Gas Mileage Calculator

MPG = miles driven ÷ gallons added

mi

Miles driven since the previous fill-up

gal

Fuel added at this fill-up, in US gallons

L/100 km

The same fuel economy, written the other way round

Use it at every fill-up to track fuel economy over time, and before a long drive to work out the cost per mile for a route. If you already know the fuel economy and only want it in another unit, that is a conversion rather than a calculation — the same division, run backwards.

1. 300 miles on 10 US gallons

   1. Distance driven: 300 miles
   2. Fuel added: 10 US gallons
   3. Fuel economy: 300 ÷ 10 = 30 mpg
   4. In imperial gallons: 30 × 1.20095 = 36.03 mpg
   5. In metric: 300 × 1.609344 = 482.8 km, and 10 × 3.785411784 = 37.85 L
   6. That is 482.8 ÷ 37.85 = 12.75 km/L, or 100 ÷ 12.75 = 7.84 liters per 100 km

   Both numbers must come from the same pair of fill-ups. Reset the trip meter halfway through and the gallons you add at the end no longer match the miles on it.

2. 400 miles on 12 gallons — a smaller car

   1. Distance driven: 400 miles
   2. Fuel added: 12 US gallons
   3. Fuel economy: 400 ÷ 12 = 33.33 mpg
   4. In imperial gallons: 33.33 × 1.20095 = 40.03 mpg
   5. In metric: 400 × 1.609344 = 643.74 km, and 12 × 3.785411784 = 45.42 L
   6. That is 643.74 ÷ 45.42 = 14.17 km/L, or 100 ÷ 14.17 = 7.06 liters per 100 km

   Three miles per gallon better than the first example, and only 0.78 liters per 100 km apart — which is why the metric figure looks like a small difference and the mpg figure does not.

One fill-up measures a tank, not a car. A week of motorway driving and a week of short cold trips give very different numbers from the same vehicle, so a single figure is a starting point rather than a verdict. The tank has to be filled to the same level both times, or the gallons added will not match the miles between them; stopping at the pump's first click is the usual way to make that repeatable. The odometer and the pump are each slightly off, and a one percent error in either moves the answer by about one percent. Above all this is measured fuel economy, which is expected to differ from the certified figure for the model — that one comes from a test cycle, not from a road.

How do I calculate gas mileage?

Divide the miles driven since the last fill-up by the gallons added at this one. 300 miles on 10 gallons is 300 ÷ 10 = 30 mpg. Both numbers have to come from the same pair of fill-ups, and the tank should be filled to the same level each time.

What counts as good mpg?

It depends far more on the vehicle and the roads than on any single number. A compact car on a motorway will beat 40 mpg; a pickup towing a trailer will be in the teens. That is exactly why tracking your own fuel economy matters — it is the only baseline that says anything about your driving rather than about a test cycle.

How do I convert mpg to liters per 100 km?

Divide 235.2146 by the mpg figure. So 30 mpg is 235.2146 ÷ 30 = 7.84 liters per 100 km. The constant is not a measurement: it is 100 × 3.785411784 ÷ 1.609344, the liters in a US gallon over the kilometers in a mile.

Why do imperial mpg figures look higher?

Because the imperial gallon is bigger — 4.54609 liters against 3.785411784 for the US gallon. The same consumption is therefore about 20% more miles per gallon in imperial units, so 30 mpg here is 36.03 mpg in the UK. Nothing about the car has changed.

What does the trip cost per mile?

Multiply the gallons you used by the price per gallon, then divide by the distance. Ten gallons at 3.50 is 35.00, and over 300 miles that is 35.00 ÷ 300 = about 0.12 a mile. Keep the currency in whatever your pump quotes — the arithmetic is the same in any of them.

• Fuel Economy — U.S. Department of Energy and Environmental Protection Agency
• NIST Guide to the SI, Special Publication 811 — US National Institute of Standards and Technology