Pounds, Stone and Kilograms: Where Weight Units Come From and Who Uses Them
The kilogram is the world’s standard, but a British doctor might still note your weight in stone, an American recipe calls for ounces, and gold is priced in troy ounces everywhere. Here’s how these units came about and where they’re used today.
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Who uses what today
| Country or context | Everyday weight units |
|---|---|
| Most of the world | Grams, kilograms, metric tons |
| United States | Ounces, pounds and short tons (2,000 lb) for everyday life; metric in science and medicine |
| United Kingdom | Grams and kilograms in shops, but body weight is often given in stone and pounds |
| Ireland | Metric in shops; stone is still common for body weight |
| Canada | Metric officially, but many people give their body weight in pounds, and grocery prices are often shown per pound and per kilogram |
| Gold, silver, platinum (worldwide) | Troy ounces |
| Boxing and some other sports | Weight classes in pounds |
The kilogram
Like the meter, the kilogram was born in revolutionary France. In 1795 the gram was defined as the mass of one cubic centimeter of water at the temperature of melting ice. A gram was too small to be practical as a standard, so in 1799 a platinum cylinder representing 1,000 grams, the mass of a liter of water, became the reference: the “Kilogram of the Archives”.
In 1889 it was replaced by the International Prototype of the Kilogram, a platinum-iridium cylinder kept in a vault near Paris and nicknamed Le Grand K. For 130 years, every kilogram on Earth was ultimately compared to that one object, which was awkward because it could gain or lose atoms over time.
On 20 May 2019, the kilogram was redefined in terms of a constant of nature, the Planck constant. Le Grand K is still kept in its vault, but it no longer defines anything: the kilogram can now be reproduced in any suitably equipped lab.
The pound, and why it’s “lb”
The abbreviation “lb” comes from the Roman libra, a unit of about 329 grams. (Libra also means “scales”, and it’s why the British pound sterling uses the £ sign: it began as a pound of silver.)
Medieval England used several different pounds at once: the Tower pound for coins, the troy pound for precious metals, and the avoirdupois pound (from Old French for “goods of weight”) for everyday trade, which became standard. The avoirdupois pound of 16 ounces is the one we use today. In 1959, the US, UK and other Commonwealth countries agreed to define it as exactly 0.45359237 kilograms.
The ounce
“Ounce” shares its root with “inch”: both come from the Latin uncia, “one twelfth”. A Roman ounce was a twelfth of a libra. The everyday (avoirdupois) ounce is now 1/16 of a pound, about 28.35 grams.
The troy ounce is different: about 31.1 grams, named after the French trading town of Troyes. It’s still the worldwide standard for pricing gold, silver and platinum, so “an ounce of gold” is heavier than an ounce of flour.
The stone
The stone was literally a stone used as a weight, and its value varied by town and by what was being weighed: wool, meat and cheese each had their own stone. Britain standardized it at 14 pounds (about 6.35 kg). It survives today almost only for body weight in the UK and Ireland: someone might say they weigh “11 stone 4”, meaning 11 × 14 + 4 = 158 pounds.
The ton
“Ton” comes from the tun, a huge wine barrel. A tun of wine weighed roughly 2,000 pounds, and the word became a large unit of weight. Three different tons survive:
- Short ton (US): 2,000 lb, about 907 kg
- Long ton (traditional UK): 2,240 lb, about 1,016 kg
- Metric ton or tonne: 1,000 kg, used everywhere else and in international trade
The three are close enough to be confused, so when precision matters, it’s worth checking which one is meant.
Quick reference
- 1 pound = 16 ounces = 0.45359237 kg (exact)
- 1 ounce ≈ 28.35 g
- 1 troy ounce ≈ 31.10 g
- 1 stone = 14 pounds ≈ 6.35 kg
- 1 kg ≈ 2.2046 pounds
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