Article — Cubic Inches to Gallons Converter
Cubic inches to gallons converter: the exact 231 in³ factor
A cubic inches to gallons converter swaps in³ for gallons using exact factors. For the US gallon: 1 US gal = 231 in³ by US statute. For the imperial gallon: 1 imp gal = 277.4194 in³ (derived from 4.54609 L). The relationships are not measured, they are defined. Forward: gallons = in³ divided by 231 (US) or 277.42 (UK). Reverse: in³ = gallons times 231 or 277.42. The 350-cubic-inch V8 is 1.515 US gallons. The 454 big block is 1.965 US gallons. 1 cubic foot (1728 in³) is exactly 7.4805 US gallons.
Default 231 in³ = 1 US gallon, the cleanest one-to-one in the system. The system toggle switches between US and imperial. Quick picks cover engine displacements and tank sizes.
The cubic inches to gallons formula
US gallons = cubic inches divided by 231. Imperial gallons = cubic inches divided by 277.4194. Both divisors are exact. The 231 is fixed by US Code Title 15 and by NIST Handbook 44. The 277.4194 derives from the imperial gallon's definition as 4.54609 liters (Weights and Measures Act 1985, UK), converted via the exact 1 inch = 2.54 cm. There is no rounding in the formula itself, only in the input value or the displayed result. For engineering work, use 3 to 6 decimal places. For everyday tank or fuel calculations, 2 decimals is enough.
231 in³ = 1 US gal exact277.42 in³ = 1 imp gal exact1728 in³ = 1 ft³ = 7.48 US gal350 in³ = 5.7 L = 1.52 US gal454 in³ = 7.4 L = 1.97 US galCubic inches to gallons for engines
American V8 engines are specified in cubic inches. The classic 350 small-block Chevy is 350 in³ (5.735 liters). The 454 big block is 454 in³ (7.438 liters). The hemi 426 is 426 in³ (6.981 liters). Dividing by 231 converts each to US gallons of swept volume per full engine cycle: a 350 sweeps 1.515 US gallons through its 8 cylinders. The conversion is not directly useful for fuel mileage (which is gallons of fuel burned, not displaced) but matters for things like oil capacity calculations and for converting US spec sheets to metric (in³ to liters).
The cubic-inch numbers on American muscle cars were marketing shorthand. A "350" was almost never exactly 350 cubic inches. The Chevy 350 was actually 5.733 L = 349.8 in³. The Ford 351 Windsor was 5.766 L = 351.9 in³. The Chrysler "Hemi 426" measured 6.981 L = 426.0 in³, which is one of the few engines where the round-number badge was almost exact. The rounding stuck because customers liked simple numbers.
Cubic inches to gallons for aquariums
Aquarium dimensions are usually in inches. A 20-gallon long tank is 30 × 12 × 12 inches = 4320 in³. Dividing by 231 gives 18.7 US gallons, which is close enough to the nominal 20 — the difference is the glass thickness and the gap between the water line and the rim. A 55-gallon tank is 48 × 13 × 21 in = 13104 in³ = 56.7 nominal US gallons, similarly bumped down to 55 by glass and headspace. The cubic inches to gallons conversion is how aquarists check whether a tank is being filled to the named capacity, and it sets the medication, salt, or chemical dose for the water volume.
For a real-water aquarium volume, multiply nominal capacity by 0.85 to 0.90. A 55-gallon tank holds about 48 US gallons of actual water once gravel, decor, and headspace are subtracted. Medication and dechlorinator doses should match the real volume, not the label, for accurate concentration.
Cubic inches to gallons for tanks
Storage tanks (fuel, water, oil) are commonly dimensioned in inches in the US. A rectangular tank with internal dimensions 24 × 18 × 30 in holds 24 × 18 × 30 = 12960 in³ = 56.1 US gallons. A cylindrical tank with 18 inch diameter and 36 inch height holds pi × 9² × 36 = 9161 in³ = 39.7 US gallons. These calculations matter for fuel-truck pricing, propane tanks, and rainwater catchment systems. For pressurised tanks (LPG, compressed air) the ratings refer to gross volume; useful volume is reduced by 20 percent for the gas-headspace requirement.
US vs imperial gallon
The US gallon and the imperial gallon are different sizes by definition. The imperial gallon (UK, Canada informally, some Commonwealth countries) is 4.54609 L = 277.42 in³. The US gallon is 3.78541 L = 231 in³. The imperial is 20 percent larger. A car listed at "30 mpg UK" gets 25 mpg US — the same physical efficiency, just measured against a larger gallon. When converting cubic inches, always check which gallon the receiving system uses. A 500-in³ tank holds 2.165 US gallons or 1.802 imperial gallons. Mixing the two is the single most common source of fuel-economy confusion across the Atlantic.
Why is the gallon 231 cubic inches?
The 231 traces to 1707, when Queen Anne of England set the wine gallon as a cylinder 7 inches in diameter and 6 inches tall. The volume of that cylinder is pi times the radius squared times the height: pi × 3.5² × 6 = 230.907 in³. The number was rounded to 231 by Parliament for ease of trade, and the resulting wine gallon became the basis of US measurement after independence. Britain replaced the wine gallon with the larger imperial gallon (10 lb of water at 62 deg F) in the Weights and Measures Act of 1824, but the United States kept Queen Anne's number. NIST Handbook 44 reaffirms 231 as the exact US gallon, and US Code Title 15 makes it federal law.
Cubic inches to gallons quick table
- 100 in³ = 0.433 US gal = 1.639 L
- 231 in³ = 1.000 US gal = 3.785 L
- 277.4 in³ = 1.201 US gal = 1.000 imp gal
- 350 in³ (Chevy V8) = 1.515 US gal = 5.735 L
- 454 in³ (big block) = 1.965 US gal = 7.438 L
- 1000 in³ = 4.329 US gal = 16.39 L
- 1728 in³ (1 ft³) = 7.481 US gal = 28.32 L
- 2000 in³ = 8.658 US gal = 32.77 L
Common cubic inches to gallons mistakes
The first mistake is mixing US and imperial gallons. A British workshop manual that says "3 gallons of oil" means 3 imperial gallons = 13.6 L; a US manual that says the same means 3 US gallons = 11.4 L. Always verify the system. The second mistake is forgetting that cubic inches and inches cubed are the same thing, but cubic feet and cubic inches differ by a factor of 1728, not 12. A tank labelled "10 cubic feet" is 17,280 in³, not 120.
The third mistake is treating cubic inches as a fluid measure when the contents are pressurised gas. A 500 in³ compressed-air tank holds 2.165 US gallons of liquid water, but at 100 psi it stores ten times that volume of free atmospheric air. Always convert to atmospheric volume before applying gallons.
Using 230 or 232 instead of the exact 231 introduces a 0.4 percent error per conversion. In a long calculation chain (tank → gallons → liters → pounds of water), this compounds. Use the exact factor in spreadsheets and engineering work. The calculator above uses 231 to full precision.