The formula
UK gal = US gal × 0.832674184629
Where the factor comes from — each unit by its definition, then its size in SI:
1 US gal = 231 × in³ = 0.003785411784 m³1 UK gal = 4.54609 × L = 0.00454609 m³
Dividing the two SI sizes gives the factor: 0.832674184629, exact — every step above is a definition, not a measurement. The reverse factor is 1.2009499255.
The cubic metre is the SI unit of volume; the litre is exactly one thousandth of it. The US and UK gallons are different sizes, which is why a fuel-economy figure in mpg depends on which country printed it.
Worked examples
- A soft-drink bottle (2 L): 0.528344 US gal = 0.439938 UK gal — 0.528344 × 0.8326741846 = 0.439938
- A paint bucket: 5 US gal = 4.16337 UK gal — 5 × 0.8326741846 = 4.16337
- A water tank (1 m³): 264.172 US gal = 219.969 UK gal — 264.172 × 0.8326741846 = 219.969
- A drinks can (330 mL): 0.0871768 US gal = 0.0725899 UK gal — 0.0871768 × 0.8326741846 = 0.0725899
Frequently asked questions
How many UK (imperial) gallons is 1 US gal?
1 US gal = 0.832674184629 UK gal, exactly, from the unit definitions.
How do I convert UK (imperial) gallons back to US gallons?
Divide by 0.832674184629 — or multiply by 1.2009499255: 1 UK gal = 1.2009499255 US gal.