What this calculator does
Find out how many gallons a space holds when you know its size in cubic feet, or the other way around. Containers are often measured one way and filled another. You can measure a pool, pond, raised bed or tank with a tape measure and get cubic feet. But water, chemicals and pumps are rated in gallons. This conversion connects the two.
Both units are set in cubic inches, which makes the ratio a clean fraction. A cubic foot is 12 × 12 × 12 = 1,728 cubic inches. A US gallon is exactly 231 cubic inches. So one cubic foot holds 1,728 ÷ 231 = 7.48051948… US gallons. And one gallon is 231 ÷ 1,728 = 0.13368… cubic feet.
Where it's used
Why this math matters outside the classroom:
- Pools. A pool holding 2,000 ft³ of water contains about 14,961 US gallons.
- Aquariums. A tank 36 × 18 × 18 in is 6.75 ft³, which is about 50.5 gallons.
- Water bills. Utilities that bill per 100 ft³ of water are charging for about 748 gallons at a time.
- Hot tubs. A hot tub 7 × 7 ft filled 3 ft deep holds 147 ft³, roughly 1,100 gallons.
- Exact ratio. Students build the factor from definitions: 1,728 in³ in a cubic foot divided by 231 in³ in a gallon is about 7.48.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter a volume in cubic feet or in US gallons.
- Choose cubic feet to gallons, or gallons to cubic feet.
- Read the result and the working. Then compare it with the reference table.
What the inputs mean
- Value to convert
- Cubic feet for ft³ → gal, or US liquid gallons for gal → ft³.
- Direction
- Which unit your number is in. The table switches to match.
Formula and method
US gal = ft³ × 1728 ÷ 231 ≈ ft³ × 7.48051948 ft³ = US gal × 231 ÷ 1728 ≈ US gal × 0.133680556
In the formulas, ft³ means cubic feet and US gal means US gallons. Neither number ever ends as a decimal. 1728/231 cuts down to 576/77, and dividing by 77 (which is 7 × 11) gives digits that repeat forever. The calculator works from the exact metric sizes of both units. So its answer matches the fraction to full precision. The steps show the factor to 12 significant digits.
The ratio comes straight from how both units are defined in inches. So it's the same today as before the inch was redefined in metric terms.
Worked example
A stock tank measures 10 cubic feet inside. How many gallons does it hold?
- 1 ft³ = 1728/231 US gal ≈ 7.48051948052 US gal.
- 10 × 7.48051948052 = 74.8051948052 US gal.
Result: 10 cubic feet holds about 74.8 US gallons.
Understanding the result
Here's a handy shortcut: multiply cubic feet by 7.5 to get gallons. Or divide gallons by 7.5 to get cubic feet. The answer is only 0.26% off. That's close enough for any estimate of filling, or of how much chemical to add.
Pool example: a rectangular pool 20 ft × 40 ft, with an average depth of 5 ft, holds 4,000 ft³. That's about 29,922 US gallons. In reverse, a 1,000-gallon tank needs about 133.7 cubic feet of space.
Water weight: a cubic foot of water weighs about 62.4 lb, and a gallon about 8.34 lb. That helps when you're checking whether a deck or floor can hold a filled hot tub or aquarium.
Assumptions and limitations
- Gallons are US liquid gallons (231 in³).
- Cubic feet are measured inside the container.
- The imperial gallon is larger: one cubic foot is about 6.23 imperial gallons, not 7.48.
- It doesn't work out the volume of a shape for you. Measure length × width × depth first, or use the volume, pool-volume or tank-volume calculators.
Frequently asked questions
How many gallons are in a cubic foot?
1728/231 ≈ 7.48052 US gallons. That is about 6.229 imperial gallons.
How many cubic feet is 1 gallon?
231/1728 ≈ 0.13368 cubic feet, or 231 cubic inches.
Why is the number not round?
The gallon wasn't designed to fit the foot. It comes from an old English wine gallon of 231 cubic inches. The cubic foot is 1,728 cubic inches, and the two don't divide evenly.
How this calculator is checked
Method: 1 ft³ = 1,728 in³; 1 US gal = 231 in³ (exact definitions). Ratio 1728/231. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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