How to convert a weight
- Enter the value you want to convert.
- Choose the unit it is currently in.
- Choose the unit you want.
- Read the result and the conversion factor in both directions.
- Check the list below for the same mass in every other unit.
What this converter covers
- Metric mass from milligrams to tonnes.
- Imperial mass including ounces, pounds and stones.
- Both the US short ton and the UK long ton, which differ substantially.
- Exact conversion factors from the international agreement.
- Every other unit listed alongside, so no second conversion is needed.
- Live results with nothing transmitted.
Mass and weight are not the same thing
Strictly, mass is how much matter something contains and weight is the force gravity exerts on it. Your mass is identical on the Moon; your weight is about one sixth. Kilograms and pounds as used here measure mass, even though everyone calls it weight.
The distinction only matters in physics and engineering, where the newton is the unit of force. In every practical context, shipping, cooking, medicine, the bathroom scale, mass is what is meant and the loose usage causes no problems.
Stones, and why only some countries use them
A stone is 14 pounds, which is roughly 6.35 kilograms. It survives almost exclusively in Britain and Ireland, and almost exclusively for body weight, nobody weighs luggage or ingredients in stones.
The awkwardness is that it is a compound unit in practice. "Eleven stone four" is 11 × 14 + 4 = 158 pounds, and converting requires handling both parts. That is why forms asking for weight in stones and pounds trip people up, and why this converter works in decimal stones with the pound conversion available alongside.
Two different tons
- Tonne (metric ton), 1,000 kilograms exactly.
- US ton (short ton), 2,000 pounds, about 907 kilograms.
- UK ton (long ton), 2,240 pounds, about 1,016 kilograms.
The three are close enough to be confused and far enough apart to matter. A shipment quoted in tons can differ by more than 10 per cent depending on which is meant, which is a genuine source of dispute in freight and commodities. When a contract says "tons", it is worth confirming which.
Ounces are ambiguous too
The ounce here is the avoirdupois ounce, about 28.35 grams, which is what recipes and shipping use. But a troy ounce, used for precious metals, is about 31.1 grams, roughly 10 per cent heavier. Gold quoted per ounce always means troy ounces.
There is also the fluid ounce, which measures volume rather than mass and is a different quantity entirely. Worse, the US fluid ounce and the imperial fluid ounce differ by about four per cent. Recipes that mix mass ounces and fluid ounces without saying which are a known source of baking failures.
The kilogram's recent redefinition
Until 2019 the kilogram was defined by a physical cylinder of platinum-iridium kept near Paris, the last SI unit still tied to an object. The problem was that it slowly changed mass relative to its own copies, which meant the definition of the kilogram was drifting.
It is now defined in terms of the Planck constant, which is fixed by definition and reproducible in any suitably equipped laboratory. Nothing changed in practical terms, but the unit is now as stable as physics allows. For lengths the length converter covers the equivalent ground, and everything here runs in your browser.