Convert st to g

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.

Grams are the unit of recipes and postage outside the United States, and the unit nutrition labels use nearly everywhere.

The factor is 6350.29318, and almost nobody carries that around. Rounded to 6400 it is off by 0.78 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 st.

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.

A stone is exactly 14 pounds, and the pound has been fixed in metric terms since the international yard and pound agreement of 1959. In this page's units that is 453.59237 g to the pound and 6350.29318 g to the stone — stipulated figures rather than measured ones, which is why every digit of them stops.

The stone never received a metric definition of its own; it inherits the pound's, 14 times over. That is also where you meet it — Britain and Ireland quote body weight this way and almost nowhere else does, so a figure in stone has usually crossed a border to reach you.

Multiplying 11 stone by 6,350.29318 gives 69,853.2 g, and every digit of that is arithmetically correct. What none of them is is measured. A weight in stone comes from a bathroom scale reading to the nearest pound at best, and more often from a person quoting the last number they remember — so the honest content of 69,853.2 is something like "about seventy kilograms".

The conversion cannot add information and it can hide the absence of it. Once 69,853 g is sitting in a database field, nothing about it says it began as a rounded figure, and anything computing from it will treat it as exact. That is the case for rounding on the way in rather than on the way out: enter 69,900 or 69.9 kg, and the number stays honest about itself.

British weights arrive as two numbers, and the reliable route is to collapse them into one before converting. 11 stone 4 lb is 158 lb, and 158 × 453.59237 is 71,667.6 g. Doing it that way needs one multiplication and one addition rather than two multiplications and an addition, which is a small saving with a real effect on how often it comes out wrong.

The mistake the compound form invites is treating the second number as a decimal. "Eleven four" is not 11.4 stone — it is 11 + 4/14, which is 11.286 — and the difference between those two readings is about 730 g. Any weight where the pounds figure exceeds nine is safe from the confusion by inspection; anything below that has to be read rather than assumed.

A gram is 0.00016 stone, which puts it several orders of magnitude below anything the unit is used for. The meaningful steps on a British scale are the pound at 453.59 g and the half stone at 3,175 g, and everything finer is below the noise of an ordinary day — a glass of water is 250 g, a meal and its drink comfortably more than a pound.

That sets a sensible resolution for anything derived from the figure. Reporting a change of 200 g in a body weight is reporting a measurement narrower than the thing being measured varies by, and a chart drawn at that scale shows mostly hydration. Where the underlying number came from a stone figure in the first place, half a pound — about 227 g — is the finest step worth plotting.

Stone figures are conversational in a way kilogram figures are not. People know their weight in stone the way they know their height in feet: to the nearest familiar step, updated occasionally, and rounded downward slightly more often than upward. A self-reported "about twelve stone" is a band rather than a point, and it converts to a band.

When the number is going somewhere that matters — a dosage, a load rating, a fitness calculation — the useful question is whether it was read off a scale that morning or recalled. If it was recalled, the conversion should be to a range: twelve stone give or take half a stone is 76,203 g with about 3,175 g either side, and stating it that way is more useful than any single figure would be.

Weight bands are common in the places a converted figure ends up — equipment ratings, clothing sizes, sports categories, guidance tables — and they are usually published in stone in British sources. An eleven-to-twelve stone band is 69,853 to 76,203 g, and both ends need converting rather than the midpoint.

Rounding a band inward is the error to avoid. Taking 69,853 up to 70,000 and 76,203 down to 76,000 narrows the range by 350 g at the two points where a range matters most: its edges. Round a lower bound down and an upper bound up, or leave both alone, and the converted band will still contain everything the original one did.

No conversation, no medical form and no scale display uses grams for an adult. The unit appears here because software chose it: an integer count of grams is a clean way to store a mass, and the choice is made by developers rather than by users. That is a good reason to convert accurately and a bad reason to think in the unit.

So the useful shape of this conversion is: keep the stone-and-pounds figure as the human record, store the gram figure as the machine one, and convert in one direction only. A weight that gets converted to grams, rounded, converted back to stone, rounded again and stored will drift by a pound or so per round trip, and the drift will be invisible because both numbers always look plausible.