Converting cubic feet to gallons comes up constantly in real life: pool capacity, fish tanks, water tanks, fuel storage, septic systems, irrigation. The formula is simple, but it's surrounded by enough confusion that it's worth a proper explanation.
The basic conversion
US gallons = Cubic feet × 7.48052
For most purposes, round to 7.48:
Cubic feet × 7.48 = US gallons
Examples:
- 10 ft³ × 7.48 = 74.8 gallons
- 100 ft³ × 7.48 = 748 gallons
- 1,000 ft³ × 7.48 = 7,480 gallons
Where the 7.48 comes from
Derived from the definitions:
- 1 US gallon = 231 cubic inches
- 1 cubic foot = 12 × 12 × 12 = 1,728 cubic inches
So:
1 ft³ ÷ 1 gallon = 1,728 ÷ 231 = 7.4805 gallons per ft³
This number is exact. The 7.48 in tables is just this rounded.
The US vs UK gallon trap
Here's where everyone gets confused. The "gallon" isn't a single unit:
| Unit | Volume | Where used |
|---|---|---|
| US gallon | 3.7854 liters = 231 in³ | United States |
| UK gallon (Imperial) | 4.5461 liters = 277.4 in³ | UK, Caribbean |
A UK gallon is about 20% larger than a US gallon:
1 ft³ = 7.48 US gallons = 6.23 UK gallons
If you're converting cubic feet to gallons, ask. For pool chemistry in the US, US gallons. For UK car fuel economy ("miles per gallon"), Imperial. For shipping or international contexts, always specify.
Older US conversion tables sometimes show "1 ft³ = 6.25 gallons." That's the UK conversion accidentally applied to US contexts. If your conversion gives a smaller number than expected, check which gallon factor you're using.
Worked examples
Pool volume
A 30 × 15 × 5 ft pool:
Volume = 30 × 15 × 5 = 2,250 ft³ US gallons = 2,250 × 7.48 = 16,830 gallons
Hot tub
A 7 × 7 × 3 ft hot tub:
Volume = 7 × 7 × 3 = 147 ft³ US gallons = 147 × 7.48 = 1,099 gallons
Fish tank
A 4 × 1.5 × 2 ft fish tank:
Volume = 4 × 1.5 × 2 = 12 ft³ US gallons = 12 × 7.48 = 89.8 gallons
Matches the "90-gallon tank" advertised size.
Reverse conversion
Cubic feet = US gallons ÷ 7.48
Or equivalently:
Cubic feet = US gallons × 0.1337
A 50-gallon water heater: 50 ÷ 7.48 = 6.68 ft³.
Quick reference table
| Cubic feet | US gallons | UK gallons | Liters |
|---|---|---|---|
| 1 | 7.48 | 6.23 | 28.32 |
| 5 | 37.4 | 31.1 | 142 |
| 10 | 74.8 | 62.3 | 283 |
| 25 | 187 | 156 | 708 |
| 50 | 374 | 311 | 1,416 |
| 100 | 748 | 623 | 2,832 |
| 500 | 3,740 | 3,116 | 14,158 |
| 1,000 | 7,481 | 6,229 | 28,317 |
Cubic feet to other units
| 1 cubic foot equals | Value |
|---|---|
| cubic inches | 1,728 |
| US gallons | 7.481 |
| UK gallons | 6.229 |
| liters | 28.32 |
| cubic centimeters | 28,317 |
| cubic meters | 0.02832 |
| cubic yards | 0.03704 |
| US quarts | 29.92 |
| US fluid ounces | 957 |
Common errors
Mixing inches and feet
If you measure a tank as 36 × 24 × 30 inches, plug into feet first:
36 in = 3 ft, 24 in = 2 ft, 30 in = 2.5 ft V = 3 × 2 × 2.5 = 15 ft³
Or compute in inches and divide by 1,728:
V = 36 × 24 × 30 = 25,920 in³ ÷ 1,728 = 15 ft³
Mixing US and UK gallons
Always know which gallon you're using.
Forgetting density (when going to weight)
Cubic feet to gallons converts volume. Gallons to pounds requires density:
Water: 1 US gallon ≈ 8.34 lb Gasoline: 1 US gallon ≈ 6.2 lb Heating oil: 1 US gallon ≈ 7.2 lb
Why 7.48 instead of a "nicer" number
Metric gives nice round conversions. The US system doesn't, because units were defined independently:
- The gallon descended from medieval English measures, codified at 231 in³ for wine in 1707.
- The foot descended from various local measures, standardized as 12 inches.
- Nobody designed these to convert cleanly to each other.
So 7.48052 is what it is.
The takeaway
Cubic feet × 7.48 = US gallons. The most useful conversion factor in American liquid volume math. Memorize it, double-check whether you need US or UK gallons, and always convert all measurements to a common unit before calculating.