How this instrument works
Weigh anything in pounds and you are using an avoirdupois pound of 7000 grains, the merchants' unit that reached England through medieval wool markets and never quite left. It drifted slightly between American and British practice until 1 July 1959, when standards keepers in America, Britain, Canada, Australia, South Africa and New Zealand signed an agreement fixing one pound at 0.45359237 kilograms. Those eight decimals are definitional rather than experimental, so no uncertainty attaches to them.
Why that particular string? It settled an old disagreement. America had defined its pound by reference to the kilogram since Mendenhall's order of 1893, arriving at 0.4535924277 kg; Britain still kept its 1844 platinum cylinder, sibling to standards immured in walls at Westminster. Rather than argue over artefacts, delegates rounded to eight places and adopted 0.45359237 kg, shaving about one part in eight million off American pounds — nothing any butcher would notice, plenty for the laboratories that calibrate kilogram standards. Parliament wrote that same figure into British law with the Weights and Measures Act 1963.
Kilograms travelled another road entirely. Until May 2019 one was whatever the platinum-iridium cylinder outside Paris happened to be; then delegates pinned Planck's constant — 6.62607015 × 10⁻³⁴ joule-seconds — and let the kilogram fall out of physics instead of metalwork. Pounds rode along untouched, because 1959 tied them to kilograms by ratio. Today pounds carry American groceries, gym plates, birth announcements and boxing cards, plus British body weight in stones; kilograms carry almost everything else, including every scientific journal on earth.
- Type your figure into Pounds (lb). The field opens at 150 to start you off.
- Kilograms (kg) answers below, refreshed as fast as you can type — nothing to submit, nothing to click.
- Working in stones and pounds? Multiply stones by 14, add loose pounds, enter that total.
- Pounds and ounces? Divide ounces by 16 first — 12 oz is 0.75 lb — because this field takes decimal pounds.
- Going kilograms to pounds instead? Divide by 0.45359237, or multiply by 2.2046226218.
Worked example — 150 lb lifter hunts for her weight class
One coach in Ohio has an athlete at 150 lb and needs to know which international powerlifting class that is. Type 150 into Pounds (lb); Kilograms (kg) returns 68.0388555. That sits above the 66 kg class, so she lifts at 74 kg unless she sheds 2.0388555 kg — 4.49 lb, which is two or three weeks of patient work rather than one hard weekend.
Why does such an answer stop dead instead of running on? Both numbers are finite decimals: 150 × 45359237 = 6803885550, and shifting the decimal point eight places yields 68.0388555 kg with nothing discarded. Divide back by 0.45359237 and 150 returns precisely. Round for the scoresheet and you have 68.04 kg, or 68 kg where officials weigh to whole kilograms.
Questions
Is one pound exactly 0.45359237 kilograms?
Yes, exactly — that value is definitional rather than experimental, so it carries no uncertainty. Standards bodies of six English-speaking nations agreed it in 1959, and Britain gave it statutory force in 1963. Any result you read here is limited only by how many decimals the display can show, never by any wobble in that factor.
Did redefining the kilogram in 2019 change what one pound weighs?
No. Replacing that metal artefact with Planck's constant left kilogram size unchanged to far better than anyone can weigh, and since 1959 pounds have been tied to kilograms by fixed ratio rather than by any independent lump of platinum. Shop scales, luggage allowances and lifting records all read identically before and after 20 May 2019.
Is the British imperial pound identical to the US customary pound?
For mass, yes — identical since 1959, both exactly 0.45359237 kg. That agreement is unusual, because imperial and US customary volumes never reconciled: the UK pint is 568.26 ml against 473.18 ml for its US liquid namesake, and the UK gallon runs about 20 percent larger. Weight is where two systems genuinely match; capacity is where mixing them quietly ruins dinner.
Why does dividing by 2.2 give slightly wrong answers?
Because one kilogram is 2.2046226 pounds, not 2.2 — an error near 0.21 percent, worth 0.14 kg on 150 lb of bodyweight and 21 kg across ten tonnes of freight. For mental arithmetic, halve then subtract one tenth: 150 halved is 75, less 7.5 leaves 67.5 kg, roughly 0.8 percent under. Fine for guessing luggage; for anything billed, dosed or entered on shipping papers, multiply by 0.45359237 properly.
Does this converter work for troy ounces of gold or silver?
No. Bullion uses the troy system, where one ounce is 31.1034768 g and twelve of them make the troy pound of 373.2417216 g. An avoirdupois pound of 453.59237 g is heavier, which is why feathers beat gold by the pound while gold beats feathers by the ounce. Convert troy weights with their own factors; this field expects ordinary commercial pounds of 16 ounces.
Does lb here mean mass or force?
Mass — which is what commerce, medicine and shipping mean by it. Engineers needing force write lbf, pound-force — whatever one pound presses down with under standard gravity, 9.80665 m/s², which works out near 4.448 newtons. Confusing those two is the classic source of wrong answers in mechanics, and it is why US engineering also carries the slug, whose mass is near 14.594 kg.