Cubic Inches to Gallons Converter
This converter turns a volume given in cubic inches into gallons, a switch you commonly need when working from US engineering drawings, engine displacement figures (a "350 cubic inch" V8, for example), tank and vessel specifications, or older technical documents that were never converted to metric. Because "gallon" means two different things depending on where you are, the tool lets you choose between the US liquid gallon and the UK Imperial gallon before it calculates, using the exact statutory definitions rather than a rounded approximation: 1 US gallon equals exactly 231 cubic inches, while 1 UK gallon equals exactly 4.54609 litres, or about 277.419 cubic inches. Enter a figure in the cubic inches field, or work the other way by entering a value in the gallons field, and the two update each other automatically. Alongside the headline gallon figure, you get the equivalent in litres and US fluid ounces, plus a full breakdown showing the calculation used, the cubic inches per gallon for your chosen unit, and every equivalent unit (US and UK gallons, litres, millilitres and cubic centimetres) side by side. A reference table of common cubic inch values is included so you can sanity-check unusual figures at a glance. Because the UK gallon is roughly 20 percent larger than the US gallon, always confirm which one your source material is actually using before you rely on the result.
1. Volume to Convert
2. Reverse Conversion
Enter a value in either field. The other updates automatically.
Common Cubic Inch Reference Values
| Cubic Inches (in³) | US Gallons | UK Gallons | Litres |
|---|
Conversion Breakdown
All Unit Equivalents
How to Convert Cubic Inches to Gallons
The conversion from cubic inches to gallons depends on which gallon you are using. There are two common definitions:
- US liquid gallon: exactly 231 cubic inches (by definition under US law). To convert, divide cubic inches by 231.
- UK Imperial gallon: defined as exactly 4.54609 litres, which equals approximately 277.419 cubic inches. To convert, divide cubic inches by 277.419.
The formula is straightforward:
US gallons = cubic inches ÷ 231
UK gallons = cubic inches ÷ 277.419
Conversion Reference Table
| Cubic Inches | US Gallons | UK Gallons | Litres |
|---|---|---|---|
| 1 in³ | 0.004329 | 0.003605 | 0.016387 |
| 10 in³ | 0.04329 | 0.03605 | 0.16387 |
| 50 in³ | 0.21645 | 0.18025 | 0.81935 |
| 100 in³ | 0.43290 | 0.36047 | 1.63871 |
| 231 in³ | 1.00000 (1 US gal) | 0.83267 | 3.78541 |
| 277.419 in³ | 1.20095 | 1.00000 (1 UK gal) | 4.54609 |
| 500 in³ | 2.16450 | 1.80233 | 8.19353 |
| 1,000 in³ | 4.32900 | 3.60465 | 16.38706 |
| 1,731 in³ | 7.49351 | 6.23965 | 28.36601 |
US Gallon vs UK Gallon: Key Facts
The US gallon (231 in³) and the UK Imperial gallon (277.419 in³) are different units with a shared history that diverged in the early 19th century. The key facts:
- 1 US gallon = 231 in³ = 3.78541 litres = 128 US fluid ounces
- 1 UK gallon = 277.419 in³ = 4.54609 litres = 160 UK fluid ounces
- 1 UK gallon = 1.20095 US gallons (the UK gallon is about 20 percent larger)
- The US gallon is subdivided into 4 US liquid quarts, each of which is 2 US pints
- The UK gallon is subdivided into 4 UK quarts, each of which is 2 UK pints
Worked Example
Convert 100 cubic inches to US gallons:
US gallons = 100 ÷ 231 = 0.4329 US gallons (approximately)
Convert 100 cubic inches to UK gallons:
UK gallons = 100 ÷ 277.419 = 0.3605 UK gallons (approximately)
Convert 100 cubic inches to litres:
1 cubic inch = 16.387064 millilitres (exactly, from the definition of the inch as 25.4 mm)
100 in³ = 100 × 16.387064 mL = 1,638.7064 mL = 1.6387 litres
Why Cubic Inches?
Cubic inches are still commonly used in engineering, automotive, and industrial contexts, particularly in the United States. Engine displacement is often quoted in cubic inches (for example, a "350 cubic inch" V8 engine). Tank and vessel volumes in older US documentation are often expressed in cubic inches. When working from technical drawings or specifications that use cubic inch measurements, this converter saves you from manual arithmetic.
Related Calculators
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- Volume Conversion Calculator: convert between litres, gallons, pints, cups, and more.
- Cubic Metre Calculator: calculate and convert cubic metres.
- Ounces to Millilitres Converter: fluid ounce conversions.
Sources and method: US gallon definition: NIST Handbook 44, Appendix C, Section 4 (1 US liquid gallon = 231 in³ exactly). UK Imperial gallon: UK Weights and Measures Act 1985, Schedule 1 (1 gallon = 4.54609 litres exactly). Cubic inch to millilitre conversion: 1 in = 25.4 mm exactly, therefore 1 in³ = 25.4³ mm³ = 16,387.064 mm³ = 16.387064 mL exactly.