pound
Massexact by definition
The international avoirdupois pound is exactly 0.45359237 kilograms. That value is exact by definition, set by the 1959 agreement among the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, which also fixed the international yard at exactly 0.9144 m. Every customary mass unit on this page is derived from it.
Watch out: The pound is a unit of mass. Pound-force is a unit of force, equal to the weight of one pound under standard gravity. They are numerically equal on Earth's surface, which is exactly why the confusion survives, and they are not interchangeable in any equation containing an acceleration.
| 1 lb | 0.45359237 kg |
The pound descends from the Roman libra, a unit of about 329 g, which is why the abbreviation is lb and why the British currency symbol £ is a stylised L. The word pound itself comes from libra pondo, meaning a pound by weight.
About the pound
The pound is unusual among customary units in being defined by an exact metric value rather than the other way round. Since 1959 the international avoirdupois pound has been exactly 0.45359237 kg, which means every pound measurement is really a metric measurement in disguise. The number is not round in either system, a fossil of the fact that the definition was chosen to disturb existing US and British standards as little as possible while forcing them to agree.
"Avoirdupois" is Anglo-Norman for goods of weight, the system used for ordinary commerce. It coexisted for centuries with the troy system for precious metals and the apothecaries' system for medicine, all three sharing the same grain but dividing it differently: 7000 grains to the avoirdupois pound, 5760 to the troy pound. That is why a troy ounce is heavier than an avoirdupois ounce while a troy pound is lighter than an avoirdupois pound, a genuinely confusing pair of facts that only makes sense once you know the grain is the common ancestor.
The trap that matters in engineering is pound versus pound-force. In \(F = ma\) with SI units, one newton accelerates one kilogram at 1 m/s². In customary units, one pound-force accelerates one pound of mass at 32.174 ft/s², not at 1 ft/s². Practitioners escape this either by using the slug, the mass that one pound-force accelerates at 1 ft/s², or by carrying the conversion constant \(g_c = 32.174\ \mathrm{lb\,ft/(lbf\,s^2)}\) through the algebra. Silently treating lb and lbf as the same thing works only when nothing is accelerating.