How to calculate MPG
Fill the tank completely and reset the trip meter. At the next fill-up, record the distance traveled and the amount of fuel required to fill the tank again.
US MPG = miles driven ÷ US gallons used
For a more representative average, repeat the process over several fill-ups and divide total distance by total fuel. Do not simply average individual MPG results when the fill-up distances differ.
MPG and L/100 km conversion
MPG measures how far a vehicle travels on one gallon, so higher is better. L/100 km measures how much fuel is used over 100 kilometers, so lower is better.
L/100 km = 235.214583 ÷ US MPG
US and imperial MPG are not interchangeable because an imperial gallon is approximately 20% larger than a US gallon.
Example gas-mileage calculation
A car travels 350 miles and takes 11.15 US gallons to refill. Its measured fuel economy is 350 ÷ 11.15 = 31.39 US MPG, equivalent to approximately 7.49 L/100 km. At $3.50 per gallon, that fill-up costs $39.03 and the fuel cost is about $0.112 per mile.
Getting a reliable fill-up measurement
Consistency matters more than trying to make one tank perfectly precise. Refill at the same station and pump when practical, park with the vehicle facing the same direction, and stop at the first automatic pump shutoff rather than repeatedly topping off. Record the trip distance and fuel quantity immediately so neither value is lost or rounded from memory. If a fill is unusually small, incomplete, or affected by a tilted forecourt, combine it with the next fill before judging the result.
Seasonal fuel blends, cold starts, idling, roof racks, towing, tire pressure, traffic, and short trips can shift real-world consumption even when the vehicle is operating normally. Keep several consecutive records and calculate total distance divided by total fuel for the most useful trend. Compare like-for-like periods, such as similar routes or seasons, and investigate a persistent change rather than reacting to one tank. The calculator estimates fuel use from the values entered; it cannot determine whether a mechanical fault, measurement difference, weather, or driving pattern caused the result.
Frequently asked questions
Why is my measured MPG different from the dashboard?
Dashboard estimates may use injector and distance data, while fill-up calculations depend on pump shutoff, tank angle, and complete refill consistency. Compare several tanks rather than a single fill-up.
What changes gas mileage?
Speed, traffic, short journeys, temperature, wind, terrain, payload, towing, tire pressure, fuel blend, maintenance, and driving style can all affect consumption.
Should I use regular or imperial gallons?
Fuel sold in the United States is measured in US gallons. UK historical MPG figures use imperial gallons. Choose the unit that matches the amount you recorded.
Is this an official fuel-economy rating?
No. It calculates measured consumption from your inputs. Official test-cycle ratings use controlled procedures and may differ from real-world driving.
Privacy and limitations
All calculations run locally in your browser. Results depend on accurate distance and refill measurements and should be treated as estimates.