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MPG calculator

Fill the tank, reset the trip meter, drive, then fill up again. Enter the miles and the gallons it took to see your real fuel economy in mpg, L/100 km, km/L and UK mpg.

Your fill-up

Distance driven

The trip meter reading since the last full tank. Enter a number above zero.

Fuel to fill up

gal

From the pump or the receipt. Enter a number above zero.

Fuel economy converter

Value

Enter a number above zero.

Unit

How to measure your mpg

Fill the tank until the pump clicks off, and reset the trip meter.
Drive as you normally do, ideally for most of the tank.
Fill up again the same way, at the same pump if you can, and note the gallons.
Enter the trip meter reading and the gallons above.

The fuel you put in the second time is the fuel you burned since the first fill. Repeat over a few tanks for a steadier number.

The formulas

mpg = miles driven ÷ gallons used

The other units follow from the exact definitions of the mile (1.609344 km), the US gallon (3.785411784 L) and the UK gallon (4.54609 L):

L/100 km = 235.21 ÷ mpg (US) km/L = 100 ÷ (L/100 km) mpg (UK) = mpg (US) × 1.20095

Watch the direction: mpg and km/L go up as the car gets more efficient, but L/100 km goes down.

Worked example

You drove 320 miles and it took 11.5 gallons to fill up. 320 ÷ 11.5 = 27.8 mpg. That is 8.45 L/100 km, 11.83 km/L and 33.4 mpg in UK gallons.

In metric: 500 km on 40 liters is 40 ÷ 500 × 100 = 8.00 L/100 km, or 29.4 mpg (US).

US and UK mpg are not the same

A UK gallon is about 20% bigger than a US gallon, so the same car shows a higher mpg in UK figures. 30 mpg (US) is 36.0 mpg (UK). When you compare numbers from a UK source, convert them first with the converter above.

Why one tank can mislead

Pumps click off at slightly different levels, so a single fill-up can be a few percent off. Short trips, cold starts, idling and a roof box pull the number down. Keep a note of each fill-up and add the miles and gallons across several tanks for a better average.

To turn your mpg into a trip budget, use the fuel cost calculator.

Questions

How do I calculate my gas mileage?

Divide the miles driven since your last full tank by the gallons it took to fill up again. 320 miles on 11.5 gallons is 27.8 mpg.

How do I convert mpg to L/100 km?

Divide 235.21 by the mpg (US). 30 mpg is 7.84 L/100 km. The same formula works the other way: 8 L/100 km is 29.4 mpg.

Why is UK mpg higher than US mpg?

The UK gallon is 4.54609 liters and the US gallon is 3.785411784 liters, so a UK mpg figure is 1.20095 times the US one. 30 mpg (US) equals 36.0 mpg (UK).

Why does my mpg differ from the trip computer?

The trip computer estimates fuel use from engine data; the pump measures it. Small differences are normal. A fill-to-fill check over a few tanks is the more direct measurement.