Gas Mileage Calculator
Fill the tank, note the trip distance until the next fill-up and how much fuel it took to fill again. Enter those two numbers; the calculator gives your economy in miles per US gallon and litres per 100 km.
Answer
Fuel economy
30mpg
That is 7.84 litres per 100 km.
- miles per US gallon
- 30
- litres per 100 km
- 7.84
- km per litre
- 12.75
Check the work
Every line comes from the same calculation as the answer.
mpg = miles ÷ gallons · L/100 km = 235.215 ÷ mpg
| mpg | 360 ÷ 12 | 30 |
|---|---|---|
| L/100 km | 235.2146 ÷ 30 | 7.8405 |
How it works
The most reliable way to measure fuel economy is the fill-to-fill method. Fill the tank completely and reset the trip meter. When you next fill up, the fuel the pump puts in is the fuel you used for the distance on the meter.
Miles per gallon divides distance by fuel: 360 miles on 12 gallons is 30 mpg. Litres per 100 km divides fuel by distance and scales to 100 km: 39 litres over 520 km is 7.5 L/100 km. One is the reciprocal of the other, so a higher mpg is a lower L/100 km.
Converting between them uses two exact definitions: a US gallon is 3.785411784 litres and a mile is 1.609344 km. Together they give 235.215 ÷ mpg = L/100 km, which is the conversion shown in the working.
Formula
mpg = miles ÷ gallons · L/100 km = litres ÷ km × 100 · L/100 km = 235.215 ÷ mpg
Economy is distance per fuel in mpg, or fuel per distance in L/100 km; the conversion between them comes from the exact sizes of the gallon and the mile.
Worked examples
- Try these numbers
360 miles on 12 gallons
- 360 ÷ 12 = 30 mpg.
- 235.215 ÷ 30 = 7.84 L/100 km.
30 mpg, which is 7.84 L/100 km.
- Try these numbers
520 km on 39 litres
- 39 ÷ 520 × 100 = 7.5 L/100 km.
- 235.215 ÷ 7.5 = 31.36 mpg.
7.5 L/100 km, which is 31.36 US mpg.
- Try these numbers
10 litres per 100 km in mpg
- 10 ÷ 100 × 100 = 10 L/100 km.
- 235.215 ÷ 10 = 23.52 mpg.
10 L/100 km equals 23.52 US mpg.
Inputs and results
- Distance driven
- Miles or kilometres since the last fill-up.
- Fuel used
- US gallons or litres needed to fill the tank again.
- Miles per gallon
- Distance in miles divided by US gallons.
- Litres per 100 km
- Litres divided by kilometres, times 100.
When to use it
- Tracking whether your car’s economy changes after a service, new tyres or a change of driving style.
- Checking a trip computer’s figure against the pump.
- Converting an economy figure between mpg and L/100 km when comparing cars from different markets.
Common mistakes
- Not filling to the same level both times, which makes the fuel figure wrong for that distance.
- Comparing UK mpg with US mpg as if they were the same unit.
- Reading a lower L/100 km figure as worse economy; for that unit, lower is better.
What it does not do
- One tank gives one sample; economy varies tank to tank, so average several fill-ups for a steadier figure.
- It uses US gallons. A UK (imperial) gallon is larger, so UK mpg figures are about 20% higher for the same car.
- It does not apply to electric vehicles, which are measured in energy per distance.
Method and assumptions
Miles per gallon is distance divided by fuel; litres per 100 km is fuel divided by distance, multiplied by 100.
The two are converted with the factor 100 × 3.785411784 ÷ 1.609344 = 235.2145833…, using the exact legal definitions of the US gallon and the international mile.
Assumptions
- Fuel added at the second fill-up equals fuel used since the first.
- Gallons are US gallons.
Edge cases
- Distance or fuel of 0 is refused, because economy is undefined without both.
- Very small amounts of fuel give large mpg figures; check the tank was filled to the same level.
- Results are not rounded before conversion, so converting back returns the original figure.
Sources and corrections
This calculator uses an elementary formula, so it needs no outside data source. The method, the rounding rules and how a reported error is corrected are set out in the site standards.
Result looks wrong?
Send the inputs you used and the answer you expected. Every report is checked against the formula and its tests, and confirmed errors are corrected in public.