| US Gallons (gal US) | Cubic millimeters (mm³) |
|---|---|
| 1 US Gallon | 3785411.784 mm³ |
| 2 US Gallons | 7570823.568 mm³ |
| 3 US Gallons | 11356235.352 mm³ |
| 4 US Gallons | 15141647.136 mm³ |
| 5 US Gallons | 18927058.92 mm³ |
| 10 US Gallons | 37854117.84 mm³ |
| 20 US Gallons | 75708235.68 mm³ |
| 25 US Gallons | 94635294.6 mm³ |
| 50 US Gallons | 189270589.2 mm³ |
| 100 US Gallons | 378541178.4 mm³ |
| Reference | US Gallons (gal US) | Cubic millimeters (mm³) |
|---|---|---|
| A teaspoon | 0.00132086 gal US | 5000 mm³ |
| A can of soft drink | 0.0871768 gal US | 330000 mm³ |
| A wine bottle | 0.198129 gal US | 750000 mm³ |
| A bathtub | 39.6258 gal US | 150000000 mm³ |
| An Olympic swimming pool | 660430 gal US | 2.5 × 1012 mm³ |
The United States gallon is a unit of volume equal to exactly 231 cubic inches, or 3.785411784 litres. Its symbol is gal US. It contains four quarts, eight liquid pints and 128 United States fluid ounces, and it is about a fifth smaller than the imperial gallon still used in Britain and Ireland.
It is older than its British rival. The figure of 231 cubic inches comes from the English wine gallon standardised in 1707 under Queen Anne, which was the common measure across the American colonies. When Britain abolished its several gallons in 1824 and replaced them with a single imperial gallon based on ten pounds of water, the United States simply kept the one it already had, and the two systems have diverged ever since.
Fuel makes it the most politically visible unit in American life. The price of a gallon of gasoline is quoted daily, watched by governments and used as shorthand for the cost of living, and vehicle efficiency is stated in miles per gallon on every window sticker. Because the American gallon is smaller, a car rated at 30 miles per gallon in the United States would be described as 36 in Britain, which is the same car and the same fuel.
The petroleum industry counts in a multiple of it. The barrel of crude oil is defined as 42 United States gallons, a figure fixed in the Pennsylvania oil fields in the 1860s when a 40-gallon barrel plus two gallons of allowance for leakage became the trade standard. Every published figure for world oil production and reserves rests on that convention.
Domestic quantities follow the same measure. Milk, paint, water for the garden, propane and swimming pool chemistry are all sold and dosed by the gallon in the United States, and a gallon of fresh water weighs 8.34 pounds against the imperial gallon's ten. That difference in weight is a useful check when a figure of uncertain origin needs to be identified.
Care with labelling matters when the two gallons meet. A liquid described simply as a gallon in an American document is 3.785 litres and in a British one 4.546, and nothing in the word itself distinguishes them. Writing gal US or specifying litres removes an ambiguity that has caused real errors in shipping, aviation and chemical dosing.
One United States gallon equals 3.785411784 litres, 231 cubic inches, 128 US fluid ounces, or about 0.833 imperial gallons.
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.