Mass & weight: the exact defined values
1 avoirdupois pound = 0.453 592 37 kilogram exactly — fixed by the 1959 international yard and pound agreement.
How the base unit is defined
Since 2019 the kilogram has been defined by fixing the Planck constant at exactly 6.626 070 15 × 10⁻³⁴ J s (SI Brochure, 9th edition).
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 1 pound (avoirdupois) | 0.45359237 kg ≈ 0.4536 kg | Exact by definition. Every other avoirdupois unit derives from it. Open converter → |
| 1 ounce (avoirdupois) | 0.028349523125 kg ≈ 28.35 g | 1/16 pound, therefore also exact. Open converter → |
| 1 stone | 6.35029318 kg ≈ 6.35 kg | 14 pounds. Still standard for body weight in the UK and Ireland. Open converter → |
| 1 tonne (metric ton) | 1000 kg | Accepted for use with the SI. Not the same as a US short ton (2000 lb = 907.18474 kg). Open converter → |
| 1 kilogram | 2.20462262184878 lb ≈ 2.2046 lb | Reciprocal of 0.45359237. Non-terminating, so this one is genuinely rounded for display. Open converter → |
Rounding traps that actually change your result
Short ton, long ton and tonne are three different units
A US short ton is 2000 lb = 907.18474 kg. A UK long ton is 2240 lb = 1016.0469088 kg. A metric tonne is 1000 kg. Freight and commodity figures are wrong by up to 12% if these are mixed up.
Troy ounces are heavier than the ounces on your kitchen scale
A troy ounce, used for precious metals, is 31.1034768 g exactly. The avoirdupois ounce used for food is 28.349523125 g. That is a 9.7% difference — enough to badly misprice gold or silver.
Where these values come from
- NIST Handbook 44, Appendix C
Publishes the avoirdupois pound as exactly 0.45359237 kg.
- BIPM SI Brochure, 9th edition
2019 redefinition of the kilogram via the Planck constant.
Frequently asked questions
Is a pound exactly 0.45359237 kg?
Yes, for the avoirdupois pound used in the US and UK. It is a defined value from the 1959 international yard and pound agreement, not a measurement.
Why is kg to lb never exact?
The pound is defined as an exact number of kilograms, so kg → lb is the reciprocal 1 / 0.45359237 = 2.204622621848... which does not terminate. Converting lb → kg is exact; kg → lb always involves display rounding.
How many kilograms is a stone?
Exactly 6.35029318 kg, because a stone is 14 pounds and the pound is exactly 0.45359237 kg.
Convert mass & weight now
These converters use the exact factors listed above.