How this instrument works
An avoirdupois pound weighs exactly 0.45359237 kilograms. That figure was never measured; it was agreed. On 1 July 1959, standards laboratories in Washington, London, Ottawa, Canberra, Wellington and Pretoria adopted one international pound, ending decades in which American and British pounds parted company somewhere around their eighth decimal. Invert that definition and 2.20462262185 falls out — this sheet's multiplier.
Underneath both traditions sits one grain. Seven thousand grains make an avoirdupois pound, and grains are pinned at 64.79891 milligrams each, so 7000 × 64.79891 mg rebuilds 0.45359237 kg digit for digit. Bullion dealers count from that same grain toward another pound entirely: 5760 grains, twelve troy ounces, 0.3732417216 kg. Gold quoted per ounce means troy ounces of 31.1034768 g, never grocery ounces of 28.349523125 g.
Both units still map onto real lives. Body weight in America and Britain arrives in pounds — and British scales add stones of 14 lb, so 70 kg reads as eleven stone — while clinics, gyms and sports federations from Lyon to Tokyo record kilograms. Airline desks hedge between systems: 23 kg checked bags get sold to US travellers as 50 lb bags, though they really allow 50.7. Grocery shelves in Ohio price by pound with gram figures alongside, and their Olympic barbell plates are cast in kilograms.
- Type your mass into Kilograms (kg); 70 is preloaded, so results appear before you touch anything.
- Read Pounds (lb) directly beneath. It recomputes on every keystroke — no button, no submit.
- Going backwards? Divide any pound figure by 2.20462262185, or multiply it by 0.45359237, to land in kilograms.
- Want British stones? Divide Pounds (lb) by 14; multiply whatever remains by 16 for ounces.
- Entry steps in thousandths, so one gram — 0.001 kg — is as fine as this field will accept.
Worked example — 70 kg at the weigh-in
Clinics in Lyon log patients at 70 kg. Type 70 into Kilograms (kg) and Pounds (lb) answers 154.323583529 lb. US charts asking for whole pounds take 154; those asking for one decimal take 154.3.
Now watch shortcuts fail. Multiplying by 2.2 gives exactly 154 lb — short by 0.32 lb, near five and one quarter ounces, plenty to shove fighters across weight-class ceilings or skew doses calculated per kilogram. Read that same result British-style and 154.323583529 lb becomes eleven stone plus 0.32 lb.
Questions
Is 2.20462262185 exact, or rounded?
Rounded, though only at an eleventh decimal. Exactness sits on the far side of this pair: one pound is defined as precisely 0.45359237 kg, with no uncertainty whatever. Its reciprocal is rational, 100000000⁄45359237, whose decimal form runs 2.2046226218487757… without ever settling, so any written version must stop somewhere. Stopping here costs roughly five parts in ten trillion — about twenty micrograms across forty tonnes of freight.
Why do pounds depend on kilograms and not the reverse?
Because 1959 made pounds derived quantities: fixed fractions of one kilogram. Kilograms, meanwhile, now answer to physics. On 20 May 2019 the SI redefined them through Planck's constant, fixed at 6.62607015 × 10⁻³⁴ joule seconds, retiring Le Grand K — that platinum-iridium cylinder kept near Paris since 1889, which had drifted tens of micrograms against its own official copies. Any pound weighed on US postal scales therefore traces back, via 0.45359237, to quantum mechanics.
Does an American pound differ from an imperial one?
Not since 1959, no. Beforehand an American pound worked out near 0.4535924277 kg under the 1893 Mendenhall Order, while Britain's imperial pound sat slightly lower — some tenth of one milligram per pound, invisible at butchers' counters and maddening inside standards labs. Both now share 0.45359237 kg. Imperial and US customary still diverge on volume, where pints, gallons and fluid ounces genuinely differ, but mass units match exactly.
Do pounds measure mass or weight?
Mass — here and in every legal definition. Confusion springs from pound-force, another unit entirely, equal to standard gravity (9.80665 m/s²) pulling on one pound of mass. Someone massing 70 kg still masses 70 kg on the Moon, and still converts to 154.32 lb on this sheet, even though spring scales up there would show about one sixth as much. Bathroom scales sense force and are calibrated to display mass.
How wrong is multiplying by 2.2?
Short by 0.21 percent, which sounds harmless until quantities grow. At 70 kg it costs 0.32 lb, roughly five ounces; at 500 kg of freight it hides more than one pound. Baggage rules show that trap from another angle: airlines label 23 kg allowances as 50 lb, yet 23 kg is 50.706 lb, so anybody rounding at 2.2 talks themselves out of luggage they had already paid for.