Convert lb to st

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.

A stone is fourteen pounds, and it is used for body weight in Britain and Ireland and essentially nowhere else — which is why an American reading a British weight usually needs this conversion.

Going this way is a division, and by a whole number: 14 of these fit into one stone with nothing left over. The awkwardness is only that the answers come out as fractions — a third of a stone, a twelfth of one — rather than as the round figures the other direction gives you.

Nothing is lost to rounding even so. The division is exact, and if your answer will not sit still as a decimal — 0.0833… and friends — that is the fraction showing through, not an error creeping in.

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 0.0714285714 st 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 not the whole of it, though: this page stops at nine significant digits and the ratio carries on past them.

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.

Divide the pounds by fourteen and the whole number is the stone. What is left over is already in pounds, so there is nothing further to convert: 176 divided by 14 is twelve with 8 remaining, and the answer is twelve stone eight. Coming from kilograms the same job needs the decimal multiplied back up by fourteen, which is an extra step and an extra place to round.

That is worth knowing because it changes how to do it by hand. Rather than dividing, subtract: fourteens out of 176 — 140 leaves 36, another 14 leaves 22, another 14 leaves 8 — ten, eleven, twelve stone and 8 lb. Anyone raised with the unit does the multiplication table instead, and the table is short enough to learn.

The whole conversion for adult weights lives between about 100 and 300 lb, which is seven stone to twenty-one. The stone marks are 98, 112, 126, 140, 154, 168, 182, 196, 210, 224, 238, 252, 266, 280 and 294 pounds. Find the largest one at or below your figure, count how many fourteens it is, and subtract.

Two of them are worth memorising because they anchor the rest. 140 lb is ten stone exactly and 210 lb is fifteen stone exactly, so anything near those is a short step away: 145 lb is ten stone five, 205 lb is fourteen stone nine. Working from a known landmark rather than from zero also catches the error where the stone figure comes out one too high, which is what happens when the remainder is allowed to reach fourteen.

The two forms look alike written down and mean different weights. Twelve stone seven is 175 lb; 12.7 stone is 177.8 lb. Somebody typing a spoken weight into a decimal field, or reading a decimal field aloud, moves the number by 0.4 lb for every pound of the remainder, in either direction, so the error is largest just below the next whole stone — 3.6 lb at nine pounds over — and vanishes at a round stone.

The conversion between the two forms is the fourteen again: pounds part divided by fourteen gives the decimal, decimal times fourteen gives the pounds part. Seven pounds is 0.5 stone, three pounds is 0.214, eleven is 0.786. Nothing in the spoken form is decimal, which is why a form asking for "weight in stone" as a single number is asking a question the unit does not naturally answer.

British weight-loss culture runs on this unit, and it runs on it in a specific way: targets are half a stone, a stone, two stone, and progress is reported in pounds until it reaches the next seven. That is why a loss of 7 lb is a milestone and a loss of 6 is not, and why the same slimming group would find "3.2 kg" an unrecognisable way to describe the same achievement.

It has a practical consequence for anyone converting an American figure into that conversation. A starting weight of 196 lb and a current weight of 175 lb is a loss of 21 lb, which is a stone and a half — the number the group will want. Converting each weight to stone separately and subtracting gives fourteen stone and twelve stone seven, and the difference between them is the same stone and a half, arrived at more slowly.

It is a translation for a conversation rather than for a form. Almost nothing official in Britain takes stone: medical records, prescriptions and BMI calculations are metric, driving and insurance forms want kilograms. What wants stone is a person — a relative comparing figures, a forum, a slimming group, a sports club with divisions set before metrication.

That is the argument for converting to the spoken form and not to a decimal. A decimal stone figure satisfies nobody: it is not what a British listener expects and it is not what any metric system will accept. If the figure is going into a field, convert the pounds to kilograms instead; if it is going into a sentence, convert to stone and pounds and say the two numbers.

A stone weight is a stone number and a pounds number between zero and thirteen. Fourteen would be another stone, which sounds obvious and is the single most common arithmetic slip when converting by subtraction: taking 168 from 182 leaves 14, and the answer is thirteen stone, not twelve stone fourteen.

The check is quick. If the pounds part is 14 or more, take another fourteen out and add one to the stone. If it is negative, you have taken one too many. Between them those two rules catch every mistake this conversion can make, which is the advantage of a unit whose divisor is a whole number — nothing here is ever off by a fraction, only by a whole stone.

Written forms vary and all of them are read the same way aloud. "12 st 8 lb" is the full version and the one a form or a racecard uses. "12st 8" drops the second unit because the remainder can only be pounds. "12–8" appears in sports listings. In running text people write "twelve stone eight", and the unit stays singular however many there are.

What none of the written forms uses is a decimal point, which is why a spreadsheet column of stone weights is awkward to build. The usable options are two columns, or a single column in pounds with the stone worked out for display — storing "12.8" is the version that looks tidy and is arithmetically meaningless, since it is neither twelve stone eight nor 12.8 stone.

The reason the unit fits weight loss so comfortably is the size of it. Guidance in Britain puts a sensible rate at one to two pounds a week, so a stone takes between seven and fourteen weeks — a target far enough away to be meaningful and near enough to be visible. A kilogram-based target does not divide as neatly into that rhythm, which is part of why the unit has outlived every attempt to retire it.

For somebody converting an American figure into that framing, the arithmetic is worth doing in pounds throughout. Take the difference in pounds, divide by fourteen for the stone, and keep the remainder: 231 lb down to 196 lb is 35 lb, which is two stone seven. Converting each weight separately and subtracting the stone figures gives the same result and loses the pounds along the way, and the pounds are what the next week is measured in.