Weight and Mass Converter

Details

About this tool

Converts between metric and imperial mass: kilograms to pounds, grams to ounces, stones to kilograms, tonnes to short and long tons. Also covers the units used in jewellery and medicine - carats, grains and troy ounces - which are easy to confuse with their everyday namesakes.

Like length, the imperial definitions are exact rather than approximate: the pound has been defined as exactly 0.45359237 kg since 1959. So kilograms to pounds is an exact conversion, and any imprecision is in the rounding you choose.

The ton is where money gets lost. A metric tonne is 1000 kg, a US short ton is 907.18 kg, and a British long ton is 1016.05 kg - a spread of 12 per cent between the extremes, all called "a ton" in ordinary speech. On a shipping quote or a commodity contract that gap is the whole margin, which is why all three are listed by name here rather than folded into one entry.

Ounces have the same problem in miniature, and it is worse because the difference runs the other way from what people expect. A troy ounce, used for gold, silver and platinum, is 31.103 g. An ordinary avoirdupois ounce is 28.35 g. Pricing precious metal with the wrong one is a 10 per cent error, and the words look identical on a price list.

Mass and weight are not the same thing, though everyday language treats them as one. Mass is how much matter there is and does not change; weight is the force gravity exerts on it and would be different on the Moon. Scales report mass, which is what everyone means, so this converter works in mass throughout.

Good to know

Questions people actually ask

Is a ton the same everywhere?

No, and the differences are large enough to matter commercially. A metric tonne is 1000 kg, a US short ton is 907.18 kg, and a British long ton is 1016.05 kg. That is a 12 per cent spread between the extremes, all called "a ton" in conversation, which is why they are listed separately here.

Why is a troy ounce different from an ordinary ounce?

They are different units that happen to share a name. A troy ounce, used for precious metals, is 31.103 g; the ordinary avoirdupois ounce is 28.35 g. Pricing gold with the wrong one is a 10 per cent error, and nothing on a price list distinguishes them.

What is the difference between mass and weight?

Mass is how much matter something contains and is the same everywhere. Weight is the force gravity exerts on that mass and would be about a sixth as much on the Moon. Everyday speech uses "weight" for both, scales actually report mass, and this converter works in mass throughout - which is what people mean.

How many pounds is a kilogram, exactly?

2.20462262... - the pound is defined as exactly 0.45359237 kg, so the conversion is exact in that direction and a recurring decimal in this one. For rough mental arithmetic, doubling and adding 10 per cent is within half a per cent.

What is a stone?

14 pounds, or 6.35 kg. It is used almost exclusively in Britain and Ireland, and almost exclusively for body weight - which is why a British weight is often given as "11 stone 4" rather than as 158 pounds or 71.7 kg.