US Gallons to cubic inches
1 US gal = 231 in³, derived from exact unit definitions.
| US gal | in³ |
|---|---|
| 1 US gal | 231 in³ |
| 5 US gal | 1,155 in³ |
| 10 US gal | 2,310 in³ |
| 55 US gal | 12,705 in³ |
| 100 US gal | 23,100 in³ |
| 275 US gal | 63,525 in³ |
| 500 US gal | 115,500 in³ |
cubic inches = US gallons × 231Exact, and an integer. There is no rounding to worry about anywhere in this conversion — 231 is the legal definition of the US liquid gallon.How to convert US gallons to cubic inches
Multiply by 231. This is the cleanest conversion on the site: an exact integer factor, straight from the definition. It is also the conversion you need whenever you are working backwards from a rated capacity into physical dimensions — designing a tank, cutting a dip stick, or checking whether a nominal size is honest.
- Multiply the gallon figure by 231 to get the required internal volume in cubic inches.
- Divide by the tank's internal cross-sectional area (in in²) to get the required liquid height in inches.
- Add ullage and any dead volume below the outlet to get an overall height.
Gallons per inch of a 48-inch vertical cylinder
- Area: π × (48/2)² = 1,809.56 in²
- 1 inch of depth = 1,809.56 in³
- 1809.56 ÷ 231 = 7.834 US gal
7.83 gallons per inch of depth
For straight-walled tanks this number is constant top to bottom, which is what makes a stick reading trivially convertible. The gallons-per-inch calculator does this for any diameter.
Designing a 60-gallon rectangular tank
- Required volume: 60 × 231 = 13,860 in³
- Chosen footprint: 36 in × 20 in = 720 in²
- Liquid height: 13,860 ÷ 720 = 19.25 in
- Plus 2 in ullage → 21.25 in internal height
60 US gal needs 13,860 in³ — 36 × 20 × 19.25 in of liquid
Checking a “275 gallon” tank against its dimensions
- Claimed: 275 US gal → 275 × 231 = 63,525 in³
- Typical 275V internal: 27 in × 44 in × 60 in (obround, ends rounded)
- Rectangular bound: 27 × 44 × 60 = 71,280 in³ = 308.6 gal
- The rounded ends remove roughly 11 % → ≈ 275 gal
275 US gal = 63,525 in³ — consistent with the standard shell
Where the factor comes from
1 US liquid gallon ≡ 231 in³ exactly, by definition
= 4 quarts = 8 pints = 128 fl oz
1 quart = 57.75 in³
1 pint = 28.875 in³
1 fluid ounce = 1.8046875 in³NIST Handbook 44, Appendix C tabulates this chain. The whole US customary liquid system hangs off the single integer 231.FAQ
How many cubic inches in a gallon?
Exactly 231 cubic inches, by definition in NIST Handbook 44, Appendix C. It is an exact integer, so the conversion introduces no rounding at all.
How do I work out gallons per inch of my tank?
Take the internal cross-sectional area in square inches and divide by 231. A 48-inch-diameter vertical cylinder has an area of 1,809.56 in², giving 7.83 gallons per inch.
Why 231 and not a round number?
It is inherited from the English wine gallon, a container of roughly 7 inches diameter and 6 inches depth, standardised at 231 cubic inches long before anyone designed a coherent unit system. The US kept it; Britain later replaced it with the imperial gallon.
Can I use this to check a manufacturer's rating?
As a cross-check, yes. Convert the rated gallons to cubic inches, compare against the internal geometry, and expect a few percent of difference from rounded ends, ribs and tolerance. A large discrepancy is worth querying.
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.
- ISO 7507-1 — Petroleum and liquid petroleum products: Calibration of vertical cylindrical tanks, Part 1: Manual strapping methodInternational Organization for StandardizationPaid standard — cited by designation; obtain from the publisher.Defines how a tank's capacity table (its “strapping table”) is produced from physical measurement, including corrections for plate thickness, shell expansion and tank tilt. It is the reason a real tank's chart rarely matches naive geometry to the last gallon.
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.