Mass units and their equivalents
| Unit | Symbol | In kilograms |
|---|---|---|
| Tonne | t | 1,000 kg |
| Kilogram | kg | 1 kgbase unit |
| Gram | g | 0.001 kg |
| Milligram | mg | 0.000001 kg |
| Pound | lb | 0.453592 kg |
| Ounce | oz | 0.0283495 kg |
| Stone | st | 6.35029 kg |
| Carat | ct | 0.0002 kg |
| Short ton (US) | ton US | 907.185 kg |
| Long ton (UK) | ton UK | 1,016 kg |
| Microgram | µg | 0.000000001 kg |
Frequently asked questions
What is the difference between mass and weight?
Mass measures the amount of matter and never changes; weight is a force that depends on gravity. A 70 kg person keeps the same mass on the Moon but weighs six times less there. In everyday use the two terms are treated as interchangeable.
Is the stone still used?
Yes, in the UK and Ireland for body weight: scales and medical records often show stones and pounds (for example 10 st 10 lb for 68 kg). The United States does not use it and states weight in pounds alone.
Is a ton the same everywhere?
No. The metric tonne is 1,000 kg, the American short ton is 2,000 pounds or 907.18 kg, the British long ton is 2,240 pounds or 1,016.05 kg. Twelve per cent separates the last two: a load quoted at 50 tons weighs anywhere from 45.4 to 50.8 tonnes depending on who wrote the figure. American documents say metric ton when they mean 1,000 kg, and freight, emissions and commodity figures are almost always metric tonnes.
Is the carat of a diamond the carat of gold?
No, and only one of the two is a mass. The metric carat has been exactly 0.2 g since 1907: a one carat diamond weighs 200 mg, and jewellers split it into points of 0.01 carat, that is 2 mg. The carat of gold is a purity fraction out of 24: 18 carat means 18 parts gold in 24, so 75% fine metal. A ring described as 22 carat therefore tells you nothing about its weight.
What do net weight, drained weight and the ℮ sign mean?
Net weight is the contents alone, without the jar, the tin or the wrapping, and it is the figure the law requires. Drained weight is what is left once the liquid is poured off: a 400 g tin of chickpeas usually declares about 240 g drained. The small ℮ beside the figure is the European estimated sign: it states that the average of the batch meets the printed weight rather than every single pack, a tolerance set by a 1976 directive.
Mass or weight, the most common confusion
A bathroom scale does not show a mass but a force, the one gravity exerts on you. The two blur together on Earth because gravity varies little here, from 9.78 m/s² at the equator to 9.83 at the poles: the same body reads two hundred grams apart without having lost anything. On the Moon it would weigh a sixth and keep its mass intact, which is why the kilogram measures mass and the newton measures force.
The kilogram changed nature on 20 May 2019. Since 1889 it had rested on a platinum-iridium cylinder kept at Sèvres, which was losing a few micrograms over the decades. It is now defined through the Planck constant, fixed at 6.62607015 × 10⁻³⁴ joule-second. The practical value did not shift by a hair, but no scale in the world depends on a single object any more.