The factor, and a shortcut
One cubic foot holds 7.48 US gallons — it comes straight from the definitions, 1728 cubic inches per cubic foot divided by 231 cubic inches per gallon. For a rough figure, multiply by 7.5: it is within 0.3% of exact.
Common volumes
| Cubic feet | US gallons | UK gallons | Litres |
|---|---|---|---|
| 1 ft³ | 7.48 | 6.23 | 28.32 |
| 5 ft³ | 37.40 | 31.14 | 141.6 |
| 10 ft³ | 74.81 | 62.29 | 283.2 |
| 50 ft³ | 374.0 | 311.4 | 1,415.8 |
| 100 ft³ | 748.1 | 622.9 | 2,831.7 |
| 1,000 ft³ | 7,480.5 | 6,228.8 | 28,316.8 |
Water bills often meter in HCF (hundred cubic feet): 1 HCF = 748.05 US gallons, so a "50 HCF" line is about 37,400 gallons.
US and UK gallons are not the same size
The same cubic foot comes out as 7.48 US gallons but only 6.23 imperial gallons. Read the wrong system and you are off by a fifth — enough to badly misjudge a fuel or tank order.
A UK imperial gallon is about 20% larger than a US gallon, so a cubic foot fills fewer of them — 6.23 versus 7.48.
Common questions
How many gallons are in a cubic foot?
One cubic foot is 7.48 US gallons (exactly 1728 / 231). In UK imperial gallons it is 6.23, because the UK gallon is the larger of the two.
How do I find the gallons in a pool or tank?
Multiply length by width by average depth, all in feet, to get cubic feet, then multiply by 7.48 for US gallons. A 20 x 10 ft pool at 5 ft average depth is 1,000 cubic feet, or about 7,480 gallons.
Why is a US gallon smaller than a UK gallon?
They were defined differently. A US gallon is 231 cubic inches (3.785 litres); the UK imperial gallon is 277.4 cubic inches (4.546 litres). The UK gallon is about 20% larger, so the two are not interchangeable.
