Cubic Inches to US gallons
1 in³ = 0.004329 US gal, derived from exact unit definitions.
| in³ | US gal |
|---|---|
| 100 in³ | 0.4329 US gal |
| 231 in³ | 1.0000 US gal |
| 500 in³ | 2.1645 US gal |
| 1,000 in³ | 4.3290 US gal |
| 5,000 in³ | 21.6450 US gal |
| 10,000 in³ | 43.2900 US gal |
| 57,750 in³ | 250.0000 US gal |
US gallons = cubic inches ÷ 231
= cubic inches × 0.004329004329… (repeating)Divide by 231. That integer is the definition of the US liquid gallon, so the division is exact; the reciprocal 0.004329004329… is not.How to convert cubic inches to US gallons
Divide by 231. This is the most direct conversion on the site, because 231 is not a derived factor — it is the legal definition of the US liquid gallon in NIST Handbook 44. Everything else on these pages is downstream of it.
- Compute the volume in cubic inches from internal dimensions.
- Divide by 231.
- Check the magnitude: gallons should be a little under one two-hundredth of the cubic-inch figure.
A rectangular reservoir, 30 in × 18 in × 14 in inside
- Volume: 30 × 18 × 14 = 7,560 in³
- 7560 ÷ 231 = 32.7272…
- Round to 32.73 US gal
7,560 in³ = 32.73 US gallons
Inside dimensions. On a moulded plastic tank the wall can be 3/8 in, which removes about 2.5 gallons from a box this size — roughly 8 % of the answer.
A vertical cylinder, 22 in diameter × 44 in tall
- Area: π × (22/2)² = 380.13 in²
- Volume: 380.13 × 44 = 16,725.7 in³
- 16,725.7 ÷ 231 = 72.41 US gal
= 72.4 US gallons brim-full
Brim-full is not usable capacity. Subtract the outlet height at the bottom and the ullage you keep at the top.
Sanity-checking an engine displacement
- A 350 cubic inch V8
- 350 ÷ 231 = 1.515 US gal
- Or metric: 350 × 16.387064 = 5,735.5 cm³ ≈ 5.7 L
350 in³ = 1.52 US gal = 5.74 L
The same two exact definitions — 231 in³ per gallon and 2.54 cm per inch — produce both answers. If the 5.7 L badge matches, your arithmetic is sound.
Where the factor comes from
1 US gallon ≡ 231 in³ (definition — NIST HB 44, App. C)
1 in³ = 1/231 US gal
= 0.004329004329004… repeating, period 6231 = 3 × 7 × 11. None of those primes divides a power of ten, which is why the reciprocal never terminates. Divide by 231 and the problem disappears.The number is a historical artefact rather than a design: it descends from the English wine gallon, a cylinder nominally 7 inches across and 6 inches deep, which was standardised at 231 cubic inches and then inherited by the United States and written into federal legal metrology.
Rounding
Multiplying by 0.004329 is 0.0001 % low — irrelevant. Multiplying by 0.0043 is 0.67 % low, which on a 500-gallon calculation is more than three gallons. If you are going to multiply rather than divide, carry at least six figures.
FAQ
How many cubic inches are in a gallon?
Exactly 231. This is a definition in NIST Handbook 44, Appendix C, not a measured or rounded value, and it is the anchor for every US gallon conversion.
How do I convert cubic inches to gallons?
Divide by 231. A 7,560-cubic-inch box holds 32.73 US gallons.
Why not just multiply by 0.004329?
You can, but 1/231 does not terminate, so any multiplier is already rounded. Six figures (0.004329) is fine; three (0.0043) is 0.67 % low.
What is 231 cubic inches physically?
About the volume of a box 11 in × 7 in × 3 in. The figure descends from the English wine gallon, historically a cylinder roughly 7 inches in diameter and 6 inches deep.
Does this apply to the imperial gallon?
No. An imperial gallon is 4.54609 litres, which works out to about 277.42 cubic inches — a definition set in litres by UK statute, not in cubic inches.
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.
- Refinement of Values for the Yard and the Pound — Federal Register Doc. 59-5442 (24 FR 5348, 1 July 1959)National Bureau of Standards; scan hosted by NOAA's National Geodetic Surveyhttps://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdfThe International Yard and Pound Agreement, signed by the United States, United Kingdom, Canada, Australia, New Zealand and South Africa. Fixes the yard at exactly 0.9144 m, from which the inch follows as exactly 25.4 mm. Every inch-to-metric volume factor on this site descends from this one page.
- 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.