Plot No. 33 · Everyday & Shopping
Gas Mileage (MPG) Calculator
Work out your fuel efficiency from miles driven and US gallons used. Miles per gallon is simply distance divided by fuel, so 300 miles on 12 gallons is 25 MPG. It also converts that to litres per 100 km and kilometres per litre for easy comparison with metric figures.
Figures use US gallons (3.78541 litres). For a UK gallon, which is larger, your MPG number would be higher for the same fuel.
The link saves your inputs so you can bookmark or share this exact result.
MPG for common miles and gallons
Miles per gallon is one division: miles driven divided by gallons used. Drive 350 miles on 14 US gallons and that is 350 divided by 14, which is exactly 25 MPG. Any pair of numbers works in the calculator above; these are the fill-ups people look up most.
| Miles driven | US gallons used | MPG | Litres per 100 km |
|---|---|---|---|
| 200 | 10 | 20.0 | 11.76 |
| 250 | 10 | 25.0 | 9.41 |
| 300 | 12 | 25.0 | 9.41 |
| 300 | 10 | 30.0 | 7.84 |
| 350 | 14 | 25.0 | 9.41 |
| 400 | 16 | 25.0 | 9.41 |
| 400 | 12 | 33.3 | 7.06 |
| 450 | 15 | 30.0 | 7.84 |
| 500 | 20 | 25.0 | 9.41 |
| 500 | 15 | 33.3 | 7.06 |
MPG converted to litres per 100 km, km per litre and UK MPG
A US gallon is 3.78541 litres and a mile is 1.609344 km, so litres per 100 km is 235.214 divided by your MPG. That is a reciprocal, not a scale: lower litres per 100 km is better, while higher MPG is better. A UK (imperial) gallon is 4.54609 litres, about 1.201 times a US gallon, so the same car always shows a higher MPG number in UK figures than in US ones. MPG measures efficiency rather than money, so to turn one of these figures into the cost of a journey you also need the pump price: the fuel cost calculator does that part.
| US MPG | Litres per 100 km | Km per litre | UK (imperial) MPG |
|---|---|---|---|
| 15 | 15.68 | 6.38 | 18.0 |
| 20 | 11.76 | 8.50 | 24.0 |
| 25 | 9.41 | 10.63 | 30.0 |
| 30 | 7.84 | 12.75 | 36.0 |
| 35 | 6.72 | 14.88 | 42.0 |
| 40 | 5.88 | 17.01 | 48.0 |
| 45 | 5.23 | 19.13 | 54.0 |
| 50 | 4.70 | 21.26 | 60.0 |
| 55 | 4.28 | 23.38 | 66.1 |
| 60 | 3.92 | 25.51 | 72.1 |
Rounded for display. Figures are calculated from the exact conversions (3.78541 litres per US gallon, 4.54609 litres per UK gallon, 1.609344 km per mile), so a UK MPG figure quoted for a car is not directly comparable with a US one.
How miles per gallon is worked out
This calculator divides the miles you drove by the US gallons of fuel you used to give your miles per gallon. Drive 300 miles on 12 gallons and you get 25 MPG. A higher MPG means your car travels further on each gallon.
It also converts that figure into metric terms so you can compare against European fuel data. Using a US gallon of 3.78541 litres and a mile of 1.609344 km, 12 gallons over 300 miles is 45.42 litres over 482.8 km, which is about 9.41 litres per 100 km, or roughly 10.63 km per litre. Note this measures efficiency only, not what a trip costs, for which you would use our fuel cost calculator.
How to use the gas mileage calculator
- Fill your tank, reset the trip meter, then drive normally until your next fill-up.
- Enter the miles shown on the trip meter and the US gallons it took to refill.
- Read your MPG along with the litres per 100 km and kilometres per litre conversions.
Worked examples
A typical tank
Inputs: 300 miles driven on 12 US gallons
Result: 25.0 MPG, which is about 9.41 litres per 100 km and 10.63 km per litre, a fairly average figure for a midsize car.
An efficient run
Inputs: 420 miles driven on 10 US gallons
Result: 42.0 MPG, around 5.60 litres per 100 km, the kind of figure a small or hybrid car returns on a motorway.
Limitations and common mistakes
Edge cases and limitations
- Figures use US gallons (3.78541 litres). A UK gallon is larger, so the same fuel would show a higher MPG number.
- A single tank reflects that journey only; mixed city and motorway driving over several tanks gives a truer average.
- It measures efficiency, not cost, so it does not factor in the price of fuel.
Common mistakes
- Mixing up US and UK gallons, which throws the result off by about 20 percent.
- Forgetting to reset the trip meter at the previous fill-up, so the distance does not match the fuel.
Frequently asked questions
How do I calculate miles per gallon?
Divide the miles you drove by the gallons of fuel you used. Driving 300 miles on 12 US gallons gives 300 divided by 12, which is 25 MPG. Fill up, reset the trip meter, then refill and note both numbers for an accurate reading.
How does MPG convert to litres per 100 km?
A US gallon is 3.78541 litres and a mile is 1.609344 km. 25 MPG means 12 gallons (45.42 litres) over 300 miles (482.8 km), which works out to about 9.41 litres per 100 km. Lower litres per 100 km means a more efficient car.
Does this measure my fuel cost?
No, this measures efficiency only, not what a journey costs. For the price of a trip you need the fuel price too. Use our separate fuel cost calculator for that; this page tells you how far your car goes on each gallon.
What is 30 MPG in litres per 100 km?
About 7.84 litres per 100 km. The shortcut is to divide 235.214 by your MPG figure, so 30 MPG gives 235.214 divided by 30. It works the other way too: 235.214 divided by litres per 100 km gives you MPG back. Note that it is a reciprocal, so higher MPG is better but lower litres per 100 km is better.
Why is UK MPG higher than US MPG for the same car?
Because a UK (imperial) gallon is 4.54609 litres against a US gallon's 3.78541, about 1.201 times larger. A bigger gallon carries the car further, so the same car scores about 20 percent higher in UK figures. A car rated 30 MPG in the US is roughly 36 MPG in the UK. This calculator uses US gallons, so multiply by 1.201 to compare with a UK figure.
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