| Gallons (gal) | Cubic inches (in³) |
|---|---|
| 1 Gallon | 277.419432792 in³ |
| 2 Gallons | 554.838865583 in³ |
| 3 Gallons | 832.258298375 in³ |
| 4 Gallons | 1109.67773117 in³ |
| 5 Gallons | 1387.09716396 in³ |
| 10 Gallons | 2774.19432792 in³ |
| 20 Gallons | 5548.38865583 in³ |
| 25 Gallons | 6935.48581979 in³ |
| 50 Gallons | 13870.9716396 in³ |
| 100 Gallons | 27741.9432792 in³ |
| Reference | Gallons (gal) | Cubic inches (in³) |
|---|---|---|
| A teaspoon | 0.00109985 gal | 0.305119 in³ |
| A can of soft drink | 0.0725899 gal | 20.1378 in³ |
| A wine bottle | 0.164977 gal | 45.7678 in³ |
| A bathtub | 32.9954 gal | 9153.56 in³ |
| An Olympic swimming pool | 549923 gal | 152559360 in³ |
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 inch is a unit of volume equal to 16.387064 cubic centimetres exactly. Its symbol is cu in, sometimes written in³. It is the volume of a cube one inch on each side, and 1728 of them make a cubic foot. Its most consequential role is legal rather than descriptive: the United States gallon is defined as exactly 231 cubic inches.
That definition is the reason American liquid measure is the shape it is. The figure of 231 comes from the English wine gallon standardised under Queen Anne in 1707, which the United States kept when Britain replaced its own gallons with a single imperial gallon in 1824. Every American fluid ounce, pint and quart descends from that one number, and it is why the two gallons differ by a fifth.
Engines made the unit famous. American car engines were quoted in cubic inches until the 1980s, and the numbers became names in their own right: a 350 is 5.7 litres, a 426 is 7.0, and a 302 is 4.9. Motorcycles still carry both figures, and the conversion is worth knowing, since 61 cubic inches make a litre.
Machinery uses it for swept volume. Pump and compressor displacement per revolution, hydraulic cylinder volume and metering pump strokes are all specified in cubic inches in American engineering, because the bores and strokes that produce them are already in inches and squaring or cubing an inch keeps the calculation in one system.
Freight pricing uses it in an unexpected way. American parcel carriers calculate a dimensional weight by dividing the package volume in cubic inches by a fixed divisor, commonly 139, to obtain a chargeable weight in pounds. A light bulky parcel is therefore priced by its cubic inches rather than by what it weighs, which changes how goods are packed.
The conversion to metric is exact but unhelpful. Because the inch is defined as exactly 2.54 centimetres, a cubic inch is exactly 16.387064 cubic centimetres, and no rounding is involved. That precision does not make the number easier to use, and it is the main reason engineering drawings outside the United States moved to millimetres.
One cubic inch equals 16.387 cubic centimetres, about 0.0164 litres, about 0.554 US fluid ounces, or 1/1728 of a cubic foot.