Guide
How to calculate your car's fuel consumption
To calculate fuel consumption in liters per 100 km, use the full-to-full method: fill the tank, note the odometer, and at the next full fill-up divide the liters you filled in between by the kilometers driven, then multiply by 100.
The formula
consumption [l/100 km] = liters filled ÷ km driven × 100
Use the liters shown on the pump, not the amount paid — fuel prices change, liters don’t. Cost per kilometer is a separate formula: money spent ÷ kilometers driven.
Step by step
- Fill the tank — to the first click of the nozzle, without topping up.
- Note the odometer reading or reset the trip meter.
- Drive as usual. If you make partial fill-ups along the way, note the liters from each of them.
- At the next full fill-up, note the liters and the odometer.
- Add up the liters from every fill-up after the first one (including the last), divide by the kilometers driven and multiply by 100.
Worked example
You fill the tank at 84,200 km. Along the way you add 20 l, and at 84,810 km you fill up again and it takes 22.5 l.
Distance: 84,810 − 84,200 = 610 km. Fuel used in that stretch: 20 + 22.5 = 42.5 l. Consumption: 42.5 ÷ 610 × 100 = 6.97 l/100 km.
At €1.65 per liter, that stretch cost 42.5 × 1.65 ≈ €70.13, or about €0.11 per kilometer (example prices).
Watch out for
- Both ends must be full fill-ups. If the last one was partial, the result is random — wait for the next full fill-up.
- The first full fill-up is only the starting point. Don’t count its liters — that fuel was burned before.
- Partial fill-ups in between count towards the total — leaving them out makes the result too low.
- A single stretch can be skewed by winter, traffic, the route or a different nozzle click. An average over several stretches is more reliable.
- The trip computer gives an estimate. The liters on the pump tell you how much fuel you actually bought.
- In a car running on LPG, track gas and petrol separately — two tanks, two consumption figures.
- Prefer miles per gallon? Divide 235.2 by the l/100 km figure for US mpg, or 282.5 for UK mpg.
How TankLens does it
TankLens uses the same method: between two full-tank fill-ups with an odometer reading, it divides the liters filled in that stretch by the kilometers driven. It reads the liters, price and amount from a photo of the pump display and checks that the numbers add up. Until there are two such fill-ups it shows “—”, not a made-up zero.