| Reference | Grains (gr) | Tonnes (t) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 77.1618 gr | 0.000005 t |
| One litre of water | 15432.4 gr | 0.001 t |
| Average adult human | 1080265 gr | 0.07 t |
| A small car | 21605302 gr | 1.4 t |
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.
The tonne is a unit of mass equal to 1000 kilograms. Its symbol is t. It is not an SI unit but is accepted for use with the SI, and in most of the world it is simply the working unit for anything too heavy to state comfortably in kilograms.
The name causes trouble in English. A tonne, sometimes written metric ton, is 1000 kilograms. A short ton, the American ton, is 2000 pounds or about 907 kilograms. A long ton, the British imperial ton, is 2240 pounds or about 1016 kilograms. The three differ by up to twelve per cent, which is large enough to matter in shipping and commodity contracts, so international documents normally spell out which is meant rather than relying on the word alone.
Freight and industry are its home. Ship cargo, rail wagons, steel production, cement, grain harvests and vehicle gross weights are all quoted in tonnes. Bridge and crane load ratings use it, and so does the deadweight tonnage of a ship, although that particular term measures carrying capacity rather than the mass of the vessel itself.
Climate accounting has given the tonne a second public life. National and corporate emissions are reported in tonnes of carbon dioxide equivalent, and carbon markets price allowances per tonne. A single average petrol car emits roughly two tonnes of carbon dioxide a year, which is why the unit appears in policy documents far outside engineering.
Energy accounting borrows the word for a quantity that is not a mass at all. The tonne of oil equivalent is defined as the energy released by burning one tonne of crude oil, about 41.87 gigajoules, and national energy balances are published in millions of them. A tonne of coal equivalent serves the same purpose at a different value. These are conventions rather than measurements: no particular tonne of oil delivers exactly that figure, but fixing one lets gas, coal, nuclear output and renewables be added together on a single scale. Readers of energy statistics therefore meet the tonne twice, once as a mass of fuel and once as a package of energy.
One tonne equals 1000 kilograms, about 2204.62 pounds, or approximately 1.10231 short tons.