Fuel Average Calculator
Work out your real fuel economy from distance and fuel used, or from odometer readings, as km/L, L/100 km, US MPG and UK MPG.
The formula
Fuel economy is distance divided by fuel used. In metric that is km/L = km ÷ liters, and L/100 km = liters ÷ km × 100. In miles and gallons, MPG = miles ÷ gallons. The calculator converts everything to kilometers and liters first (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L) and then shows the same trip in all four common forms.
In odometer mode the distance is the end reading minus the start reading.
Worked example
You drive 500 km and the pump then takes 40 liters to refill the tank. That is 500 ÷ 40 = 12.50 km/L, and 40 ÷ 500 × 100 = 8.00 L/100 km. In miles per gallon the same trip is 12.5 × 3.785411784 ÷ 1.609344 = 29.40 MPG (US) and 12.5 × 4.54609 ÷ 1.609344 = 35.31 MPG (UK).
How to measure it properly
- Fill the tank until the pump clicks off, and note the odometer. Drive as normal.
- At the next fill-up, fill until the pump clicks off again. Enter the odometer reading from the first fill as the start, the current reading as the end, and the liters or gallons the second fill took.
- The fuel you enter is the amount added at the second fill, not the amount from the first.
- Averaging over a few tanks gives a steadier figure than one tank, because the pump's click-off point varies.
US gallons, UK gallons and why the MPG numbers differ
An imperial gallon is about 20% bigger than a US gallon, so the same car has a higher MPG figure in UK units. The calculator shows both so you can compare with published figures from either country. When you choose US or UK units, the gallons you enter are taken to be that kind of gallon.
Limits
- It treats your numbers as exact. If you did not fill to the same level both times, the result is only an estimate.
- Results update as you type, and each figure is rounded to two decimals.
- A trip computer or dashboard average may differ from this figure, because it estimates what the engine used rather than what the pump delivered.
- Changing the units box does not convert the numbers already typed in.
More worked examples
US units: 300 miles on 10 US gallons is 30.00 MPG (US), which is also 36.03 MPG (UK), 12.75 km/L and 7.84 L/100 km.
UK units: 300 miles on 8 imperial gallons is 37.50 MPG (UK), 31.23 MPG (US), 13.28 km/L and 7.53 L/100 km. The same trip looks better in UK MPG because the gallon is bigger.
Odometer mode: 0 to 350 km on 28.5 liters gives 12.28 km/L, 8.14 L/100 km and 28.89 MPG (US). An efficient car, 120 km on 3 liters, reaches 40.00 km/L, which is 2.50 L/100 km or 94.09 MPG (US).
Converting between the forms
The four figures describe the same trip: 8.00 L/100 km is 12.50 km/L, 29.40 MPG (US) and 35.31 MPG (UK). Because L/100 km is the inverse of km/L, a bigger MPG or km/L is better, while a smaller L/100 km is better.
Errors you may see
- In odometer mode, an end reading equal to or lower than the start shows "The end reading must be higher than the start reading."
- Fuel of 0 shows "Please enter valid, positive values."
- A blank distance shows "Please fill in the distance driven and the fuel used." A start reading of 0 is fine.
- In odometer mode a "Distance driven" line is added at the top of the results.
Frequently asked questions
How do I find my car's real fuel consumption?
Fill the tank until the pump clicks off and note the odometer. Drive, then fill until it clicks off again, at the same pump if you can. Enter the two odometer readings and the amount of fuel the second fill took.
What is the difference between km/L and L/100 km?
They describe the same thing from opposite ends. km/L is how far you go on one liter, so a bigger number is better. L/100 km is how much fuel you use for 100 km, so a smaller number is better. 100 ÷ km/L gives L/100 km: 100 ÷ 12.5 = 8.
Why is my MPG different in US and UK units?
An imperial gallon is 4.54609 liters and a US gallon is 3.785411784 liters, so the same car scores about 20% higher in UK MPG. Check which gallon a quoted figure uses before comparing.
Can I use it for a motorbike or a truck?
Yes. It only needs a distance and an amount of fuel, so any vehicle works. On a bike with a small tank, measure over several fill-ups, because the pump's rounding is a bigger share of a small fill.
What is 8 L/100 km in MPG?
29.40 MPG in US gallons and 35.31 MPG in imperial gallons (500 km on 40 liters). The two MPG figures differ because the gallons differ.
Do I need to change the units box if I only know the odometer?
Choose the unit your odometer and fuel gauge use. Metric takes kilometers and liters, US takes miles and US gallons, and UK takes miles and imperial gallons. The labels on the input boxes change with your choice.
Can the odometer start at zero?
Yes. A start reading of 0 is accepted, for example 0 to 350 km. Only an end reading that is not above the start is rejected.