 4 as 11.4 stone is wrong: the correct conversion is to multiply the stone by fourteen, add the loose pounds to get 158 pounds total, then convert. Treating the pounds as a decimal fraction produces an error of several kilograms.

The unit's history explains its awkwardness. Before standardisation, a stone varied by trade and by region, ranging from around eight pounds for some goods to well over twenty for others, with different values for wool, meat, and cheese. The fourteen-pound stone was fixed by statute in the nineteenth century, and the others gradually disappeared. Its persistence for body weight but nothing else reflects habit rather than any practical advantage.

The United States, despite sharing the pound, does not use the stone at all — American body weights are given in pounds alone. So a British weight in stone means nothing to an American audience without conversion, and vice versa, which is why this conversion sits between two systems that otherwise look similar.