Mass versus weight
Strictly, mass is the amount of matter in an object and weight is the force gravity exerts on that mass. Mass is measured in kilograms; weight, being a force, is properly measured in newtons. In everyday use the two are treated as interchangeable because gravity is effectively constant across the Earth's surface.
The distinction becomes real away from Earth. An object with a mass of 70 kg has that same mass on the Moon but weighs about one sixth as much, because lunar gravity is weaker. Bathroom scales measure force and report an implied mass, which is why they would read differently on the Moon despite the body being unchanged.
Key conversion factors
As with length, the metric-imperial link is exact by definition: the international pound is defined as exactly 0.45359237 kilograms.
- 1 kilogram = 2.20462 pounds
- 1 pound = 0.45359237 kg exactly = 16 ounces
- 1 ounce = 28.3495 grams
- 1 stone = 14 pounds = 6.35029 kg
- 1 US ton (short ton) = 2,000 lb = 907.185 kg
- 1 metric ton (tonne) = 1,000 kg = 2,204.62 lb
| kg | lb | st and lb |
|---|---|---|
| 50 | 110.2 | 7 st 12.2 lb |
| 55 | 121.3 | 8 st 9.3 lb |
| 60 | 132.3 | 9 st 6.3 lb |
| 65 | 143.3 | 10 st 3.3 lb |
| 70 | 154.3 | 11 st 0.3 lb |
| 75 | 165.3 | 11 st 11.3 lb |
| 80 | 176.4 | 12 st 8.4 lb |
| 85 | 187.4 | 13 st 5.4 lb |
| 90 | 198.4 | 14 st 2.4 lb |
| 95 | 209.4 | 14 st 13.4 lb |
| 100 | 220.5 | 15 st 10.5 lb |
| 110 | 242.5 | 17 st 4.5 lb |
| 120 | 264.6 | 18 st 12.6 lb |
Stone, and the three kinds of ton
The stone, equal to 14 pounds, remains in common use for body weight in the UK and Ireland, where someone might describe themselves as 11 stone 4 rather than 158 pounds. It is essentially unused in the United States.
Ton is genuinely ambiguous and worth clarifying in any context where it matters. The US short ton is 2,000 pounds, the UK long ton is 2,240 pounds, and the metric tonne is 1,000 kilograms, or about 2,205 pounds. The long ton and the tonne differ by only about 1.6%, which makes the ambiguity easy to overlook and occasionally expensive.
| Ton | Pounds | Kilograms | Where used |
|---|---|---|---|
| US short ton | 2,000 lb | 907.185 kg | United States |
| UK long ton | 2,240 lb | 1,016.05 kg | United Kingdom, shipping |
| Metric tonne | 2,204.62 lb | 1,000 kg | Everywhere metric; often written t |
Troy weight for precious metals
Gold, silver, and platinum are traded in troy ounces, which are heavier than the ordinary avoirdupois ounces used for food and shipping. A troy ounce is about 31.103 grams against 28.350 grams for a standard ounce — roughly 10% more.
The troy pound compounds the confusion by containing 12 troy ounces rather than 16, making it lighter than a standard pound overall. When a price is quoted per ounce for a precious metal, it is virtually always a troy ounce, and this converter uses standard avoirdupois units throughout.
| Unit | Grams | Subdivision | Used for |
|---|---|---|---|
| Avoirdupois ounce | 28.3495 g | 16 to the pound | Food, shipping, body weight |
| Troy ounce | 31.1035 g | 12 to the troy pound | Gold, silver, platinum prices |
| Avoirdupois pound | 453.592 g | 16 oz | Everyday weight |
| Troy pound | 373.242 g | 12 oz t | Rarely used; lighter than the everyday pound |
Ounces to grams for the kitchen
Recipe conversions are the other everyday case. An ounce is 28.35 grams, so a rough rule of 30 g per ounce overstates by about 6% — fine for most cooking, not for baking.
| oz | g | lb |
|---|---|---|
| 1 | 28.3 | 0.0625 |
| 2 | 56.7 | 0.125 |
| 3 | 85 | 0.1875 |
| 4 | 113.4 | 0.25 |
| 6 | 170.1 | 0.375 |
| 8 | 226.8 | 0.5 |
| 12 | 340.2 | 0.75 |
| 16 | 453.6 | 1 |