How do I calculate my MPG?

Fill the tank, reset the trip meter, drive, then refill. Divide the miles driven by the gallons added at the second fill-up.

It depends on the car type. Compare your result with the EPA rating for your model on FuelEconomy.gov rather than with other kinds of cars.

How do I convert MPG to L/100km?

Divide 235.21 by US MPG (or 282.48 by UK MPG). For example, 30 US MPG is about 7.8 L/100km.

Is the car's trip computer accurate?

Often it's a few percent optimistic. The full-tank method over several fill-ups is usually more accurate.

Why is my MPG lower in winter?

Cold engines run less efficiently, winter fuel blends contain slightly less energy, and heaters, defrosters and lower tire pressure all add to fuel use.

MPG Calculator: Miles per Gallon & km/L / CarO

Written by Muneeb Rana · Last updated 21 October 2026

Quick answer: An MPG calculator divides the miles you drove by the gallons needed to refill the tank. Fill up completely, reset the trip meter, drive normally, then fill up again: MPG = miles driven ÷ gallons added. For example, 330 miles ÷ 11 gallons = 30 MPG. The same method gives km/L, or L/100km = litres ÷ km × 100.

TL;DR

Use the full-tank method across at least two fill-ups.
MPG = miles ÷ gallons; L/100km = litres ÷ km × 100.
Average several tanks for a reliable figure.

Use this MPG calculator to find your car's real fuel economy from your own fill-ups. Real-world MPG is usually different from the window-sticker figure, and knowing it helps you budget fuel, spot problems early and compare cars. It also works in km per litre and L/100km.

How to use the MPG calculator

Fill the tank completely and note the odometer, or reset the trip meter.
Drive normally until you need fuel again.
Fill the tank completely again and note the litres or gallons added.
Enter the distance and the fuel added.
Repeat over several tanks; the tool averages them.

How the MPG calculator works

mpg      = miles_driven / gallons_added
km_per_l = km_driven / litres_added
l_per_100km = litres_added / km_driven * 100

The fuel you add at the second fill-up is what the car used since the first one, which is why both fill-ups must be to full.

Worked example

Odometer at first fill: 24,210 miles. At the second fill: 24,540 miles, and 11.0 gallons went in. 330 ÷ 11.0 = 30.0 MPG (about 12.8 km/L or 7.8 L/100km).

Why your MPG may be lower than the sticker

EPA ratings come from standard lab tests. Short trips, cold weather, aggressive driving, roof boxes, under-inflated tires and extra weight all reduce real economy. FuelEconomy.gov says keeping tires properly inflated can improve mileage by about 0.6% on average and up to 3%, and using the manufacturer's recommended oil grade can improve it 1-2%. More in how to improve fuel economy.

A sudden MPG drop is a warning sign

If your economy falls noticeably with no change in driving, check tire pressures, look for a dragging brake, and consider a diagnostic check. See our warning lights guide and the step-by-step how to calculate MPG.

Logging fill-ups in CarO

CarO's fuel log records each fill-up with cost, odometer and fuel type, calculates MPG, and shows refueling reports per car. Use this calculator to check a single tank or convert units with the fuel economy converter. Estimate trips 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: Keeping Your Vehicle in Shape: www.fueleconomy.gov (accessed 21 October 2026)
FuelEconomy.gov: Driving More Efficiently: www.fueleconomy.gov (accessed 21 October 2026)
FuelEconomy.gov: Find and Compare Cars: 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.

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