Weight Calculator

One pound is exactly 0.45359237 kilograms. Not approximately: the figure was fixed by treaty in 1959 and has not moved since, which is why this converter can give it to the last digit rather than rounding. Enter an amount, pick the unit you have and the unit you want, and the result appears alongside the same quantity in all thirteen units the calculator covers, from grains up to metric tons.

Weight and mass are not the same thing

Strictly, mass is how much matter something contains and weight is the force gravity exerts on it. Mass is measured in kilograms, weight in newtons. This calculator, like almost every tool called a weight calculator, converts mass.

The distinction is easy to see once you leave Earth. A 70 kg astronaut has a mass of 70 kg everywhere. On Earth, where gravity pulls at 9.80665 m/s², that mass weighs 686 newtons. On the Moon, where gravity is 1.62 m/s², the same astronaut weighs 113 newtons, about a sixth as much, while the mass on the label is unchanged. In orbit the weight is effectively zero and the mass is still 70 kg, which is why bumping into something in free fall still hurts.

In daily use nobody minds. Bathroom scales say kilograms or pounds, both units of mass, and since gravity varies by only about 0.5% across the Earth's surface, treating the two as interchangeable costs nothing. If you need the force rather than the quantity of matter, multiply the mass in kilograms by 9.80665 to get newtons.

Why there are so many units in the first place

Because weight standards grew out of trade, and every trading culture built its own. The earliest records, Babylonian and Egyptian, describe weighing by filling a vessel with seed and counting it, so the first standards were volumes of grain rather than reference objects. The carat still carries that history: it comes from the carob seed, and it is now fixed at exactly 200 milligrams.

The Babylonians used the talent as their base unit and split it on their sexagesimal, base-60 system. The Romans used the libra, roughly 329 grams, divided into twelve unciae. That one word produced two English words: uncia as a twelfth of a foot became the inch, and as a twelfth of a pound became the ounce. The Roman abbreviation is also why a pound is written lb, from libra, rather than anything resembling the English word.

Medieval England ran on silver. A penny weighed about 22.5 grains, 240 pennies made the tower pound of 5400 grains, and the coinage weight and the trade weight stayed tied together for centuries. That is the root of the two pounds still in use for different goods: the troy pound of 5760 grains for precious metals, and the avoirdupois pound of 7000 grains for everything else. A troy ounce is 31.1034768 g and an ordinary ounce is 28.349523125 g, which is why gold quoted per ounce is quoted in a heavier ounce than the one on a food label.

Britain tidied all of this into the Imperial system with the Weights and Measures Act of 1824. The physical standards were destroyed when fire gutted the Houses of Parliament in 1834, and replacements were not legalised until 1855. The United States, independent by then, kept the earlier English units, which is how the two systems drifted apart on the hundredweight and left us with a US quarter of 25 lb and a UK quarter of 28 lb.

The metric system, and what changed in 2019

The metric system was designed rather than inherited. France adopted it in 1795, defining the gram as the mass of a cubic centimetre of water and the kilogram as a thousand of those, which is why a litre of water still weighs almost exactly a kilogram. Napoleon reintroduced older customary measures alongside it in 1812, and metric units were made compulsory again in 1840.

For most of its life the kilogram was a physical object: a platinum-iridium cylinder held near Paris, with official copies distributed worldwide. The trouble is that objects change. Comparisons over the twentieth century showed the prototype and its copies drifting apart by tens of micrograms, and since the prototype was the definition, the world's standard of mass was whatever that cylinder happened to be.

That ended on 20 May 2019. The kilogram is now defined by fixing the Planck constant at exactly 6.62607015 × 10^-34 joule seconds, which ties mass to time and length through physics rather than to a lump of metal. Nothing changed at the checkout: the redefinition was chosen so the new kilogram matched the old one to within the measurement uncertainty. What changed is that any properly equipped laboratory can now realise the unit from first principles.

The thirteen units in this calculator

Every imperial and US customary entry below is an exact multiple of the pound, so the conversions are exact rather than measured. Only the atomic mass unit carries experimental uncertainty.

The stone and quarter are worth a note. A UK stone is 14 lb and is still the normal way to state body weight in Britain and Ireland: 12 stone 7 lb is 79.38 kg. The US stone of 12.5 lb is a trade unit and is not used for people. In both systems a quarter is two stones, which is the check that tells you the pair belongs together.

A ton is three different weights

Three units share the name, and they differ by up to 12%. The metric ton, or tonne, is 1000 kg. The US short ton is 2000 lb, or 907.18474 kg. The UK long ton is 2240 lb, or 1016.0469088 kg. The unqualified ton in this calculator is the metric one.

The gap matters at scale. A shipment quoted as 500 tons is 500,000 kg metric, 453,592 kg short, or 508,023 kg long, a spread of more than fifty metric tons. Freight, mining and emissions figures are usually metric; older US industrial figures are usually short tons; UK shipping historically used long tons. If a document does not say which, the number is not usable.

Reference weights worth memorising

A few anchors make it possible to sanity-check any conversion without a calculator.

• 1 millilitre of water weighs about 1 gram, so 1 litre is about 1 kilogram and 1 cubic metre is about 1000 kg, which is 1 metric ton.
• A US nickel weighs exactly 5 grams and a penny 2.5 grams, so loose change makes a usable field scale check.
• 1 kilogram is 2.2046226218488 lb. The rounded 2.2 is within 0.2%, which is close enough for mental arithmetic.
• 1 ounce is about 28.35 g, and 1 stone (UK) is 6.35 kg.
• A carat is 200 mg exactly, so a 1 carat diamond is a fifth of a gram.
• A grain is 64.79891 mg. It survives in bullet and gunpowder charges, and in some medicine dosing.

Weights from galaxies to electrons

The units here span more than 60 orders of magnitude, from the atomic mass unit at 1.66 × 10^-27 kg to metric tons. The figures below are current CODATA and IAU values; the parenthesised digits are the standard uncertainty in the last places shown.

The galaxy figure is the loosest number on the page. Estimates of the mass of the Milky Way range from about 0.8 to 1.5 × 10¹² solar masses depending on the method, because most of it is dark matter inferred from how fast the outer stars orbit rather than anything observed directly.

Using this calculator

Type the amount, choose the unit you are converting from and the unit you want, and the answer updates as you type. Swap units reverses the pair without retyping. Results are given to fourteen significant figures with trailing zeros removed, so an exact conversion such as 1 lb to kg prints as 0.45359237 rather than a rounded 0.4536.

Below the result, the same quantity is listed in all thirteen units at once, with the unit you selected highlighted, which is quicker than converting one pair at a time. For length, area, volume and temperature, the general Conversion Calculator covers the same ground, and if you are working out body weight rather than converting it, the Ideal Weight Calculator and BMI Calculator are the ones you want.