| Reference | Tonnes (t) | Stones (st) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 0.000005 t | 0.000787365 st |
| One litre of water | 0.001 t | 0.157473 st |
| Average adult human | 0.07 t | 11.0231 st |
| A small car | 1.4 t | 220.462 st |
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.
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.