Gallons to cubic feet
Multiply by 0.133681. Exact, from the gallon's definition.
Material estimate
Enter a figure in gallons.
You need to buy
Use this figure
Worth knowing
What this assumes
Capacity is sold in gallons. Space is measured in feet.
That is the whole reason this conversion gets searched. A cistern, a pool, a hot tub, a septic tank — all rated in gallons, all installed into a space measured in feet. A 500 gallon tank is 66.84 cubic feet, and that is the figure that answers whether it fits.
The factor is exact: the US liquid gallon is defined as 231 cubic inches and a cubic foot is 1,728, so a gallon is 0.1337 cubic feet. Nothing here was measured.
Stated capacities as real volumes
| Vessel | Gallons | Cubic feet | Cubic yards |
|---|---|---|---|
| Rain barrel | 55 gal | 7.35 ft³ | 0.27 yd³ |
| Small cistern | 250 gal | 33.42 ft³ | 1.24 yd³ |
| Large cistern | 500 gal | 66.84 ft³ | 2.48 yd³ |
| Hot tub | 400 gal | 53.47 ft³ | 1.98 yd³ |
| Small pool | 5,000 gal | 668.4 ft³ | 24.76 yd³ |
| Average pool | 20,000 gal | 2673.61 ft³ | 99.02 yd³ |
Exact conversions at US liquid gallons. The cubic yard column is there because that is the unit you will be quoted in if you are digging a hole for it.
US or imperial, again
An imperial gallon is about 20 per cent larger than a US one, so it is 0.1605 cubic feet rather than 0.1337. On imported equipment that gap is not academic — a tank rated in imperial gallons holds a fifth more than the same number of US gallons implies, and the hole has to suit.
And the hole is bigger than the tank
Always. Bedding underneath, working space around the sides, cover over the top — and the excavation is measured in bank volume while the spoil leaves loose and bigger. The excavation calculator handles the swell and the battered sides, and the cubic yard calculator turns the shape into yards. For a pool, the pool volume calculator works the sloping floor properly, and cubic feet to gallons runs this conversion the other way.
Common questions
How many cubic feet is a gallon?
0.1337 cubic feet. The US liquid gallon is defined as exactly 231 cubic inches and a cubic foot is 1,728, so the factor is 231 ÷ 1,728 — exact, from the definitions rather than from a measurement.
How do I convert gallons to cubic feet?
Multiply by 0.133681, or divide by 7.48052 — the same operation either way. Cubic feet is always the much smaller number, because a cubic foot holds about seven and a half gallons. If your answer got bigger, the arithmetic went the wrong way round.
Why would I need this?
Because capacity is quoted in gallons and space is measured in feet. A 500 gallon cistern is 66.84 cubic feet, and that is the number that tells you whether it fits in the crawlspace or how big a hole to dig. Pools, rain harvesting, septic and hot tubs all live at this boundary — sold by the gallon, installed by the foot.
Is this a US or an imperial gallon?
US liquid. An imperial gallon is around 20 per cent larger, so an imperial gallon is 0.1605 cubic feet rather than 0.1337. On imported equipment the difference is real: a tank rated in imperial gallons holds a fifth more than the same number of US gallons would suggest.
How deep a hole does a tank need?
More than the tank's own volume, always. You need bedding underneath, working space around the sides, and cover over the top — and the excavation itself is measured in bank volume while the spoil comes out loose and bigger. The excavation calculator handles the swell and the battered sides; this page just gets you from the gallon rating to the cubic feet.