| Reference | Stones (st) | Grains (gr) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 0.000787365 st | 77.1618 gr |
| One litre of water | 0.157473 st | 15432.4 gr |
| Average adult human | 11.0231 st | 1080265 gr |
| A small car | 220.462 st | 21605302 gr |
The stone is a unit of mass equal to 14 pounds, or about 6.35029 kilograms. Its symbol is st. It is a British unit with an unusual property: it is almost entirely confined to one purpose, human body weight, and almost entirely confined to Britain and Ireland.
Before standardisation the stone was not a fixed quantity at all. It varied by commodity and by region, from eight pounds for meat to twenty-four for certain wools, and a stone of one good was simply not a stone of another. The Weights and Measures Act of 1835 fixed it at fourteen pounds for all purposes, ending a genuine source of commercial dispute.
Its survival is a study in how measurement habits actually change. Britain converted trade to metric units over decades, and shops now weigh in kilograms, but personal weight remained stubbornly in stones and pounds. A person who buys potatoes by the kilogram will still say they weigh eleven stone four. Medical practice sits awkwardly between the two, with charts in kilograms and patients who think in stones, so most clinical software displays both.
The convention is to give a whole number of stones followed by the remaining pounds, never a decimal, so twelve stone six means 174 pounds. This makes mental conversion to metric awkward and is one reason the unit has no traction outside its home. Ireland uses it similarly, and Australia and New Zealand abandoned it in the 1970s.
Sport keeps the unit visible where medicine has quietly dropped it. British boxing reports fighters at, say, twelve stone seven, horse racing states the weight a jockey must carry in stones and pounds, and rowing crews are selected against a stone-based limit. Clinical practice has moved the other way: hospital notes, drug dosing and growth charts are all metric, because a dose calculated per kilogram cannot safely pass through a conversion. The result is that many British patients hear their weight in kilograms from a nurse and repeat it in stones to a friend an hour later, translating in their heads at roughly six and a third kilograms to the stone.
One stone equals 14 pounds, about 6.35029 kilograms, or 224 ounces.
The grain is a unit of mass equal to exactly 64.79891 milligrams. Its symbol is gr. It is the smallest unit in the imperial and US customary systems, and the only one shared identically by the avoirdupois, troy and apothecaries systems, which makes it the hinge on which conversions between them turn.
Its origin is agricultural and literal: a grain of barley, or in some traditions wheat, taken as a natural standard because seeds from a single crop are remarkably uniform. The English statute of 1266 defined the penny as the weight of thirty-two wheat grains taken from the middle of the ear. That definition survived in principle until the nineteenth century, when the grain was fixed by law against the pound rather than against any seed. Seven thousand grains make an avoirdupois pound, and 5760 make a troy pound.
Firearms are where the unit is most alive today. Bullet mass is quoted in grains almost universally, so a 55-grain rifle bullet or a 230-grain pistol bullet is standard language among shooters worldwide, including in metric countries. Propellant charges are weighed in grains as well, and reloading scales are calibrated in them.
Pharmacy used the grain for centuries, and the five-grain aspirin tablet is the ancestor of today's 325-milligram tablet, a figure that looks arbitrary in metric units and obvious in the old ones. Dentistry, arrow and archery components, and the pearl and gemstone trades all retain it in places.
Its most remarkable property is that it is the same in every English weight system. Troy, avoirdupois and apothecaries' weights each divide differently above the grain — a troy pound is 5760 grains and an avoirdupois pound 7000 — but all three rest on the identical grain of 64.79891 milligrams. That made the grain the common currency by which a goldsmith's weights and a grocer's weights could be compared, and it is the reason the unit survived standardisation while its larger relatives were pruned away. Handloaders exploit the same precision today: powder charges are weighed to a tenth of a grain, about six milligrams, because a small error in charge produces a large change in pressure.
One grain equals 64.79891 milligrams exactly, one seven-thousandth of an avoirdupois pound, or about 0.002286 ounces.