How many cubic inches are in a US gallon?

There are exactly 231 cubic inches in one US liquid gallon. This is the statutory definition established in the US Weights and Measures Act. To convert cubic inches to US gallons, divide the number of cubic inches by 231. For example, 462 cubic inches equals 2 US gallons.

How many cubic inches are in a UK (Imperial) gallon?

One UK (Imperial) gallon equals approximately 277.419 cubic inches. The UK gallon is defined as exactly 4.54609 litres, which converts to approximately 277.4194 cubic inches. The UK gallon is about 20 percent larger than the US liquid gallon. To convert cubic inches to UK gallons, divide by 277.419.

Why are US and UK gallons different sizes?

The US and UK gallon diverged historically. The US gallon is based on the old English wine gallon of 231 cubic inches, which was the standard before the UK redefined its gallon in the Weights and Measures Act 1824. The UK Imperial gallon was redefined as the volume of ten pounds of water at a specific temperature and pressure, which works out to about 277.42 cubic inches. As a result, one UK gallon is approximately 1.2009 US gallons.

Cubic Inches to Gallons Converter

Convert cubic inches to US liquid gallons or UK Imperial gallons instantly. Enter your volume in cubic inches and choose your gallon type. The converter uses the exact statutory definitions: 1 US gallon = 231 cubic inches exactly; 1 UK gallon = 277.419 cubic inches (4.54609 litres).

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Exact definitions NIST Handbook 44 (US gallon = 231 in³ exactly). UK gallon = 4.54609 L per Weights and Measures Act 1985.

1. Volume to Convert

Cubic inches (in³)

in³

Gallon type

2. Reverse Conversion

Gallons (convert back to cubic inches)

gal

Enter a value in either field. The other updates automatically.

Conversion Result

Gallons
-
US liquid gallons

Litres

Metric equivalent

US fl oz

US fluid ounces

Cubic Inches

Your input

Common Cubic Inch Reference Values

Cubic Inches (in³)	US Gallons	UK Gallons	Litres

Conversion Breakdown

Input volume-

Gallon type selected-

Cubic inches per gallon-

Calculation-

Result-

All Unit Equivalents

US liquid gallons-

UK Imperial gallons-

Litres-

Millilitres-

US fluid ounces-

Cubic centimetres (cm³)-

Result: Enter a value above to see the conversion.

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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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.

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