To calculate fuel consumption, divide the litres added after your starting full tank by the kilometres driven, then multiply by 100. Finish the measurement by filling the tank completely again. Count every fill-up in between and the final full refill; exclude the litres in the starting fill. This gives an approximate result in litres per 100 kilometres for one vehicle using one fuel.
Fuel consumption calculator
The inputs you need
Use these two figures:
- Distance between full fills (km): the ending odometer reading minus the starting reading.
- Fuel added after the starting full fill (L): the total litres from every intermediate refill and the final full refill, excluding the starting fill.
Both values must be greater than zero. A decimal comma or point is acceptable, such as 50,5 or 50.5. You need a completed full-to-full period, every refill recorded, and records for one vehicle using one fuel. Display the result to two decimal places.
The L/100 km formula
Fuel consumption (L/100 km) = included litres ÷ distance driven (km) × 100
“Included litres” means all fuel added after the starting full fill, up to and including the final full refill. Distance must cover exactly the same period. Subtract the starting odometer reading from the ending reading to find it.
For example, 50 L ÷ 700 km × 100 = 7.14 L/100 km. Over that stretch, the car used approximately 7.14 litres for every 100 kilometres driven. This measures fuel use, rather than a cost in euros or the full cost of owning the car.
How to use the full-to-full method
Hong Kong’s Electrical and Mechanical Services Department (EMSD) describes measuring between two full tanks and using the same litres-and-distance formula. Apply it to your own records as follows:
- Fill the tank completely and record the odometer reading. This establishes the starting point. Leave the litres from this fill out of your total.
- Drive and record every subsequent fill-up. Keep the litres from each receipt, including any partial refills.
- Fill the tank completely again to end the measurement. Record the new odometer reading and the litres added.
- Add all litres purchased after the starting fill. Include every intermediate fill-up and the final full refill.
- Calculate the distance and apply the formula. Use records for the same vehicle, the same fuel and the same measurement period.
Worked example with a partial refill
Suppose the odometer reads 80,000 km when you establish your starting full tank. Later, you add 20 L without filling the tank completely. At 80,700 km, you fill it completely again, adding another 30 L.
- Starting full fill at 80,000 km
- Starting odometer reading only; exclude its litres
- Intermediate partial refill: 20 L
- Include all 20 L
- Final full refill: 30 L at 80,700 km
- Include 30 L and the ending odometer reading
Distance: 80,700 − 80,000 = 700 km. Included fuel: 20 + 30 = 50 L. Consumption: 50 ÷ 700 × 100 = 7.14 L/100 km.
Using only the final 30 L would leave out some of the fuel added during the measurement. Adding the starting fill would count fuel that established your starting tank level.
When the records cannot give a reliable result
If your latest refill is partial, the measurement is still open. You do not yet know how much fuel is needed to return to the starting tank level. Keep recording until you fill the tank completely again. If a fill-up is missing and you cannot recover its litre amount, start a new measurement with a full tank.
The amount paid or the position of the fuel gauge cannot replace the required litre total. You also need a positive distance. If the distance is zero or negative, check that the odometer readings are in the correct order.
Keep different cars and fuels separate. For a car using petrol and LPG, use the petrol and LPG record guide to track both fuels’ purchases over one period and distance. Purchases do not by themselves show fuel actually consumed.
Comparing several measurements
Each result describes its own stretch of driving. Note the routes you used when comparing records, so one result does not become a permanent assumption about the car.
For a closer look at a change, see what to check when fuel consumption increases.
For an overall average across completed, non-overlapping periods, divide the total included litres by the total distance, then multiply by 100. Do not simply average the individual L/100 km figures when the distances differ: that would give a short period the same weight as a long one.
Fuel consumption and cost: common questions
Does the price per litre tell you the trip cost?
No. You need your car’s consumption, the price actually paid per litre and the trip distance. Use the trip fuel cost calculator for the whole journey.
What do you need for fuel cost per kilometre?
Your car’s consumption in litres per 100 km and the price paid per litre. Multiply consumption by that price, then divide by one hundred: the approximate fuel-only cost in euros per kilometre. Try the 100 km fuel cost calculator.
LEA reported Lithuania’s average morning petrol price as 1,958 Eur/l on 2026-10-27; use your receipt price.
Turning consumption into a useful record
You can keep a fill-up history in MyFuel, a free browser app available in English and Lithuanian. Sign in with Google to manually record fill-ups and other expenses for one car and see approximate fuel consumption. Apply the full-to-full method to your own records: the app’s approximate consumption summary does not confirm that these measurement conditions were met. You can attach a receipt photo, but you still need to enter the fields yourself. Start with your next full tank and keep a record of every refill that follows.