Gas Mileage Calculator
Gas mileage, or MPG, tells you how far your car travels on each gallon of fuel — the headline number for how efficient it is. This calculator works it out from two figures you can read straight off the trip computer or the pump: the miles you drove and the gallons you used. It also shows the same efficiency as litres per 100 km and as UK (imperial) MPG, so you can compare cars sold in different markets.
Calculate
Default result: 30.0
Gas Mileage Calculator · Result
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Gas mileage (US MPG)
300 × 10
- L/100km
- 7.84
- UK MPG (imperial gallon)
- 36.0
Reviewed by the calculators.dev team · Last updated 2026-06-24
Formula reviewed against U.S. EPA / fueleconomy.gov — MPG, and the 235.215 MPG↔L/100km conversion
How to calculate
Reset your trip meter when you fill up, drive normally, then refuel and note how many US gallons it took. Enter the miles driven and the gallons used, and the calculator divides miles by gallons for your MPG. Driving 300 miles on 10 gallons is 30 MPG. The L/100km figure is the inverse measure used in most of the world — lower is better — and the UK MPG figure uses the larger imperial gallon.
US MPG = miles ÷ gallons. The metric equivalent is L/100km = 235.215 ÷ US MPG, an INVERSE relationship — as MPG goes up, L/100km goes down. UK MPG = US MPG × 1.20095, because one imperial gallon is about 1.2 US gallons. For 300 miles on 10 gallons: 30 US MPG, about 7.84 L/100km, and about 36.0 UK MPG.
Example calculation
Driving 300 miles on 10 US gallons gives 30 miles per gallon. That is the same as about 7.84 litres per 100 km and roughly 36.0 UK MPG, because the imperial gallon is larger than the US gallon. A higher MPG — or a lower L/100km — means a more efficient car.
- mpgUS
- 30.0
- lPer100km
- 7.84
- mpgUK
- 36.0
Assumptions
- Gallons are US gallons (3.785 litres); the UK MPG figure converts to the larger imperial gallon (4.546 litres).
- The result reflects the specific stretch of driving you measured; city and highway figures differ.
- A single tank's measurement is a snapshot — average several tanks for a more reliable figure.
Common mistakes
- Mixing up US and UK gallons — the same car shows a higher MPG in UK gallons because the imperial gallon is larger.
- Forgetting to reset the trip meter at the fill-up, so the miles do not match the gallons used.
- Treating L/100km like MPG — with L/100km a LOWER number is more efficient, the opposite of MPG.
Frequently asked questions
How do I calculate my car's MPG?
Fill the tank and reset the trip meter, drive, then refuel and note the gallons. Divide the miles driven by the gallons used. 300 miles on 10 gallons is 30 MPG.
How do I convert MPG to L/100km?
Divide 235.215 by the US MPG. It is an inverse relationship, so 30 MPG becomes about 7.84 L/100km. A higher MPG always gives a lower L/100km.
Why is UK MPG higher than US MPG?
The imperial (UK) gallon is about 20% larger than the US gallon, so a car travels further on one UK gallon. Multiply US MPG by 1.20095 to get UK MPG.
Is one tank enough to judge efficiency?
It gives a rough figure. Because driving conditions vary, averaging several tanks gives a more reliable picture of your real-world fuel economy.