Mass, Not Weight
Mass is the quantity of matter in an object. It does not change with location: a 70 kg person has 70 kg of mass on Earth, on the Moon, or in orbit. Weight is the force gravity exerts on that mass, and it does change — which is why the same person weighs about a sixth as much on the Moon.
Everyday language uses "weight" for both, and bathroom scales are labelled in kilograms, which are units of mass. The distinction matters in physics and engineering, and almost nowhere else.
Exact Definitions
Since 1959 the imperial units have been defined in terms of the kilogram, so these conversions are exact rather than approximate:
| Unit | Exact in kilograms |
|---|---|
| 1 pound (lb) | 0.45359237 |
| 1 ounce (oz) | 0.028349523125 |
| 1 stone (st) | 6.35029318 |
| 1 short ton (US) | 907.18474 |
| 1 long ton (UK) | 1,016.0469088 |
| 1 tonne (metric) | 1,000 |
| 1 carat | 0.0002 |
| 1 grain | 0.00006479891 |
The kilogram itself was redefined in 2019 in terms of the Planck constant, retiring the platinum-iridium cylinder held near Paris that had served as the standard since 1889 — and which had drifted measurably against its own copies.
Common Conversions
| Kilograms | Pounds | Stones and pounds |
|---|---|---|
| 1 | 2.205 | 0 st 2.2 lb |
| 5 | 11.02 | 0 st 11.0 lb |
| 10 | 22.05 | 1 st 8.0 lb |
| 50 | 110.2 | 7 st 12.2 lb |
| 60 | 132.3 | 9 st 6.3 lb |
| 70 | 154.3 | 11 st 0.3 lb |
| 80 | 176.4 | 12 st 8.4 lb |
| 90 | 198.4 | 14 st 2.4 lb |
| 100 | 220.5 | 15 st 10.5 lb |
Quick Mental Conversions
- kg to lb: double it and add 10%. 70 kg → 140 + 14 = 154 lb (exact: 154.3).
- lb to kg: halve it and subtract 10%. 150 lb → 75 − 7.5 = 67.5 kg (exact: 68.0).
- st to kg: multiply by 6.35, or by 6 and add a bit.
- oz to g: multiply by 28.35, or by 28 for a close estimate.
The Three Different Tons
| Ton | Kilograms | Pounds | Used in |
|---|---|---|---|
| Short ton | 907.2 | 2,000 | United States |
| Long ton | 1,016.0 | 2,240 | United Kingdom, shipping |
| Tonne (metric) | 1,000 | 2,204.6 | Almost everywhere else |
The three differ by up to 12%, which is a substantial gap in freight, mining or construction. When a figure matters, ask which ton is meant.
Stones
The stone — 14 pounds — survives almost exclusively in Britain and Ireland, and almost exclusively for body weight. A British person will give their weight as "eleven stone four" and their luggage allowance in kilograms without noticing the inconsistency. It has no legal standing for trade in the UK and is not used in the United States at all.
Specialist Units
| Unit | Equivalent | Used for |
|---|---|---|
| Carat | 200 mg | Gemstones |
| Grain | 64.799 mg | Ammunition, medicine, precious metals |
| Troy ounce | 31.103 g | Gold, silver, platinum |
| Atomic mass unit (u) | 1.6605 × 10−27 kg | Atoms and molecules |
Note that a troy ounce is about 10% heavier than the ordinary avoirdupois ounce used for food. Precious metals are always quoted in troy ounces, which is why a "pound" of gold is lighter than a pound of feathers — the old riddle has a real answer.
Everyday Reference Masses
| Object | Approximate mass |
|---|---|
| Paperclip | 1 g |
| AA battery | 24 g |
| Tennis ball | 58 g |
| Smartphone | 180 g |
| Litre of water | 1 kg |
| House cat | 4.5 kg |
| Car tyre | 10 kg |
| Adult human | 62–80 kg |
| Grand piano | 400 kg |
| Small car | 1,200 kg |
| African elephant | 5,400 kg |
Frequently Asked Questions
How many pounds in a kilogram?
2.20462. For mental arithmetic, double and add 10%.
Is a kilogram a unit of mass or weight?
Mass. The SI unit of weight, being a force, is the newton — and 1 kg weighs about 9.81 N on Earth.
Why does gold use a different ounce?
Historical convention. The troy system, named after Troyes in France, has been used for precious metals since medieval trade fairs and never converted to the avoirdupois system that governs everything else.
How many kg in a stone?
6.35029318, or 14 pounds. Eleven stone is about 69.9 kg.