| Cubic millimeters (mm³) | Gallons (gal) |
|---|---|
| 1 Cubic millimeter | 0.000000219969248299 gal |
| 2 Cubic millimeters | 0.000000439938496598 gal |
| 3 Cubic millimeters | 0.000000659907744897 gal |
| 4 Cubic millimeters | 0.000000879876993196 gal |
| 5 Cubic millimeters | 0.0000010998462415 gal |
| 10 Cubic millimeters | 0.00000219969248299 gal |
| 20 Cubic millimeters | 0.00000439938496598 gal |
| 25 Cubic millimeters | 0.00000549923120748 gal |
| 50 Cubic millimeters | 0.000010998462415 gal |
| 100 Cubic millimeters | 0.0000219969248299 gal |
| Reference | Cubic millimeters (mm³) | Gallons (gal) |
|---|---|---|
| A teaspoon | 5000 mm³ | 0.00109985 gal |
| A can of soft drink | 330000 mm³ | 0.0725899 gal |
| A wine bottle | 750000 mm³ | 0.164977 gal |
| A bathtub | 150000000 mm³ | 32.9954 gal |
| An Olympic swimming pool | 2.5 × 1012 mm³ | 549923 gal |
The cubic millimetre is a unit of volume equal to one thousandth of a cubic centimetre, written mm³. It is the volume of a cube one millimetre on each side, and it is exactly one microlitre. Which of the two names is used depends on whether the thing being measured is a solid or a liquid, and the distinction is entirely a matter of professional habit.
Machining measures productivity in it. The material removal rate of a milling cutter or a grinding wheel is quoted in cubic millimetres per minute, and that single figure captures what a machine and a tool can achieve better than spindle speed or feed rate can on their own. Comparing two cutting strategies, two tool coatings or two machines means comparing how many cubic millimetres of metal each turns into swarf per unit time.
Additive manufacturing inverts the same measurement. A fused-filament printer deposits ten to twenty cubic millimetres a second, a laser powder-bed machine rather less, and build time follows directly from the part volume divided by that rate. Because the volume of a component is known exactly from its model, the estimate is unusually reliable.
Wear and durability testing uses the unit to describe loss. A specific wear rate is stated in cubic millimetres removed per newton of load and metre of sliding, which allows materials of very different hardness to be compared on one scale. Bearing life, brake pad testing and coating evaluation all reduce to counting cubic millimetres that are no longer there.
Medical imaging works at the same magnitude. Micro-computed tomography of bone reports trabecular volume and porosity in cubic millimetres, tumour volumes in early imaging are given the same way, and voxel sizes in high-resolution scans are quoted as fractions of one. In each case the unit is small enough to describe structure rather than merely size.
Its identity with the microlitre is worth remembering when moving between fields. An engineer discussing a cubic millimetre of material and a biologist discussing a microlitre of solution are talking about the same quantity, and any calculation that mixes the two needs no conversion factor at all.
One cubic millimetre equals 1 microlitre, 0.001 cubic centimetres, 0.000000001 cubic metres, or about 0.000061 cubic inches.
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.