Weight might be the oldest measurement people trade in: grain, metal, salt and spices were sold by weight long before anyone measured land precisely. The two units that dominate today, the pound and the kilogram, come from very different traditions.
The pound: a Roman libra
The pound descends from the Roman libra, which is why its abbreviation is “lb” even though the word is pound (from the Latin pondo, “by weight”). Medieval England used several pounds at once. The one we use today is the avoirdupois pound, from Anglo-Norman French for “goods of weight”, divided into 16 ounces and used for everyday goods.
Precious metals kept their own system: the troy pound has 12 troy ounces, and a troy ounce (about 31.1 g) is heavier than an avoirdupois ounce (about 28.35 g). Gold is still priced per troy ounce. The weight converter uses avoirdupois ounces and pounds, the ones on kitchen and bathroom scales.
Stone and tons
The stone, 14 pounds, was once common for weighing wool and produce, and survives in Britain and Ireland for body weight. The ton grew from the tun, a large wine cask. The US short ton is 2,000 lb, the British long ton 2,240 lb, and the metric ton (tonne) 1,000 kg, about 2,205 lb, so “a ton” needs its country.
The kilogram: a liter of water
When France created the metric system in the 1790s, the unit of mass was tied to the new unit of length: a kilogram was the mass of one liter of water. In 1799 a platinum cylinder, the “kilogram of the Archives”, was made to match it. In 1889 the Metre Convention adopted a new platinum-iridium cylinder, the International Prototype of the Kilogram, nicknamed Le Grand K, kept under three glass bell jars in a vault near Paris.
A unit that drifted
Every kilogram in the world was ultimately compared with that one object. But when the official copies were brought back for checks over the 20th century, they had drifted relative to the original by tens of micrograms, about the weight of a grain of sand. Nobody could say which had changed, because by definition Le Grand K always weighed exactly one kilogram.
On 20 May 2019 the kilogram was redefined by fixing the value of the Planck constant, a fundamental constant of physics. Laboratories can now realize the kilogram with an instrument called a Kibble balance, without reference to any artifact. Le Grand K is still kept, but as a historical object.
How the two meet
Since the 1959 international agreement, the pound is defined as exactly 0.45359237 kg. So the pound is now, in effect, also defined by the Planck constant. One kilogram is about 2.20462 lb, which is why “double it and add 10%” is a good mental estimate for kilograms to pounds.
Try it
- Weight & mass converter — grams to tons, on a balance