Cubic Feet to US gallons
1 ft³ = 7.48052 US gal, derived from exact unit definitions.
| ft³ | US gal |
|---|---|
| 1 ft³ | 7.48 US gal |
| 2 ft³ | 14.96 US gal |
| 5 ft³ | 37.40 US gal |
| 7.48 ft³ | 55.95 US gal |
| 10 ft³ | 74.81 US gal |
| 25 ft³ | 187.01 US gal |
| 50 ft³ | 374.03 US gal |
| 100 ft³ | 748.05 US gal |
| 500 ft³ | 3,740.26 US gal |
US gallons = cubic feet × 1728 / 231
= cubic feet × 576 / 77
= cubic feet × 7.480519480519… (repeating)Exact as the fraction 576/77. The decimal repeats the block 480519 forever, so 7.480519481 is a rounding — an excellent one, but a rounding.How to convert cubic feet to US gallons
This is the conversion that turns a geometry result into a capacity. You calculate a tank's volume in cubic feet from its dimensions, then bring it into the unit the trade actually uses.
- Get the volume in cubic feet from internal dimensions. Shell thickness, internal baffles and dished heads all belong in the geometry step, not here.
- Multiply by 7.480519481 (or divide by 0.133680556 — same thing).
- Check the magnitude: gallons should be roughly seven and a half times the cubic-foot figure.
A horizontal cylindrical tank, 4 ft diameter × 10 ft long
- Cross-section: π × (4/2)² = 12.56637 ft²
- Volume: 12.56637 × 10 = 125.6637 ft³
- 125.6637 × 7.480519481 = 940.0 US gal
≈ 940 US gallons
Flat-ended only. Dished or elliptical heads add capacity — use the dished-end calculator rather than assuming flat.
A rectangular concrete sump, 6 ft × 3 ft × 2 ft deep
- Volume: 6 × 3 × 2 = 36 ft³
- 36 × 7.480519481 = 269.3 US gal
36 ft³ = 269 US gallons
For secondary containment under 40 CFR § 112.8(c)(2), the sump must hold the entire capacity of the largest single container plus freeboard for rain — so compare this 269 gal against the tank it serves, not against the total site inventory.
A cubic yard of liquid
- 1 yd³ = 27 ft³
- 27 × 7.480519481 = 201.974 US gal
1 cubic yard = 201.97 US gallons
Where the factor comes from
1 foot = 12 inches
1 ft³ = 12³ = 1728 in³ exactly
1 US gal ≡ 231 in³ exactly
1 ft³ = 1728 / 231 gal
= 576 / 77 gal (lowest terms)
= 7.480519480519… repeating, period 6Both inputs are exact integers, so the ratio is an exact rational number. It simply happens not to have a terminating decimal, because 77 = 7 × 11 and neither factor divides a power of ten.NIST Handbook 44's Appendix C conversion table prints this as 7.480 519, which is the same number truncated to seven figures.
Rounding: the 7.48 shortcut
Nearly every field handbook uses 7.48. It runs 0.0069 % low — genuinely negligible below a few thousand gallons, and worth a moment's thought above that.
| Volume | Exact (gal) | Using 7.48 | Shortfall |
|---|---|---|---|
| 36 ft³ (sump) | 269.30 | 269.28 | 0.02 gal |
| 125.66 ft³ (4×10 tank) | 940.00 | 939.93 | 0.07 gal |
| 1,336.8 ft³ (10,000 gal) | 10,000 | 9,999.3 | 0.7 gal |
| 13,368.1 ft³ (100,000 gal) | 100,000 | 99,993 | 7 gal |
FAQ
How many gallons are in a cubic foot?
7.480519480519… US gallons, repeating. It is the exact fraction 1728/231, which reduces to 576/77. The familiar 7.48 is that value rounded to three figures.
Why is the number 7.48 and not something rounder?
Because the foot and the gallon were standardised independently. A cubic foot is 1728 cubic inches and a gallon is 231 cubic inches, and 1728 is not a tidy multiple of 231.
Is 7.48 good enough?
For most tank work, yes — it is 0.0069 % low, about 0.7 gallons on a 10,000-gallon tank. Measurement error in the tank dimensions will normally dwarf it.
How do I get cubic feet in the first place?
From internal dimensions and the right shape formula: L × W × H for rectangular, π(D/2)²L for a flat-ended cylinder, and a partial-fill formula for a horizontal cylinder gauged part-full. The formula library lists each one.
Does this work for gases?
Not as a quantity conversion. A cubic foot of gas depends entirely on temperature and pressure, which is why gas is metered in standard cubic feet referred to fixed base conditions. The geometric conversion is unchanged; the meaning of the number is not.
Sources and standards
Every factor on this page is traced to the document that defines it. Where a value is exact, it is exact because the cited standard makes it so — not because we rounded it well.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
- Special Publication 811, Appendix B.8 — Factors for Units Listed AlphabeticallyNational Institute of Standards and Technologyhttps://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8NIST's master table of conversions into SI. Factors printed in boldface are exact by definition rather than measured or rounded — which is how you tell a defined unit from an approximated one.
- 40 CFR § 112.8(c)(2) — Spill Prevention, Control, and Countermeasure Plan requirements for onshore facilitiesU.S. Environmental Protection Agency, via the electronic Code of Federal Regulationshttps://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-112/subpart-B/section-112.8Requires secondary containment sized for the entire capacity of the largest single container, plus sufficient freeboard to contain precipitation. Note that the rule itself does not state “110 %” — that is a common design convention layered on top of the regulatory floor.
- Unit Conversion — Office of Weights and MeasuresNational Institute of Standards and Technologyhttps://www.nist.gov/pml/owm/metric-si/unit-conversionNIST's index page explaining which of its publications governs which kind of conversion: SP 811 for exact factors, Handbook 44 Appendix C for trade and commerce, Handbook 133 Appendix E for packaged goods.
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.