Fuel Mileage Calculator

Background.

Miles per gallon is the one fuel figure drivers can measure for themselves, and a measured figure is more honest than the number on a window sticker. Fill the tank until the pump clicks off, zero the trip odometer, drive as you normally would, then fill again: the gallons the second fill takes are the fuel the car burned over the miles on the trip meter. Divide one by the other and you have your actual economy, cold starts, traffic queues and short errands included.

The arithmetic is a single division, so all the accuracy lives in the measurement. Pumps shut off at slightly different points, and half a gallon either way on a 12-gallon refill moves a 360-mile result from 30 mpg to 28.8 or 31.3. Adding up several tankfuls of miles and gallons shrinks that error to a few percent of a much larger total.

The result is also given as litres per 100 kilometres, the convention used in Europe, Canada and Australia. The two scales run in opposite directions. Miles per gallon counts distance per unit of fuel, so higher is better; litres per 100 km counts fuel per unit of distance, so lower is better. The inversion matters when you compare cars. Over 1,000 miles, going from 10 to 20 mpg saves 50 gallons, while going from 30 to 50 mpg saves only about 13.3, even though the second improvement looks bigger on the mpg scale.

The gallons here are US gallons of 3.785 litres. A British imperial gallon holds about 20% more, so the same car reads about 20% higher in UK mpg: 30 US mpg is roughly 36 imperial.

What is fuel mileage calculator?

Fuel mileage, also called fuel economy, is the distance a vehicle covers per unit of fuel burned, given here in miles per US gallon. It is an average over the distance you enter, so a figure from one tankful blends highway cruising, idling and hard acceleration in whatever mix that tank saw. Its reciprocal, fuel consumption, describes the same performance as the litres needed to cover 100 kilometres. Electric and plug-in vehicles are rated in MPGe instead, which this page does not compute.

How to use this calculator.

1. 01Fill the tank until the nozzle clicks off and reset the trip odometer, or write down the main odometer reading, before you drive away.
2. 02Enter the miles shown at your next fill-up as Distance traveled. If your odometer reads in kilometres, divide by 1.609 first.
3. 03Fill to the same automatic click-off and enter the gallons on the pump as Fuel consumed. If you stopped for fuel along the way, add those gallons too, because all of it was burned over that distance.
4. 04Read the mpg figure, and use the L/100 km figure when comparing against a European or Canadian rating. For a dependable average, total the miles and gallons from three or four tankfuls and enter the totals.

The formula.

Fuel mileage is distance divided by fuel: MPG = miles ÷ gallons. The metric figure turns that ratio upside down and rescales it: L/100 km = 235.214583 ÷ MPG. The constant comes from the exact unit definitions. One US gallon is 3.785411784 litres and one mile is 1.609344 kilometres, so 100 × 3.785411784 ÷ 1.609344 = 235.2146. Because consumption is the reciprocal of economy, equal steps in mpg are not equal steps in fuel used: 25 mpg is 9.41 L/100 km, 30 mpg is 7.84 and 40 mpg is 5.88. Of the two inputs, the fuel figure usually carries the larger error, since it is the smaller number and depends on where the pump stops. Fuel must be more than zero; a distance of zero reports zero for both results.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Department of Energy FuelEconomy.gov. Fuel economy guidance current 2026. Calculating and comparing fuel economy. Retrieved 2026-08-06.
2. [2]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
3. [3]OpenStax, Rice University. University Physics, 2016. Volume 1 mechanics and Volume 2 thermodynamics. Retrieved 2026-08-06.