US Gallons to cubic feet
1 US gal = 0.133681 ft³, derived from exact unit definitions.
| US gal | ft³ |
|---|---|
| 1 US gal | 0.134 ft³ |
| 5 US gal | 0.668 ft³ |
| 10 US gal | 1.337 ft³ |
| 50 US gal | 6.684 ft³ |
| 100 US gal | 13.368 ft³ |
| 275 US gal | 36.762 ft³ |
| 500 US gal | 66.840 ft³ |
| 1,000 US gal | 133.681 ft³ |
cubic feet = US gallons × 231 / 1728
= US gallons × 77 / 576
= US gallons × 0.1336805555… (repeating 5)Exact as the fraction 77/576. The decimal repeats a final 5 forever; 0.1336805556 is that value correctly rounded to ten figures.How to convert US gallons to cubic feet
This runs the geometry backwards: you know the capacity and you need the space it occupies — for a containment berm, a pit, a floor plan or a shipping envelope.
- Multiply the gallon figure by 0.1336805556, or divide by 7.480519481.
- Check the magnitude: cubic feet should be about one-seventh of the gallon number.
- If you are sizing a physical space, add whatever margin the application requires after converting, so the margin stays visible.
Secondary containment for a 500-gallon tank
- 500 × 0.1336805556 = 66.84 ft³
- Berm footprint chosen: 8 ft × 8 ft = 64 ft²
- Wall height needed: 66.84 ÷ 64 = 1.044 ft ≈ 12.5 in
500 US gal = 66.84 ft³ → a 12.5-inch wall over an 8 × 8 ft pad
That is the bare regulatory floor: 40 CFR § 112.8(c)(2) requires the entire capacity of the largest single container plus freeboard for precipitation. Most designers add 10 % or size the freeboard to a design storm, which would push this wall to roughly 14 inches.
A 275-gallon oil tank's displaced volume
- 275 × 0.1336805556 = 36.76 ft³
- Typical 275V footprint: 27 in × 60 in = 11.25 ft²
- 36.76 ÷ 11.25 = 3.27 ft of equivalent liquid height
275 US gal = 36.76 ft³
Rainwater harvesting: roof area to fill 1,000 gallons
- 1000 × 0.1336805556 = 133.68 ft³
- 1 inch of rain on 1 ft² = 1/12 ft³ = 0.0833 ft³
- 133.68 ÷ 0.0833 = 1,604 ft² of roof per inch of rain
1,000 US gal = 133.68 ft³ ≈ one inch of rain on 1,600 ft² of roof
Before losses. Real collection efficiency after first-flush diversion, splash and gutter overflow is typically 75–90 % of that.
Where the factor comes from
1 US gal ≡ 231 in³ exactly
1 ft³ = 1728 in³ exactly
1 US gal = 231 / 1728 ft³
= 77 / 576 ft³ (lowest terms)
= 0.13368055555… repeating576 = 2⁶ × 3², and the factor of 3 is what stops the decimal terminating. Keep it as 77/576 if your spreadsheet supports fractions.Rounding
0.1337 is 0.0146 % high; 0.134 is 0.24 % high. On a 10,000-gallon containment calculation that second shortcut adds 3.2 ft³ of imaginary capacity — in the wrong direction, since it makes an undersized berm look adequate. When a rounding choice can only err one way, err toward the conservative side of the requirement.
FAQ
How many cubic feet are in a gallon?
0.1336805555… ft³, exactly 77/576. A gallon is 231 cubic inches and a cubic foot is 1728, so the ratio is 231/1728.
How many cubic feet is a 275-gallon tank?
36.76 ft³ of liquid capacity. The tank's external envelope is larger — a standard 275V oil tank occupies roughly 44 ft³ of room including the shell and legs.
How do I size secondary containment from a gallon figure?
Convert to cubic feet, then divide by the containment floor area to get the wall height. 40 CFR § 112.8(c)(2) sets the floor at the entire capacity of the largest single container plus freeboard for precipitation; the widely used 110 % rule is a design convention rather than the regulatory text.
Should I use 0.1337 or the full factor?
0.1337 is fine for estimating and 0.13368056 for anything you sign. The rounding runs high, which flatters a containment design — so prefer the full factor when the answer must be conservative.
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).
- 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.
- 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.
- 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.