Convert lb to kg

A pound is exactly 0.45359237 kilograms by the same 1959 agreement that fixed the inch. The abbreviation "lb" comes from libra, the Roman unit it descends from.

The kilogram was the last unit defined by a physical object — a cylinder in a vault near Paris that was slowly losing mass. Since 2019 it has been fixed to the Planck constant instead.

The factor is 0.453592, and almost nobody carries that around. Rounded to 0.45 it is off by 0.79 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 lb.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The pound has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 0.45359237 kg to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is the whole of it, with nothing rounded away at the end.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

The number nearly everybody carries is 2.2, and dividing by it gives an answer about 0.2 % high — 200 lb becomes 90.91 kg against a true 90.72. On a suitcase that is 190 grams and nobody cares. On a figure that will be multiplied by something else it is the first of two errors, and the second one rarely cancels it.

The multiplication is the safer habit in this direction. Pounds times 0.45359237 needs no division and no reciprocal, and if you want a short version, 0.4536 is right to a part in ten thousand. The field above carries the full factor and rounds only when it prints, which is why its answer and a hand-worked one can differ in the last place shown without either being wrong.

Medicine is written per kilogram almost everywhere, including in the United States: a dose is stated as so many milligrams per kilogram of body weight and then multiplied out. That makes the pound-to-kilogram step the input to an arithmetic chain rather than a display value, and a 10 % error at the top is a 10 % error at the bottom.

This is where halving the pounds does damage. A 40 lb child halved is 20 kg; the true figure is 18.14, and the dose worked from 20 is a tenth too large before anything else has been decided. Clinics weigh in kilograms for exactly this reason, and a weight you convert yourself should be converted with the full factor and handed over as the kilogram figure rather than as a pound figure somebody else will convert again.

Airline allowances are the commonest place the two units are printed side by side, and they are not equal. The standard economy checked limit of 23 kg is 50.71 lb; the heavier tier of 32 kg is 70.55 lb. The pound figures on the ticket are rounded down, so a bag weighed in pounds at an airport that enforces kilograms has a little under a pound of headroom that the printed number does not show.

The direction of the rounding is what matters at the desk. Packing to the pound figure keeps you inside the kilogram limit; packing to the kilogram figure and then weighing in pounds at home will show a number above the one printed on your ticket, and the bag is still legal. Where the excess-weight fee is charged in bands rather than continuously, the gap between 22.7 and 23 kg is the whole of the margin.

A competition barbell is 20 kg, which is 44.09 lb, and it is sold in the United States as a 45 lb bar. American plates are stamped 45, 35, 25, 10, 5 and 2.5 lb; metric plates are 25, 20, 15, 10, 5 and 2.5 kg. A 45 lb plate is 20.41 kg and a 20 kg plate is 44.09 lb, so the two are close enough to be confused and far enough apart to accumulate.

Loaded up, the gap is real. A bar with four 45 lb plates a side reads 405 lb, which is 183.7 kg; the nearest metric loading of four 20 kg plates a side on a 20 kg bar is 180 kg, or 396.8 lb — eight pounds lighter, at a level where eight pounds is a session-to-session decision. Converting the total after loading is the only version that holds, because the plate labels have already rounded.

Immigration paperwork, driving licences outside the United States, insurance applications and hospital admissions all take a single kilogram figure, and most of them take an integer. That is a coarser unit than the pound the number started in: rounding 168 lb to 76 kg has moved it by half a pound, which nothing downstream will notice, while rounding 168 lb to 80 kg has moved it by eight.

Enter the converted figure, not the pound figure with a kilogram label on it. The failure mode is transcription rather than arithmetic — a field labelled kg receiving 165 because that is what the scale said — and it is invisible afterwards, because 165 kg is a weight a person can have. A converted figure that lands somewhere between 45 and 140 kg for an adult is at least self-checking.

The mental version worth knowing is halving and subtracting ten per cent, because both steps are easy and the result is predictable: 0.45 rather than 0.4536, which is 0.8 % low. 150 lb gives 75, minus 7.5, so 67.5 kg against a true 68.04. 220 lb gives 110, minus 11, so 99 kg against 99.79.

Knowing which way it fails is what makes it usable. Every answer from this shortcut is slightly under the truth, by about one part in a hundred and twenty, so it is a safe estimate for anything with an upper limit and an unsafe one for anything with a lower limit. Add a per cent back if the number is going anywhere it will be checked.

The pound is defined as 45359237 hundred-millionths of a kilogram, and 45359237 is odd and not divisible by five. That is enough to guarantee that no whole number of pounds smaller than a hundred million converts to a whole number of kilograms — the first one that does is 100,000,000 lb, which is exactly 45,359,237 kg.

The practical version of that is that every kilogram answer on this page is a rounded one, and the rounding is yours to choose rather than the conversion's. It also explains why the reference table above shows tidy pounds and untidy kilograms while a kilogram-first table does the opposite: the round numbers live on whichever side you started from, and they cannot live on both.

Round a converted figure and the round trip stops closing. 180 lb is 81.65 kg; entered as 82 and converted back it is 180.78 lb. 165 lb is 74.84 kg; rounded to 75 and returned it is 165.35 lb. The drift is under a pound each way and it is entirely the rounding, not the factor, which is exact in both directions and to as many places as anyone wants.

It matters when the same weight passes through more than one system. A figure recorded in pounds at home, converted to whole kilograms by a form, converted back to pounds by an app and rounded again has picked up two roundings and can differ from the original by more than a pound. Where a weight is going to be compared with itself later, record the unit it was measured in alongside the number, and convert from that record each time rather than from the last conversion.