MPG calculator
Miles driven, gallons used, and the figure that falls out — plus the metric equivalents, because L/100 km is the fastest way to notice you have entered something wrong.
Work out your MPG
Fill the tank, drive, fill it again — then enter the two numbers.
The two starting values are a placeholder example, not a figure for any vehicle. Replace them with your own readings.
Measuring it properly
The tank-to-tank method, which is the only one that needs no trust in the trip computer
- Fill the tank until the pump clicks off, and stop there. The click is the only repeatable stopping point you have.
- Reset the trip meter, or write down the odometer reading.
- Drive normally until the tank is at least half used. A short sample exaggerates every error in the two fills.
- Fill up again at the same pump if you can, again to the first click. The gallons on that receipt are the fuel the distance consumed.
- Enter the miles and the gallons above. Repeat over several tanks and average them — one tank is a sample of one.
The dashboard readout is a useful running indicator, but it is an estimate made by the vehicle. The receipt is a measurement.
How the numbers are worked out
Including the unit conversions, spelled out
- MPG
miles driven ÷ US gallons used - Gallons for a trip
distance ÷ MPG - Gallons per 100 miles
100 ÷ MPG - L/100 km
235.2146 ÷ MPG (3.785412 L per gallon ÷ 1.609344 km per mile × 100) - km per litre
MPG × 1.609344 ÷ 3.785412
A US gallon is 3.785412 litres; an imperial gallon is 4.54609. A figure quoted in imperial gallons is about 1.2 times the same vehicle’s US figure, which is why UK and US numbers for the same car never match. This calculator works entirely in US gallons.
Divide by zero and you get an em dash, not a number. Leaving the gallons field empty produces no answer rather than a misleading one.
Why your figure will not match the sticker
Both numbers can be right and still disagree
EPA estimates come from standardised test procedures run under controlled conditions, so that every vehicle is measured the same way and the figures are comparable to each other. They are not a forecast of your commute.
- Short trips, cold starts, idling and stop-start traffic all consume fuel without adding many miles.
- Speed, roof racks, towing, a heavy load and low tyre pressure each cost economy.
- Winter fuel blends, cold air and running the heater or air conditioning change consumption seasonally.
- Two identical vehicles driven by two different people routinely return noticeably different figures.
A measured figure that is lower than the EPA estimate is not by itself evidence of a fault. If you believe a vehicle has a safety problem, the place to report it is NHTSA — and the reports other owners have already filed are on each vehicle’s page here.