How to Calculate MPG (Miles Per Gallon)
Miles Per Gallon (MPG) is the standard measurement of vehicle fuel economy in the United States and the United Kingdom. Here is the exact math behind MPG calculations and unit comparisons.
Miles driven divided by the quantity of gallons needed to replace the burned fuel.
The Basic MPG Formula
MPG is an intuitive metric because it tells you directly how much travel range you receive for each unit of fuel purchased.
To calculate MPG accurately, you need two data points from a single driving interval: the exact distance traveled in miles, and the gallons required to restore the fuel tank to its previous level.
Understanding US Gallons vs UK Imperial Gallons
A critical distinction that often confuses vehicle buyers comparing international automotive reviews is the volume of a gallon.
1 US Gallon = 3.78541 liters (231 cubic inches).
1 UK (Imperial) Gallon = 4.54609 liters (approx. 277.4 cubic inches).
Because a UK gallon is approximately 20.095% larger in volume, a car tested in the UK yielding 40 MPG UK achieves only 33.3 MPG US in identical physical conditions.
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