Weight units explained
Almost every unit here is defined against the kilogram, and almost all of them are exact. The one genuine minefield is the word "ton", which names three different masses depending on who is speaking.
Milligram mg
1 mg = 0.000001 kg Exact
A thousandth of a gram. This is the scale of medication doses and of the active ingredient in almost anything — a paracetamol tablet is 500 mg, a grain of table salt around 0.06 mg. Below this, laboratories switch to micrograms rather than adding decimal places.
Gram g
1 g = 0.001 kg Exact
The everyday small unit almost everywhere except the United States. It began as the mass of one cubic centimetre of water, which is why a litre of water still weighs almost exactly a kilogram. Recipes outside the US measure dry ingredients in grams, and by weight rather than by cup — which is why they reproduce better.
Kilogram kg
1 kg = 1 kg Exact
The base unit of mass in the SI, and the only one that was still defined by a physical object until 2019 — a platinum-iridium cylinder near Paris that was slowly, embarrassingly, losing mass. It is now defined through the Planck constant, so it cannot drift again.
Tonne t
1 t = 1,000 kg Exact
A thousand kilograms. Written "tonne" in most of the English-speaking world and "metric ton" in the United States to keep it apart from the American short ton, which is about 9% lighter. If a document says only "ton", this is the one it means outside the US roughly half the time — which is exactly the problem.
Grain grain
1 grain = 0.00006479891 kg Exact
The oldest unit here, originally the mass of a single barleycorn, now defined as exactly 1/7000 of a pound. It survives in three places: bullet and gunpowder weights, arrow shafts, and pharmacy, where aspirin was long sold in five-grain tablets — which is where the modern 325 mg dose comes from.
Carat ct
1 ct = 0.0002 kg Exact
The gemstone carat, exactly 200 milligrams. Do not confuse it with the karat that measures gold purity, which is a proportion out of 24 and not a mass at all. A one-carat diamond weighs 0.2 g; 18-karat gold is 75% gold and could weigh anything.
Ounce oz
1 oz = 0.02834952 kg Exact
A sixteenth of a pound, about 28.35 g. Watch for two traps: the fluid ounce is a volume and is a different thing entirely, and the troy ounce used for precious metals is about 10% heavier at 31.1 g. A quoted gold price is per troy ounce.
Pound lb
1 lb = 0.45359237 kg Exact
The international avoirdupois pound, defined since 1959 as exactly 0.45359237 kg. That precise-looking figure exists because the US and Commonwealth agreed to pin the pound to the kilogram rather than maintain rival physical standards. The abbreviation lb comes from the Roman libra.
Stone st
1 st = 6.350293 kg Exact
Fourteen pounds, used in Britain and Ireland almost exclusively for body weight and never for anything else. British weights are given as stone plus a remainder — 11 st 4 lb — and the plural stays "stone", not "stones". The rest of the world finds this baffling and the UK shows no sign of stopping.
Short ton short ton
1 short ton = 907.18474 kg Exact
The American short ton: exactly 2,000 pounds, or about 907 kg. This is what "ton" means by default in the United States, including in vehicle ratings and freight quotes. It is roughly 10% lighter than a tonne and 11% lighter than a long ton.
Long ton long ton
1 long ton = 1,016.05 kg Exact
The British long ton: 2,240 pounds, or about 1,016 kg. Twenty hundredweight of 112 pounds each — the odd 112 comes from a medieval hundredweight that was never quite a hundred of anything. Still seen in shipping displacement and older British industrial figures.