The three conversions people need most: cubic inches ÷ 231 = US gallons, cubic feet × 7.4805 = US gallons, and 1 mL = 1 cm³ exactly. Everything else on this page follows from those.
Cubic inches to gallons
This is the most-searched volume conversion there is, and the number to remember is 231 — a US gallon is defined as exactly 231 cubic inches. Not approximately: exactly, by definition.
| Cubic inches | US gallons | Litres | Cubic feet |
|---|---|---|---|
| 231 | 1.00 | 3.79 | 0.134 |
| 500 | 2.16 | 8.19 | 0.289 |
| 1,000 | 4.33 | 16.39 | 0.579 |
| 1,728 | 7.48 | 28.32 | 1.000 |
| 2,310 | 10.00 | 37.85 | 1.337 |
| 4,620 | 20.00 | 75.71 | 2.674 |
| 5,000 | 21.65 | 81.94 | 2.894 |
| 10,000 | 43.29 | 163.87 | 5.787 |
| 12,705 | 55.00 | 208.20 | 7.353 |
The 1,728 row is worth noticing: that is one cubic foot, and it converts to 7.4805 gallons — which is exactly where the familiar 7.48 factor comes from, since 1,728 ÷ 231 = 7.4805.
Every conversion factor
| Unit | In litres | In cubic inches | In US gallons |
|---|---|---|---|
| 1 teaspoon (US) | 0.00493 | 0.301 | 0.00130 |
| 1 tablespoon (US) | 0.01479 | 0.902 | 0.00391 |
| 1 fluid ounce (US) | 0.02957 | 1.805 | 0.00781 |
| 1 cup (US) | 0.23659 | 14.44 | 0.0625 |
| 1 pint (US) | 0.47318 | 28.88 | 0.125 |
| 1 quart (US) | 0.94635 | 57.75 | 0.25 |
| 1 litre | 1 | 61.02 | 0.26417 |
| 1 US gallon | 3.78541 | 231 | 1 |
| 1 imperial gallon | 4.54609 | 277.42 | 1.20095 |
| 1 cubic foot | 28.3168 | 1,728 | 7.48052 |
| 1 oil barrel | 158.987 | 9,702 | 42 |
| 1 cubic yard | 764.555 | 46,656 | 201.974 |
| 1 cubic metre | 1,000 | 61,024 | 264.172 |
The one identity worth memorising
Metric volume was designed around this. A millilitre is a cubic centimetre — not approximately, identically — which is why medical and laboratory work moves between mL and cc without thinking. It also means that if you measure anything in centimetres, the cubic centimetres you calculate are already millilitres, with no conversion step at all.
The imperial system has no such tidiness. A gallon is 231 cubic inches for historical reasons, a foot is 12 inches, and a cubic foot is therefore 1,728 cubic inches — three unrelated numbers you simply have to know. That is the practical case for working in metric wherever you have the choice.
US versus imperial: a 20% trap
| Unit | US | Imperial (UK) | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | +20.1% |
| Quart | 0.946 L | 1.137 L | +20.1% |
| Pint | 0.473 L | 0.568 L | +20.1% |
| Fluid ounce | 29.57 mL | 28.41 mL | −3.9% |
Note the oddity in the last row: the imperial fluid ounce is smaller than the US one, even though the imperial gallon is larger. That is because an imperial gallon contains 160 fluid ounces while a US gallon contains 128. So "a pint" is 20% bigger in Britain, but "a fluid ounce" is 4% smaller — which catches out recipe writers in both directions.
Frequently asked questions
How do I convert cubic inches to gallons?
Divide cubic inches by 231, because a US gallon is defined as exactly 231 cubic inches. So 462 in³ is 2 gallons, and 4,620 in³ is 20 gallons. For imperial gallons divide by 277.42 instead.
How many cubic inches are in a cubic foot?
1,728, because a cubic foot is 12 × 12 × 12 inches. It follows that a cubic foot holds 1,728 ÷ 231 = 7.4805 US gallons, which is where that conversion factor comes from.
How do I convert litres to gallons?
Divide litres by 3.7854 for US gallons, or by 4.5461 for imperial gallons. Going the other way, multiply US gallons by 3.7854. The two gallons differ by about 20 percent, which is enough to matter on anything larger than a bucket.
What is the difference between a US and an imperial gallon?
A US gallon is 3.7854 litres and an imperial gallon is 4.5461 litres — about 20 percent larger. The US kept the older English wine gallon of 231 cubic inches, while Britain redefined its gallon in 1824 based on ten pounds of water. Both are still in use, which is why the distinction matters.
Is a millilitre the same as a cubic centimetre?
Yes, exactly. One millilitre is defined as one cubic centimetre, so 1 mL = 1 cm³ = 1 cc with no conversion at all. This is why medical and laboratory work moves between the units freely, and it is the single most useful identity in metric volume.
How many cubic feet are in a cubic metre?
35.315. Going the other way, one cubic foot is 0.02832 cubic metres. Freight is quoted in cubic metres (CBM) internationally and cubic feet domestically in the US, so this pair comes up constantly in shipping.