How to Calculate MPG (Step by Step)
Written by Muneeb Rana · Last updated 21 October 2026
Quick answer: To calculate MPG, fill the tank completely and reset the trip meter, drive normally until you need fuel, then fill up completely again. Divide the miles driven by the gallons you just added: 330 miles ÷ 11 gallons = 30 MPG. For metric, divide km by litres for km/L, or litres ÷ km × 100 for L/100km.
TL;DR
- Full tank, reset trip, drive, full tank again.
- MPG = miles ÷ gallons added.
- Average several tanks for accuracy.
Learning how to calculate MPG gives you your car's real-world fuel economy, which is more useful than the sticker figure for budgeting and spotting problems. It takes two fill-ups and one division.
Step by step
- Fill the tank until the pump clicks off. Don't top up past the click.
- Reset the trip meter, or write down the odometer.
- Drive normally until the tank is at least half empty.
- Fill up again to the click, at the same pump if possible.
- Note the gallons or litres added and the miles or km driven.
- Divide. Or use the MPG calculator.
Formulas
| Unit | Formula | Example |
|---|---|---|
| US MPG | miles ÷ US gallons | 330 ÷ 11 = 30 |
| UK MPG | miles ÷ imperial gallons | 330 ÷ 9.16 = 36 |
| km/L | km ÷ litres | 531 ÷ 41.6 = 12.8 |
| L/100km | litres ÷ km × 100 | 41.6 ÷ 531 × 100 = 7.8 |
Convert between them with the fuel economy converter.
Common mistakes
Topping off past the click, forgetting a partial fill-up in between (add all fuel bought in the period), using the trip computer instead of real numbers, and judging from one tank. Average at least three tanks.
Why your MPG differs from the sticker
EPA ratings come from standardised tests. Real-world economy depends on speed, traffic, weather, loads and maintenance. FuelEconomy.gov lists factors like aggressive driving, roof racks and under-inflated tires; see how to improve fuel economy.
Keep a fuel log
Logging every fill-up makes the calculation automatic over time; see how to keep a fuel log. CarO's fuel log records cost, odometer and fuel type per fill-up and calculates MPG for you. Cost a trip with the fuel cost calculator.
Download CarO free on the App Store or Google Play. Free with ads; optional in-app purchases.
Sources
- FuelEconomy.gov: Find and Compare Cars: www.fueleconomy.gov (accessed 21 October 2026)
- FuelEconomy.gov: Keeping Your Vehicle in Shape: www.fueleconomy.gov (accessed 21 October 2026)
- FuelEconomy.gov: Driving More Efficiently: www.fueleconomy.gov (accessed 21 October 2026)
Disclaimer: CarO and this website give general car-care information, not professional advice. Maintenance intervals and specifications vary by vehicle, so always follow your owner's manual and a qualified mechanic. Mileage and cost figures are estimates; check local tax rules before using any log for deductions.
FAQ
How do you calculate MPG?
Divide the miles driven between two full fill-ups by the gallons added at the second fill-up.
How do you calculate L/100km?
Divide the litres added by the kilometres driven, then multiply by 100.
Is the car's MPG display accurate?
Often a few percent optimistic. The full-tank method over several tanks is more reliable.
How many tanks should I average?
At least three, ideally more, covering your normal mix of driving.
Why did my MPG suddenly drop?
Check tire pressure, recent driving changes, cold weather or extra weight. A persistent drop can signal a mechanical issue.
Related guides
Track this in CarO
CarO is a free car maintenance tracker app for iPhone and Android that keeps a full service history, reminds you about oil changes and inspections by mileage or date, and logs fuel fill-ups with MPG, odometer readings and car expenses for every vehicle you own. Logs are stored in the cloud and can be exported as CSV.
