Conversion from Grains to Hundredweights

Information about the Grain (gr)

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.

Information about the Hundredweight (cwt)

The hundredweight is a unit of mass whose value depends on which side of the Atlantic you are standing. The imperial or long hundredweight is 112 pounds, about 50.802 kilograms. The US or short hundredweight is 100 pounds, about 45.359 kilograms. Its symbol is cwt, from the Latin centum and the English weight.

The name promises a hundred and the imperial version delivers a hundred and twelve, which looks like an error until the arithmetic behind it is seen. Twenty hundredweight make a long ton of 2240 pounds, and a hundredweight is eight stone of fourteen pounds each. The chain of halvings and doublings that runs from the pound to the ton only closes neatly if the intermediate unit is 112, so the number is a consequence of the system rather than a mistake in it.

Its working life was in bulk trade. Coal, cement, grain, potatoes and fertiliser moved by the hundredweight, and sack sizes were built around it: the standard British coal sack held one hundredweight, which is why a coal merchant's cart was rated in a number of hundredweight rather than in tons. Vans and light trucks were once classified the same way, and the phrase survives in some vehicle names.

In the United States the short hundredweight is still used in agricultural markets, where livestock, grain and milk prices are quoted per hundredweight rather than per pound or per ton. Elsewhere the unit has largely given way to the tonne and the kilogram, and its main modern role is to make old records intelligible.

Continental Europe kept a cousin of it under a different name. The quintal, from the Arabic qintar by way of medieval Latin, served the same purpose in French, Spanish, Italian and Portuguese trade, and when the metric system arrived it was simply redefined as a round 100 kilograms. That metric quintal is still the unit in which grain harvests are reported across much of southern Europe, Latin America and India, and it sits close enough to the imperial hundredweight of 50.8 kilograms that old and new figures can look deceptively similar. The parallel is instructive: both units survived because a sack a strong worker could handle is a more natural quantity than any round metric mass.

One imperial hundredweight equals 112 pounds, about 50.802 kilograms, or 8 stone.