Tonne
Non-SI unit accepted for use · quantity: mass
01 · Definition
The tonne is older than the metric system: in France the word tonne meant a large barrel, and the measure came from the load a cart could carry. In 1795 the number was fixed at a thousand kilograms, and the barrel remained only in the name.
One tonne is exactly one thousand kilograms. Formally it is simply a megagram, and by the SI one ought to write Mg, but in two hundred years nobody has begun to: the tonne took root so firmly that it was left as an accepted unit with a symbol of its own. The measure is tied to water: a cubic metre at four degrees weighs exactly a tonne, and the whole of logistics rests on that — load capacity, displacement, volumes of extraction. A separate difficulty lies in the English-speaking world, where two more tons live under almost the same name.
In a contract with an American supplier «ton» without qualification means the short ton — 907 kg, nine per cent less than the metric one. The British long ton, on the contrary, is one and a half per cent heavier. The metric tonne is written t or MT to avoid confusion.
02 · Conversion
Enter a mass — the sheet will convert it
The tonne admits prefixes: the kilotonne and the megatonne apply to explosions and emissions, the gigatonne to the global carbon balance. Note that a «megatonne» formally means 10⁹ kg, that is a gigagram: the prefix hangs on the tonne, not on the gram.
The tonne is a measure of mass, not of force. A load of one tonne presses on its support with a force of 9.81 kN; the «tonne-force» used in engineering is exactly that number, but it belongs to a different quantity and has no place in the SI.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| t | tonne, 1000 kilograms | 1 | the main notation |
| kg | kilogram | 1000 | the SI base unit |
| q | quintal, 100 kg | 10 | agriculture |
| kt | kilotonne, 10⁶ kg | 0.001 | explosions, emissions |
| Mt | megatonne, 10⁹ kg | 1·10⁻⁶ | extraction, nuclear equivalent |
| Gt | gigatonne, 10¹² kg | 1·10⁻⁹ | carbon balance |
| short | American short ton, 907.18474 kg | 1.1 | USA, 2000 pounds |
| long | long ton, 1016.047 kg | 0.9842 | Britain, 2240 pounds |
| lb | pound | 2205 | English-speaking countries |
| pud | pood, 16.38 kg | 61.05 | Russia until 1918 |
| Mg | megagram — the same tonne | 1 | notation by the logic of the SI |
| t | tonne | 1 | the modern notation |
The American short ton equals 907.18474 kilograms, the British long ton 1016.0469088. In a contract without qualification «ton» means the short one: it is nine per cent lighter than the metric tonne, and on a consignment of a thousand tonnes the difference comes to ninety-three tonnes.
03 · Orders of magnitude
from a motorcycle to the atmosphere of the EarthA car — 1.5 t
04 · Measuring instruments
What tonnes are weighed with
A platform on strain gauges: up to eighty tonnes with an accuracy of twenty kilograms. Four sensors under the corners, the sum of the readings gives the mass of the lorry with its load.
A dynamometer in the sling: it shows the tension force and divides by nine point eighty-one to give tonnes. The crane operator sees mass, the instrument measures force.
Cargo is reckoned by the depth of immersion: a hull that has settled by a centimetre displaces a known volume of water. Draught marks on the side are a scale in tonnes, drawn once at the yard.
They integrate the load on the belt against the speed of travel: tonnes per hour on the way to the crusher. The accuracy is a per cent, but the counting never stops.
05 · Writing rules
t lower-case — capital T is the tesla
The symbol is written with a lower-case t, and prefixes are allowed on it: kt, Mt, Gt. In English documents the metric tonne is written t or MT, the American one short ton. In Russian «т» is right and «тн» is not. The tonne-force tf belongs to force and is not used in the SI.
In sea transport one meets the freight ton — not a mass but a volume of 1.133 cubic metres. Cargo is charged by whichever is greater, mass or volume: light bulky goods are paid for by space.
06 · Neighbouring units
The tonne is a multiple notation of the kilogram, so the whole family of mass stands beside it. Through density it is linked to the cubic metre, through the acceleration of free fall to the kilonewton. The tonne of TNT equivalent measures not mass but energy: 4.184 gigajoules.
The atmosphere of the Earth weighs 5.15·10¹⁵ tonnes, the whole biomass of the planet about 5.5·10¹¹ tonnes, yearly CO₂ emissions 37 gigatonnes.
07 · Historical section
The tons that are not a thousand kilograms
Two thousand pounds. In American contracts «ton» without qualification means exactly this one, and on large consignments the difference from the metric tonne runs to tens of tonnes.
2240 pounds — twenty hundredweight of 112 each. It holds on in shipbuilding: the displacement of old ships is given in them.
Forty pounds, about sixteen kilograms. Harvests and cargoes were counted in poods, and the word survives in sayings: to eat a pood of salt with someone.
Not a mass but a volume: a hundred cubic feet of the interior spaces of a vessel. Port dues are charged on it, so the register ton is met on every ship’s papers.
The only unit that was kept solely because people could not be weaned off it
By the logic of the SI the tonne is unnecessary: a thousand kilograms is a megagram, and the symbol Mg exists. But in two hundred years no port, no railway and no steelworks moved to megagrams, and the International Committee for Weights and Measures gave in: the tonne was put on the list of units accepted for use with the SI, as it was fixed in France in 1795.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with their own names, and non-system units alongside them. Each has its own sheet: definition, dimension, conversion, instruments, rules of writing. In 23 languages.