The pound, like the inch, is defined exactly in metric terms: one pound is 0.45359237 kilograms, fixed by the same 1959 agreement. Everything else in the imperial system follows from it. An ounce is a sixteenth of a pound. A stone is 14 pounds.
Strictly, these are units of mass rather than weight. Mass is how much matter is present and does not change; weight is the force gravity exerts on that mass and varies with location. Your mass is identical on the Moon; your weight is about a sixth. In everyday use the words are interchangeable, and every figure on this page is mass.
Quick reference
The table follows whichever units you select above.
Worth memorising
1 kg ≈ 2.205 lb1 lb ≈ 454 g— near enough half a kilo for cooking1 oz ≈ 28.35 g1 st = 14 lb ≈ 6.35 kg
Where people go wrong
Three different tons
This one costs real money in freight. A US short ton is 2,000 lb, about 907 kg. A UK long ton is 2,240 lb, about 1,016 kg. A metric tonne is 1,000 kg exactly. The gap between the short ton and the tonne is over 10 per cent, and shipping quotes that do not specify which are worth querying before you sign.
Troy ounces are heavier
Precious metals are priced in troy ounces, which are about 31.10 g rather than the usual 28.35 g — roughly 10 per cent heavier. A troy pound is 12 troy ounces, not 16, so a troy pound is actually lighter than a standard pound. Gold quoted per ounce always means troy.
Stone is not a decimal
British body weights are given as stone-and-pounds. 11 st 6 lb is 160 lb, not 11.6 stone. Multiply the stone by 14 first, then add the pounds.
Fluid ounces are not ounces. One measures volume, the other mass, and they only coincide for water by rough accident. A cup of flour and a cup of honey occupy the same volume and differ in mass by more than double. Recipes that give weights are reproducible; recipes that give cups are approximations.
Common questions
How many pounds in a kilogram?
About 2.20462. For mental arithmetic, double it and add ten per cent: 70 kg is about 140 plus 14, so roughly 154 lb. The true figure is 154.3.
What is 1 stone in kilograms?
6.35029318 kg, so a little over six and a third kilograms.
Why do recipes and scales disagree?
Usually because one is measuring volume and the other mass. A US cup of flour ranges from about 120 g to 145 g depending on whether it was scooped or spooned. If a recipe matters, weigh it.