Fuel Consumption Calculator (MPG)

How to measure your mpg

The most reliable method is the full-tank method:

1. Fill the tank until the pump clicks off and reset the trip odometer.
2. Drive normally, ideally until the tank is at least half empty.
3. Fill up again, preferably at the same pump, and note the gallons on the receipt and the miles on the trip odometer.
4. Keep the units on miles and US gallons and enter both numbers. Add the price per gallon if you want cost figures.

The gallons that refilled the tank are exactly what you burned over those miles. Averaging several tanks (add up all the miles and all the gallons) smooths out small differences in where the pump stops. If you have liters and kilometers, for example from a rental car abroad, switch the units to metric.

How the calculator works

Fuel economy is distance divided by fuel; fuel consumption is the reverse:

mpg = miles ÷ gallons
L/100 km = liters ÷ km × 100

To convert, the calculator uses 1 mile = 1.609344 km, 1 US gallon = 3.785412 liters and 1 imperial gallon = 4.54609 liters, the factors published by NIST. UK mpg uses the larger imperial gallon, so the same car shows a figure about 20% higher in UK mpg than in US mpg. With a price entered, the cost per mile is the total paid (gallons × price) divided by the miles, and the cost per 100 miles is the same figure times 100. In metric units these become cost per km and per 100 km.

Examples

1. One tank: 320 miles on 11.5 gallons

Your car averaged 27.8 mpg, which is 8.45 L/100 km or 33.4 UK mpg. At $4.35 a gallon, close to the U.S. average for regular of $4.354 reported by the EIA for the week of October 5, 2026, fuel costs $15.63 per 100 miles.

2. Three tanks averaged

Over three fill-ups you drove 1,050 miles and pumped 38.2 gallons in total: 27.5 mpg and 8.56 L/100 km, or $15.83 per 100 miles at the same price.

3. A rental car in Europe

You drove 500 km and refilled 35 liters. That is 7 L/100 km, or 33.6 US mpg and 40.4 UK mpg.

Your mpg vs. the EPA rating

EPA ratings come from standardized tests and are meant for comparing vehicles, not for predicting your own mileage. How and where you drive, vehicle condition and maintenance, fuel variations and even engine break-in all change the result. According to fueleconomy.gov, aggressive driving can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic, and an extra 100 pounds in the car can cut mpg by about 1%.

Also note that mpg is not linear. Over 10,000 miles, going from 15 to 20 mpg saves about 166.7 gallons, while going from 35 to 50 mpg saves about 85.7. Gallons per mile, or L/100 km, shows fuel use directly.