How do I calculate gas mileage?

Fill the tank, record the odometer, drive normally, refill the tank, and divide miles driven by US gallons added. The result is miles per gallon.

How do I convert MPG to L/100 km?

For US MPG, divide 235.214583 by MPG. For example, 30 US MPG is approximately 7.84 L/100 km.

What is good gas mileage?

Good mileage depends on vehicle type, load, route, weather, and driving conditions. Compare a measured result with the rating and previous fill-ups for the same vehicle.

Is US MPG the same as imperial MPG?

No. An imperial gallon is larger than a US gallon, so the same fuel economy produces a numerically higher imperial MPG result.

Automotive calculator

Gas Mileage Calculator

Enter the distance driven and fuel added at your next fill-up. Get real-world US MPG, imperial MPG, L/100 km, km/L, fuel cost, and cost per mile.

Fill-up details

Distance driven

Distance unit

Fuel used

Fuel unit

Fuel price per US gallon

Optional for fuel-cost results.

Currency symbol

Fuel economy results

Real-world fuel economy—US miles per gallon

Imperial MPG—

Liters per 100 km—

Kilometers per liter—

Total fill-up cost—

Fuel cost per mile—

Fuel cost per kilometer—

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.

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.