Gas Mileage (MPG) Calculator

How This Calculator Works

Gas mileage (MPG) is the distance a vehicle travels divided by the amount of fuel it uses, expressed in miles per gallon. Enter the distance you traveled and how much fuel you used to cover it, and this calculator returns your fuel economy — in MPG, and as the metric equivalent (liters per 100 kilometers) used in most of the rest of the world.

To measure your own vehicle’s real-world mileage: fill the tank completely, reset your trip odometer (or note the current reading), drive normally until you need to refuel, then fill up again and note exactly how much fuel it took. That fuel amount and the distance since your last fill-up are exactly what this calculator needs.

The Formula

$$\text{MPG} = \frac{\vA{\text{Distance (miles)}}}{\vB{\text{Fuel Used (gallons)}}}$$ $$\text{L/100km} = \left(\frac{\vB{\text{Fuel Used (L)}}}{\vA{\text{Distance (km)}}}\right) \times 100$$

Note that these two figures move in opposite directions for the same fuel efficiency: a higher MPG is better (more distance per gallon), while a lower L/100km is better (less fuel needed per 100 kilometers) — they aren’t simply the same number converted, since one measures “distance per fixed fuel” and the other measures “fuel per fixed distance.”

Worked Example

A trip of 100 km using 8 liters of fuel:

1. Converted to imperial units: about $\vA{62.1}$ miles using about $\vB{2.11}$ gallons.
2. MPG: $\vA{62.1} \div \vB{2.11} \approx 29.4 \text{ MPG}$.
3. Metric equivalent: $(\vB{8} \div \vA{100}) \times 100 = 8.0 \text{ L per 100 km}$.