Mass is the amount of matter in an object; weight is the gravitational force acting on that mass (measured in newtons), though both terms are used interchangeably in everyday life. The kilogram (SI base unit) was redefined in 2019 by fixing the value of the Planck constant, replacing a physical platinum–iridium prototype that had served as the reference for 130 years. The avoirdupois pound is defined as exactly 0.45359237 kg.
Postal services weigh parcels in grams and ounces; aviation loads cargo in tonnes; precious metals such as gold and silver are traded in troy ounces. Note that the troy ounce (31.1035 g) is heavier than the avoirdupois ounce (28.3495 g) — a 10% difference that matters in commodity trading. Body weight in the UK is still commonly expressed in stones (1 stone = 14 lb).
| 1 kg | = 2.20462 lb |
| 1 lb | = 453.592 g |
| 1 stone | = 14 lb |
| 1 oz | = 28.3495 g |
| 1 metric ton | = 1,000 kg |
| 1 troy oz | = 31.1035 g |