Pounds to Stones

A stone is exactly 14 pounds — still the standard for body weight in the UK and Ireland.

Nobody has ever said "eleven point four three stone"

This converter divides by 14 and gives you a decimal, which is arithmetically perfect and socially impossible. The stone survives in British usage precisely because it is spoken as a pair — eleven stone six — and the pair is what a scale displays, what a doctor writes down, and what anybody says out loud:

The decimal is not merely unidiomatic, it is actively misleading, because the fractional part counts in fourteenths and looks like tenths:

Every 0.0714 of a stone is one more pound. Someone reading 11.5 stone as "and a half, so five or six pounds" is right by accident — half a stone is 7 lb. Reading 0.0714 as "not quite a pound" is simply wrong: it is exactly one pound, and nothing about the decimal point signals that.

And writing the pair has a trap in it

Converting to the pair is two steps — take the whole stones, then round what is left into pounds — and the obvious version has a bug that stays invisible until it appears in front of somebody:

13.5 lb floors to zero stone with 13.5 pounds left over, which rounds to 14. The display reads "0 st 14 lb", a quantity that does not exist. Sweeping every tenth of a pound from 1 to 400 finds 140 weights that print it.

The fix is two lines: after rounding, check whether the remainder has reached the base and carry. Any mixed display has the same hazard, and it always arrives through the rounding rather than through the division:

It is the same bug as a height showing "5 ft 12 in" — which is a real one this site had, and fixed, on its height converter.

A smaller thing, worth knowing

One fourteenth is exact as a fraction and not as a binary number, so the decimal route does not always come home. Of the pounds from 1 to 500, 48 fail to return to themselves after a trip through stones and back — 29, 31, 58, 59, 62 among them.

Fourteen pounds is exactly one stone, so the round numbers are fine, and this page rounds its display to six decimals which hides the rest. It is not a defect to fix — it is what dividing by 14 in binary costs, and every tool that does it pays the same.

It is, though, one more argument for the pair. Stones and pounds held as two whole numbers never drift at all: every weight from 0 to 400 lb converts to a pair and back to exactly the pound it started as. Integers do not have this problem, which is why weights, currencies and durations are all better stored in their smallest unit and formatted at the last moment.