The tonne, unit of mass
The tonne is exactly 1000 kilograms. It is not an SI unit but one accepted for use alongside the SI: its compliant name is the megagram, Mg, which nobody writes.
The word comes from the tun, the wine cask, and it kept the cask's volume. A ship's capacity was counted in tuns stowed in the hold, and tonnage has never stopped being that measurement: since the convention of 1969, gross tonnage is a dimensionless figure drawn from the enclosed volume of the vessel, that volume multiplied by 0.2 plus 0.02 times its base-ten logarithm. A liner listed at 228,000 tons does not have that mass; the number measures space, and the mass sits on another line of the ship's papers under the name displacement. Then comes the trap in the bare word: the American short ton weighs 907.185 kg and the British long ton 1016.047, so a cargo of ten thousand tons moves by 1089 tonnes depending on which country wrote the bill of lading.
The tonne in other units
| 1 t | 1,000 kg |
| 1 t | 1,000,000 g |
| 1 t | 1,000,000,000 mg |
| 1 t | 2,205 lb |
| 1 t | 35,274 oz |
| 1 t | 32,151 oz t |
| 1 t | 157.473 st |
| 1 t | 5,000,000 ct |
| 1 t | 1.10231 ton US |
| 1 t | 0.984207 ton UK |
| 1 t | 1,000,000,000,000 µg |
| 1 t | 2,000 lb metric |
| 1 t | 85,735 tola |
| 1 t | 266,667 momme |
| 1 t | 266.667 kan |
Frequently asked questions
Why do trade documents write MT?
MT is the commercial shorthand for metric ton, and it contradicts the international system, where M is the prefix mega: read literally it would mean a million tonnes. The correct symbol is the lower-case t, the capital T being the tesla. Emission inventories, by contrast, do write kt and Mt for the thousand and the million tonnes.
What volume does a tonne of water take up?
A round cubic meter, and that is no coincidence: the kilogram was defined in 1795 as the mass of a liter of water, so a thousand liters weigh a tonne. The convenience stops at fresh water, since seawater runs about 1025 kilograms per cubic meter, diesel 840 and petrol 750, so a 50 cubic meter tanker carries only 37.5 tonnes of petrol.
What are the tonne of TNT and the tonne of oil equivalent?
Two units of energy that carry the name of a mass. The tonne of TNT is by convention 4.184 gigajoules, the gram of TNT having been fixed at a round thousand calories, and it is the scale of explosions and earthquakes. The tonne of oil equivalent is 41.868 gigajoules, ten times more, and serves national energy balances. Neither measures a mass.