Measure your car’s fuel economy. This free miles per gallon calculator turns miles driven and fuel used into MPG, with L/100km, km/L and imperial MPG, plus an odometer mode.
Fill the tank, drive, then refill. Enter the miles driven (or your odometer readings) and the gallons it took to refill. That gives your real-world MPG.
MPG = miles / gallons. The US gallon (3.785 L) is smaller than the UK gallon (4.546 L), so US and imperial MPG differ for the same car – both are shown. Lower L/100km means better efficiency.
What This Miles Per Gallon Calculator Does
This calculator turns miles driven and fuel used into your real-world miles per gallon (MPG), converting the result to L/100km, km/L, and imperial (UK) MPG at the same time. An odometer mode lets you enter start and end readings instead of a pre-calculated distance.
The Core Formula
MPG = miles driven / gallons used
Driving 420 miles on 12 gallons of fuel gives exactly 35 MPG – equivalent to about 6.72 L/100km, 14.88 km/L, or 42.03 MPG using the imperial (UK) gallon.
Why US and UK MPG Differ for the Same Car
A US gallon is about 3.785 litres, while a UK (imperial) gallon is larger at about 4.546 litres. Since MPG is miles per gallon, a car reported in imperial MPG will show a higher number than the exact same car reported in US MPG – not because it’s more efficient, but purely because the “gallon” unit is bigger.
How to Get an Accurate Real-World MPG Reading
- Fill the tank completely and reset or note your odometer.
- Drive normally until the tank needs refilling again.
- Refill completely and note exactly how many gallons it took.
- Enter the miles driven and gallons used for your true MPG.
Why This Often Differs From the Sticker MPG
Official EPA or manufacturer MPG ratings are measured under standardized test conditions. Real-world driving – city traffic, highway speed, weather, cargo load, and driving style – almost always produces a somewhat different figure, which is exactly why tracking your own MPG over a full tank gives a more personally accurate number.
How to Use the MPG Calculator
- Choose miles driven or odometer mode.
- Enter your distance and the fuel used to refill.
- Read your MPG in US, imperial, and metric units.
Want to estimate your fuel budget too? See our Fuel Cost Calculator.