LBS to kg

Type a weight in pounds and read the kilograms. The multiplier is 0.45359237 and it is a definition rather than a measurement, so the arithmetic is exact and only the display rounds. Below the converter there is a complete 1 to 100 pound table, a second table for the ounces that make up the part of a pound that is not a decimal, and the four places this direction actually goes wrong: ounces, baggage limits, gym plates, and the moment floating point starts printing 30.999999999999996.

Where 0.45359237 comes from, and how to do it in your head

Kilograms = pounds × 0.45359237. That constant is not rounded and not regional. It was fixed by the International Yard and Pound Agreement of 1959, the same agreement that made the inch exactly 2.54 cm, and it applies in every country that signed.

It is also not arbitrary. Underneath both the pound and the troy system sits a single shared atom: the grain, defined as exactly 64.79891 milligrams. An avoirdupois pound is 7000 grains, and 7000 × 64.79891 mg is 453.59237 g on the nose. Every other number on this page falls out of that one: an ounce is 437.5 grains, so 28.349523125 g; a troy ounce is 480 grains, so 31.1034768 g. You can check all of them with a calculator, which is the point.

For mental arithmetic, halve the pounds and then take about nine per cent off. 160 lb becomes 80, minus nine per cent is 72.8, against a true 72.57 — that shortcut runs 0.31 per cent high. If you want something faster and are willing to be about three quarters of a per cent low, multiply by 0.45 instead: 160 × 0.45 = 72.0.

Three anchors worth keeping. One pound is 454 g, which is why a US pack of butter is labelled 454 g outside the United States. Ten pounds is 4.536 kg. And 220 lb is 99.79 kg, which is close enough to 100 that people quote them as if they were the same number — they are 0.2 kg apart.

Ounces are sixteenths, not decimals — the mistake that shows up on birth records

A weight written as 7 lb 6 oz is not 7.6 pounds. There are sixteen ounces in a pound, so the 6 means six sixteenths, which is 0.375 lb. Reading it as a decimal makes a newborn 102 g heavier than they are, and that is the single most common failure in this direction of the conversion.

Worked through: 7 lb is 3175.15 g, 6 oz is 170.10 g, and the baby weighs 3345 g. American medical records give birth weight in pounds and ounces; almost every other country records grams, and the growth charts a parent is handed are in grams or kilograms. Converting the ounces separately, then adding, is the only way to move between the two without introducing an error bigger than the rounding.

The ounces-to-grams table below stops at 15 for exactly this reason: sixteen ounces is another pound, so 15 is the largest ounce figure that can appear in a correctly written weight. If you see 7 lb 18 oz, the weight was written wrong, not measured wrong.

One more ounce to keep separate. Recipes written in the United States use ounces for dry ingredients and fluid ounces for liquids, and the two are not related — a fluid ounce is a volume. Eight ounces of flour is a mass this page can convert; eight fluid ounces of milk is a volume it cannot, because that answer depends on what the liquid is.

Why the troy pound is lighter although the troy ounce is heavier

This sounds like a contradiction and it is not, because the two systems disagree about how many ounces make a pound. Both are built from the same grain. A troy ounce is 480 grains and an avoirdupois ounce is 437.5, so the troy ounce is the heavier of the two by about ten per cent. But a troy pound is only twelve ounces, or 5760 grains, while an avoirdupois pound is sixteen ounces, or 7000. Twelve heavier ounces still come to less than sixteen lighter ones.

The numbers: a troy pound is 373.2417216 g and an ordinary pound is 453.59237 g, so a troy pound is 82.3 per cent of an ordinary one. Anyone reading a historical assay record, a coin specification or an old apothecary text is looking at troy pounds, and treating them as ordinary pounds overstates the mass by more than a fifth.

The unit list on this page includes the troy ounce, because bullion is quoted in it every day and getting it wrong is a ten per cent error on a price. It does not include the troy pound, which has no current commercial use. We would rather say that than let you assume it is in there: if you need it, multiply troy pounds by 373.2417216 to get grams, and note that this is one of the few places where reaching for a converter without checking which pound you have is genuinely expensive.

Baggage and parcels: convert first, then round the way the carrier rounds

A 50 lb checked-bag limit is 22.6796 kg. Going the other way, the 23 kg allowance used on most international routes is 50.71 lb — which means a bag packed exactly to 23 kg is over a 50 lb limit, by about eleven ounces. That is not a rounding artefact. It is the reason a bag that was fine on the long-haul leg gets pulled at the desk on a domestic connection.

The general trap is that limits and prices are not symmetric operations. A limit is a hard ceiling, so you round your own weight up before comparing. A price band is charged by rounding the billable weight up into the next band, never to the nearest one, so a parcel at 2.05 kg costs the same as one at 2.5 kg on a half-kilogram scale.

Half-kilogram and one-pound bands never line up, which is why converting a rate card is more annoying than it looks. One pound is 0.4536 kg and sits inside the first half-kilo band; two pounds is 0.9072 kg and lands in the second; three pounds is 1.3608 kg and lands in the third. There is no repeating pattern to memorise, because 0.45359237 and 0.5 have no common multiple worth writing down. Convert each band edge separately and round in the direction that costs you money, not the direction that looks tidy.

Gym plates: a 45 lb bar and a 20 kg bar are different bars

An Olympic barbell is 20 kg, which is 44.09 lb. The bar sold as a 45 lb bar is 20.41 kg. They look interchangeable on a rack and they are not, and neither are the plates: 45 lb is 20.41 kg against a 20 kg plate, and 25 lb is 11.34 kg against a 10 kg plate.

This matters when a training programme written in pounds meets equipment loaded in kilograms. The three numbers that appear in almost every American programme are 135, 225 and 315 lb, because they are a 45 lb bar plus one, two and three pairs of 45 lb plates. In kilograms those are 61.23, 102.06 and 142.88 — and none of them can be built from metric plates, which come in 20, 15, 10, 5, 2.5 and 1.25 kg. The closest you can load to 135 lb is 60 kg or 62.5 kg.

The honest advice is to pick the nearest loadable weight and stop trying to hit the converted number exactly. The two per cent between 61.23 and 60 kg is smaller than the difference a night of bad sleep makes, and chasing it with fractional plates is solving a problem that the conversion created rather than one the training has.

Floating point, and how many digits are honest

Multiply 10 by 0.45359237 in any language that uses ordinary double-precision arithmetic and you get 4.535923700000001, not 4.5359237. The constant has no exact binary representation, so the product carries a tiny error. Convert 31 lb to kilograms and back without rounding and you get 30.999999999999996 — 31 is the first whole number of pounds where the round trip fails, and 82 of the first thousand do the same thing.

The fix is to round for display to a fixed number of significant figures rather than a fixed number of decimals. This page uses six by default, which is enough that a value converted out and back returns exactly what you started with. Switch to a fixed number of decimal places and you get a friendlier number but lose that property, and the page says so at the moment you change the setting rather than letting you discover it later.

How many digits are actually meaningful depends entirely on what you weighed. A domestic bathroom scale resolves to about 100 g and your own weight moves by more than a kilogram across a normal day, so one decimal is already more than the instrument earns. A newborn is the opposite case: one ounce is 28 g, so grams are the right resolution and rounding to the nearest tenth of a kilogram throws away information a paediatrician uses. For shipping, the carrier band is larger than anything you are worrying about.

We also do not insert thousands separators into the result. A number you are about to paste into a spreadsheet should not contain commas.