About

Cubic inches and gallons serve different measurement domains. Cubic inches (in³) quantify displacement in engines, hydraulic cylinders, and machined cavities. Gallons measure fluid capacity for fuel, chemicals, and beverages. Confusing US and Imperial gallons causes dosing errors, fuel miscalculations, and shipping disputes. A US gallon equals exactly 231 in³ by legal statute (US Code Title 15 §205). An Imperial gallon equals 277.4194 in³, derived from its definition of 4.54609 L. The 20% difference between the two gallon types is large enough to ruin recipes, overflow tanks, or underdeliver product.

This converter applies the exact statutory conversion factors for both US liquid gallons and Imperial gallons. It does not round intermediate values. Results are computed to up to 8 decimal places. Note: US dry gallons (268.8025 in³) are not included because they are rarely used outside grain trade contexts. Always verify which gallon standard your application requires before converting.

Formulas

The conversion from cubic inches to US gallons uses the exact legal definition established by the US National Institute of Standards and Technology (NIST):

V_{US gal} = V_in³231

For Imperial gallons, the conversion factor derives from the Imperial gallon's definition as exactly 4.54609 L, combined with the relationship 1 in³ = 0.016387064 L:

V_{imp gal} = V_in³277.4194

The reverse conversions multiply instead of divide:

V_in³ = V_{US gal} × 231

V_in³ = V_{imp gal} × 277.4194

Where V_in³ = volume in cubic inches, V_{US gal} = volume in US liquid gallons, and V_{imp gal} = volume in Imperial gallons. The factor 231 is exact by statute. The factor 277.4194 is rounded to 4 decimal places from the exact value 277.41943... which derives from 4.54609 ÷ 0.016387064.

Reference Data

Cubic Inches (in³)	US Gallons (gal)	Imperial Gallons (imp gal)	Litres (L)	Common Use Case
1	0.004329	0.003605	0.016387	Single cubic inch reference
14.4375	0.0625	0.05204	0.23659	1 US fluid ounce
28.875	0.125	0.10408	0.47318	1 US cup (half-pint)
57.75	0.25	0.20816	0.94635	1 US pint
115.5	0.5	0.41633	1.89271	1 US quart
231	1	0.83267	3.78541	1 US gallon (fuel jug)
277.4194	1.20095	1	4.54609	1 Imperial gallon
462	2	1.66535	7.57082	Small engine displacement
1155	5	4.16337	18.9271	5-gallon bucket
2310	10	8.32674	37.8541	Portable fuel tank
3465	15	12.4901	56.7812	Small aquarium
4620	20	16.6535	75.7082	Jerry can pair
5775	25	20.8168	94.6353	Industrial drum (quarter)
6930	30	24.9802	113.562	30-gallon trash can
8085	35	29.1436	132.489	Medium aquarium
9240	40	33.3070	151.416	Water heater (small)
11550	50	41.6337	189.271	50-gallon drum
12705	55	45.7971	208.198	Standard oil drum
17325	75	62.4506	283.906	Large aquarium
23100	100	83.2674	378.541	100-gallon tank
46200	200	166.535	757.082	IBC tote (half)
57750	250	208.168	946.353	IBC tote (standard)
69300	300	249.802	1135.62	Small fuel delivery
115500	500	416.337	1892.71	Large storage tank
231000	1000	832.674	3785.41	1000-gallon water tank

Frequently Asked Questions

The US gallon was inherited from the English wine gallon of 1707, legally defined as exactly 231 cubic inches. The Imperial gallon was redefined by the British Weights and Measures Act of 1824 as the volume of 10 pounds of distilled water at 62°F, which works out to approximately 277.42 cubic inches. The Imperial gallon is about 20.1% larger than the US gallon. This historical divergence means you must always specify which gallon system you are using when converting.

Engine displacement measures the total swept volume of all cylinders. A classic 350 cubic inch (5.7L) V8 engine displaces approximately 1.515 US gallons or 1.262 Imperial gallons per complete cycle. While displacement is rarely expressed in gallons in practice, the conversion is useful when calculating volumetric efficiency or comparing displacement to fuel tank capacity ratios for range estimation.

The US gallon conversion factor (231 in³/gal) is exact by legal definition and introduces zero rounding error. The Imperial gallon factor uses the value 277.4194 in³/gal, which is accurate to 4 decimal places. For volumes under 1,000,000 cubic inches, the accumulated rounding error is less than 0.001 Imperial gallons. The converter displays up to 8 decimal places, which exceeds the precision of most physical measurement instruments.

This tool focuses on the cubic inches to gallons relationship. However, you can chain the result: multiply US gallons by 3.785411784 to get litres, or multiply Imperial gallons by 4.54609 to get litres. The reference table above includes litre equivalents for common values. For direct conversion, 1 cubic inch equals exactly 0.016387064 litres by international agreement (since 1959).

The mathematical conversion factor is a fixed geometric relationship and does not change with temperature. However, the physical volume of a liquid measured in gallons does change with thermal expansion. Petroleum products expand approximately 0.06% per degree Fahrenheit. If you are converting a measured volume of fuel at a non-standard temperature (60°F is the US petroleum standard), you should apply a Volume Correction Factor (VCF) from ASTM Table 6 after converting units.

A US liquid gallon is exactly 231 cubic inches (3.78541 L). A US dry gallon is approximately 268.8025 cubic inches (4.40488 L), defined as 1/8 of a US bushel. The dry gallon is about 16.4% larger than the liquid gallon. Dry gallons are used almost exclusively in agriculture for grain and produce measurements. This converter handles liquid gallons only, as they account for over 99% of gallon usage in engineering, fuel, and everyday contexts.