Gas Mileage Calculator
This gas mileage calculator finds actual miles per gallon from miles driven and US gallons used. Its second mode estimates the fuel needed for a distance when you already know the vehicle's MPG.
The same result is shown as liters per 100 kilometers, kilometers per liter, US MPG, and imperial MPG. Fuel price is kept out of this calculation so the page answers fuel-economy questions without mixing in trip-cost intent.
Gas mileage
30 mpg
300 ÷ 10 = 30 mpg
Metric economy: 12.75 km/L
US miles per gallon: 30 mpg
Imperial miles per gallon: 36.03 mpg
mpg = miles ÷ US gallons
How to calculate miles per gallon
Divide miles driven by US gallons used. A car that travels 300 miles on 10 gallons returns 300 ÷ 10 = 30 mpg.
For a reliable fill-up calculation, start with a full tank, reset the trip meter, drive normally, and refill to the same level. The gallons added at the second fill-up approximate the fuel used over the recorded distance.
Fuel needed for a trip
Divide trip distance by MPG. A 450-mile trip in a vehicle rated at 30 mpg requires 450 ÷ 30 = 15 US gallons under the assumed conditions.
The number does not include a reserve for detours, idling, weather, or a gap between the destination and the next gas station. Add that margin separately when planning a real trip.
Gas mileage examples
| Distance | Fuel used | Gas mileage |
|---|---|---|
| 240 miles | 8 US gal | 30 mpg |
| 300 miles | 10 US gal | 30 mpg |
| 350 miles | 14 US gal | 25 mpg |
| 420 miles | 12 US gal | 35 mpg |
| 600 miles | 20 US gal | 30 mpg |
US MPG, imperial MPG, and L/100 km
- US MPG uses the US liquid gallon, exactly 3.785411784 liters.
- Imperial MPG uses the 4.54609-liter imperial gallon, so the same car has a numerically higher imperial MPG.
- L/100 km measures fuel consumed rather than distance achieved; a lower value is better.
- 30 US mpg is about 7.84 L/100 km, 12.75 km/L, and 36.03 imperial mpg.
Why actual fuel economy changes
- Cold starts, short trips, and idling use fuel without adding much distance.
- High speed, headwinds, roof cargo, and low tire pressure increase resistance.
- Stop-and-go driving normally consumes more fuel than steady highway travel.
- A multi-tank average reduces the effect of pump shutoff differences and one unusual trip.
Frequently asked questions
How do I calculate MPG?
Divide miles driven by US gallons used. Driving 360 miles on 12 gallons gives 30 mpg.
How many gallons will I need for a trip?
Divide trip miles by the expected MPG. A 600-mile trip at 30 mpg needs 20 US gallons before any reserve.
What is the difference between US and imperial MPG?
The gallons differ in size. An imperial gallon is 4.54609 liters versus 3.785411784 liters for a US gallon, so imperial MPG is about 20% higher for identical consumption.
How do I convert MPG to L/100 km?
For US MPG, divide 235.214583 by MPG. For example, 30 US mpg is about 7.84 L/100 km.
Why does my calculated MPG differ from the dashboard?
A dashboard may estimate fuel flow continuously, while a fill-up calculation is affected by pump shutoff level and the specific trip. Average several tanks for a steadier result.
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Updated: 2026-08-28