How to calculate fuel cost for a trip
A fuel cost calculator estimates how much a journey will cost in fuel. It multiplies the distance by your vehicle's fuel consumption to find the liters needed, then multiplies the liters by the price per liter. This one needs three numbers: distance in kilometers, consumption in liters per 100 km, and the fuel price per liter.
The formula, exactly as the calculator applies it:
Fuel needed (L) = distance (km) ÷ 100 × consumption (L/100 km)
Total cost = fuel needed × price per liter
Both results are shown to two decimal places. The cost is calculated from the unrounded liters, so it can differ by a cent from multiplying the displayed liters yourself. All three inputs must be greater than zero.
Worked examples
| Trip | Distance | Consumption | Price per liter | Fuel needed | Cost |
|---|---|---|---|---|---|
| Weekend drive | 450 km | 6.5 L/100 km | 1.85 | 29.25 L | 54.11 |
| Daily commute (round trip) | 25 km | 7 L/100 km | 1.70 | 1.75 L | 2.98 |
| Long road trip | 1,200 km | 8 L/100 km | 280 | 96.00 L | 26,880.00 |
Taking the first one by hand: 450 ÷ 100 = 4.5, and 4.5 × 6.5 = 29.25 liters. At 1.85 per liter, 29.25 × 1.85 = 54.11.
The commute row is worth multiplying out further. At 2.98 a day and five working days a week, that commute costs about 14.90 a week in fuel before parking or wear on the car.
Using miles, gallons and mpg
Every input is metric, so convert first if your car shows miles per gallon. The conversion depends on which gallon your figures use:
| Gallon | Size | L/100 km from mpg |
|---|---|---|
| US gallon | 3.785411784 L | 235.215 ÷ mpg |
| Imperial (UK) gallon | 4.54609 L | 282.481 ÷ mpg |
The US gallon is defined as 231 cubic inches, which works out to exactly 3.785411784 liters (NIST SP 811 lists it), and the imperial gallon is defined in UK law as 4.54609 liters in the Weights and Measures Act 1985. A UK car rated at 40 mpg is using a bigger gallon than a US car rated at 40 mpg, so it's the less efficient of the two.
Example: a 300-mile trip in a car rated at 30 US mpg, with gas at $3.50 a gallon.
- Distance: 300 × 1.609344 = 482.8 km.
- Consumption: 235.215 ÷ 30 = 7.84 L/100 km.
- Price: 3.50 ÷ 3.785411784 = 0.9246 per liter.
- Enter those and the calculator returns 37.85 L and 35.00, which matches the quick US check of 300 ÷ 30 = 10 gallons × $3.50.
The length converter and volume converter handle the distance and gallon conversions if you'd rather not do them by hand.
Where to get a realistic consumption figure
The most accurate number is one you measure yourself: fill the tank, reset the trip meter, drive normally, then fill up again and divide liters by kilometers. The mileage calculator does that division and gives the answer in L/100 km, ready to paste in here.
Manufacturer and government ratings are a reasonable fallback, but they come from standardized test cycles. The US EPA explains how its tests are run, and why on-road results vary, on fueleconomy.gov. Motorway speeds, a loaded car, towing, cold starts and stop-start traffic all push real consumption above the label, so it's sensible to add a margin when budgeting a long drive.
What the estimate leaves out
- Tolls, parking and charging fees. Only fuel is costed.
- Price changes along the route. One price is applied to every liter.
- Electric vehicles. The calculator works in liters, so it can't take kWh/100 km directly.
- Splitting the cost. Divide the total by the number of passengers yourself.
If the trip is long, the travel time calculator estimates how many hours the same distance takes, and the budget calculator helps fit the fuel cost into a monthly plan.
Fuel prices and consumption change constantly, so treat the result as an estimate for planning, not a quote.
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