Measure · 21 units
Mass
The kilogram is fixed by the Planck constant (2019). The pound has been exactly 0.453 592 37 kg since 1959, and the grain, 1/7000 pound, is shared by the avoirdupois, troy and apothecaries’ systems, which is how those three relate.
Convert mass
Then and now
For 130 years the kilogram was the mass of one metal cylinder kept near Paris; copies drifted from it by tens of micrograms. The carat once varied by country between about 188 and 213 mg before settling on 200 mg.
All mass units
“Exact” means the unit is defined as an exact multiple of the SI unit; the decimal is printed in full when it ends. Rows marked historical or obsolete are kept for reading old documents.
| Unit | Defined as | In SI | Status | Source |
|---|---|---|---|---|
| SI | ||||
| milligram mg | 1/1000 g | 0.000 001 kg | exact | BIPM SI Brochure |
| gram g | 1/1000 kg | 0.001 kg | exact | BIPM SI Brochure |
| kilogram kg | SI base unit: fixed by the Planck constant h = 6.626 070 15 × 10⁻³⁴ J s | 1 kg | exact | BIPM SI Brochure |
| SI (accepted) | ||||
| tonne (metric ton) t | 1000 kg | 1 000 kg | exact | BIPM SI Brochure |
| Imperial / US | ||||
| grain gr | 64.798 91 mg = 1/7000 pound | 0.000 064 798 91 kg | exact The grain is shared by the avoirdupois, troy and apothecaries’ systems. | Yard and pound, 1959 |
| US | ||||
| hundredweight (short) | 100 pounds | 45.359 237 kg | exact | NIST Handbook 44 |
| ton (short) | 2000 pounds | 907.184 74 kg | exact | NIST Handbook 44 |
| Imperial | ||||
| stone st | 14 pounds | 6.350 293 18 kg | exact | UK Weights and Measures Act 1985 |
| quarter (British) | 28 pounds | 12.700 586 36 kg | exact historical | NIST Handbook 44 |
| hundredweight (long) cwt | 112 pounds | 50.802 345 44 kg | exact | NIST Handbook 44 |
| ton (long) | 2240 pounds = 20 long hundredweight | 1 016.046 908 8 kg | exact | NIST Handbook 44 |
| Avoirdupois | ||||
| dram (avoirdupois) dr | 1/16 ounce = 27.343 75 grains | 0.001 771 845 195 312 5 kg | exact | NIST Handbook 44 |
| ounce (avoirdupois) oz | 1/16 pound = 437.5 grains | 0.028 349 523 125 kg | exact | Yard and pound, 1959 |
| pound (avoirdupois) lb | exactly 0.453 592 37 kg = 7000 grains | 0.453 592 37 kg | exact | Yard and pound, 1959 |
| Troy | ||||
| pennyweight dwt | 24 grains | 0.001 555 173 84 kg | exact | NIST Handbook 44 |
| troy ounce oz t | 480 grains = 31.103 476 8 g | 0.031 103 476 8 kg | exact Precious metals are still priced by the troy ounce. | NIST Handbook 44 |
| troy pound | 12 troy ounces = 5760 grains | 0.373 241 721 6 kg | exact historical | NIST Handbook 44 |
| Apothecaries’ | ||||
| scruple | 20 grains | 0.001 295 978 2 kg | exact historical | NIST Handbook 44 |
| dram (apothecaries’) | 3 scruples = 60 grains | 0.003 887 934 6 kg | exact historical | NIST Handbook 44 |
| Gemstones | ||||
| carat (metric) ct | exactly 200 mg | 0.000 2 kg | exact Before the metric carat (1907 onward) the carat varied from about 188 to 213 mg by country. | NIST Handbook 44 |
| US engineering | ||||
| slug | the mass that 1 pound-force accelerates at 1 ft/s² | 14.593 902 937 2… kg | exact historical | NIST SP 811 |