Weight & Mass Converter

Enter a mass in milligrams, metric carats, grams, kilograms, tonnes, ounces, pounds, stone, US short tons or UK long tons, and every other unit follows. Everything goes through kilograms, and the imperial side rests on the 1959 international agreement that fixed one pound at exactly 0.45359237 kilograms — an exact number, not a measurement, which is why avoirdupois conversions never drift.

The word "ton" is the one to be careful with, because three different units share it and they are close enough to be mistaken for each other. A US short ton is 2,000 pounds, about 907 kg. A UK long ton is 2,240 pounds, about 1,016 kg. A tonne is exactly 1,000 kg. The spread between the smallest and largest is twelve percent, which on a shipping quote or a freight manifest is a real amount of money, and none of the three is wrong — they are simply three answers to the same word. Spelling helps in writing and not in speech, which is why "metric ton" is the phrase to use out loud.

The ounce has the same problem in a smaller package, and this table uses only one of the two. An avoirdupois ounce is 28.35 grams and is what groceries, post and cooking mean. A troy ounce is 31.10 grams, about ten percent heavier, and is what precious metals are priced in — so a quoted ounce of gold is not the same quantity as an ounce of flour, and converting a bullion price with the ounce on this page produces a number that is wrong by that ten percent. The metric carat, on the other hand, is unambiguous: exactly 200 milligrams, standardised in 1907, which is why gemstone weights are comparable across countries in a way that ounces are not.

One conceptual note. These are units of mass, not weight, even though the everyday word for what a scale reports is "weight". Mass does not change with location; the force it produces does, which is the quantity the force converter handles. A kilogram of flour is a kilogram of flour on the Moon, and it presses on the scale with a sixth of the force.