| Gallons (gal) | Cubic meters (m³) |
|---|---|
| 1 Gallon | 0.00454609 m³ |
| 2 Gallons | 0.00909218 m³ |
| 3 Gallons | 0.01363827 m³ |
| 4 Gallons | 0.01818436 m³ |
| 5 Gallons | 0.02273045 m³ |
| 10 Gallons | 0.0454609 m³ |
| 20 Gallons | 0.0909218 m³ |
| 25 Gallons | 0.11365225 m³ |
| 50 Gallons | 0.2273045 m³ |
| 100 Gallons | 0.454609 m³ |
| Reference | Gallons (gal) | Cubic meters (m³) |
|---|---|---|
| A teaspoon | 0.00109985 gal | 0.000005 m³ |
| A can of soft drink | 0.0725899 gal | 0.00033 m³ |
| A wine bottle | 0.164977 gal | 0.00075 m³ |
| A bathtub | 32.9954 gal | 0.15 m³ |
| An Olympic swimming pool | 549923 gal | 2500 m³ |
The gallon in the imperial system is a unit of volume equal to exactly 4.54609 litres. Its symbol is gal. It contains four quarts, eight pints and 160 imperial fluid ounces, and it is about a fifth larger than the United States gallon, a difference that causes more confusion than any other single discrepancy between the two systems.
Its original definition was elegant. The Weights and Measures Act of 1824 fixed the imperial gallon as the volume occupied by ten pounds of distilled water at sixty-two degrees Fahrenheit under a barometric pressure of thirty inches of mercury. That tied liquid measure directly to the avoirdupois pound, so a gallon of water weighed ten pounds and a pint weighed a pound and a quarter. In 1985 the definition was replaced by the exact metric figure, but the old relationship still holds to within a fraction of a per cent.
Fuel economy keeps it in daily use. British and Irish drivers describe consumption in miles per gallon even though fuel has been sold in litres since the 1990s, and the figure is an imperial one. The same car scores about twenty per cent higher in British miles per gallon than in American, purely because the gallon is larger, which is why cross-Atlantic comparisons of efficiency are meaningless unless the gallon is specified.
Legal use in Britain is now narrow but real. Draught beer and cider must be sold in pints, half pints or third pints, and milk in returnable containers may be sold in pints, which keeps the whole imperial chain alive by keeping its smallest working member alive. Several Caribbean territories still sell motor fuel by the imperial gallon.
The difference from the American gallon is worth stating plainly. One imperial gallon is 4.546 litres and one United States gallon is 3.785, so an imperial gallon is 1.2 United States gallons. Recipes, fuel figures, tank capacities and chemical dilutions all break if the wrong one is assumed, and the error is large enough to matter but small enough to look plausible.
Below it the chain runs quart, pint, gill and fluid ounce, each a quarter or a fifth of the one above rather than a decimal step. That structure was designed for dividing a container by halving it repeatedly, which is a practical merit when pouring and a serious nuisance when calculating.
One imperial gallon equals 4.54609 litres, 8 imperial pints, 160 imperial fluid ounces, or about 1.201 US gallons.
The cubic metre is the SI unit of volume, written m³. It is the volume of a cube one metre on each side, and because the metre is a base unit, every other volume unit in the system is derived from it. It holds a thousand litres, and a cubic metre of fresh water has a mass of very nearly one tonne, which is the single most useful fact about it.
Utilities are billed in it. Domestic water meters almost everywhere record consumption in cubic metres, and a household of four uses somewhere between 100 and 200 of them a year. Natural gas is metered the same way and then converted to energy for billing, because the heat released by a cubic metre of gas depends on its composition and has to be corrected for temperature and pressure at the meter.
Construction orders materials by it. Ready-mixed concrete is sold and delivered by the cubic metre, a standard truck carrying six to eight; excavation and earthworks are priced per cubic metre moved; and sawn timber, aggregate, topsoil and bark are all traded the same way. Because a cubic metre of concrete weighs about 2.4 tonnes, the unit also decides what a lorry or a crane can carry.
Freight uses it in a more subtle way. Shipping and removals quote volume in cubic metres, and air freight charges by whichever is greater, the actual mass or a volumetric mass calculated at a fixed ratio, commonly 167 kilograms to the cubic metre. Light bulky cargo is therefore priced by the space it occupies rather than by what it weighs, and the whole calculation runs on this unit.
Environmental measurement lives here too. River flow is given in cubic metres per second, reservoir capacity in millions of cubic metres, air quality as micrograms of pollutant per cubic metre of air, and ventilation as cubic metres per hour. A room four metres by five with a ceiling at two and a half holds fifty cubic metres of air, which is the sort of figure ventilation standards are written against.
Against imperial and United States units it converts to 35.315 cubic feet or 1.308 cubic yards. The cubic yard matters most in practice, because American concrete and excavation are ordered in it, and the two units are close enough that a mistaken figure looks plausible while being a third out.
One cubic metre equals 1000 litres, about 35.315 cubic feet, about 1.308 cubic yards, or about 264.2 US gallons.