Fuel cost calculator
What the trip costs, what each passenger owes, and how much fuel it burns.
Trip cost
$35.00
300 miles at 30 mpg (US)
- Fuel used
- 37.85 L
- Per person
- $35.00
- Per mile
- $0.12
What this calculator works out
Given a distance, a fuel consumption figure and a price, it returns:
- the total cost of the fuel for the trip
- how many litres (and US gallons) that is
- the cost per person when shared
- the cost per kilometre or mile
It handles both metric and imperial input, because the two systems are used side by side and mixing them silently is how trip budgets go wrong.
The formula
litres = distance (km) × consumption (L/100 km) ÷ 100
total cost = litres × price per litre
per person = total cost ÷ passengersEverything is normalised to kilometres and litres first, then converted back for display. That means you can enter miles, mpg and a price per gallon in any combination and still get a correct answer.
mpg and L/100 km are inverses
This trips people up constantly. Fuel economy measured in miles per gallon goes up as the car gets better; litres per 100 km goes down. They are reciprocal, not proportional, which is why “10% better mpg” is not “10% less fuel”.
L/100 km = 235.2 ÷ mpg (US)
mpg = 235.2 ÷ (L/100 km)A worked example
A 300 mile trip in a car doing 30 mpg, with fuel at $3.50 a gallon:
- Fuel used: 300 ÷ 30 = 10 gallons (37.85 litres)
- Cost: 10 × 3.50 = $35.00
- Split four ways: $8.75 per person
- Per mile: $0.117
The same trip in metric — 482.8 km at 7.84 L/100 km and $0.92 a litre — returns the same total, because everything is converted to one set of units before the arithmetic.
Why the real number will differ
The label figure is a test-cycle number. Real consumption depends on how the car is driven and loaded:
- Cold starts and short trips — the engine runs rich until it warms up.
- Speed — consumption rises steeply above about 65 mph, because drag grows with the square of speed.
- Air conditioning — a few percent at speed, more in stop-start traffic.
- Load and roof racks — weight costs fuel on hills, drag costs it everywhere.
Treat the result as a budget, not a receipt. If you want the real number for your own car, use the distance you actually drive on a tank and the litres you actually put in.
Splitting the cost
Dividing by passengers is the fair version for a shared drive, but it is not the only fair version: some groups split by distance driven, some add a wear-and-tear allowance per mile. The calculator divides evenly and stops there, because the social rule is not an arithmetic problem.
Frequently asked questions
How do I convert mpg to litres per 100 km?
Divide 235.2 by the mpg figure. That constant comes from the gallon-to-litre and mile-to-kilometre conversions. 30 mpg is 235.2 ÷ 30 = 7.84 L/100 km. The relationship is inverse, so a higher mpg always means a lower L/100 km.
Why is my real consumption worse than the official figure?
Official figures come from a standardised test cycle with a warm engine and no air conditioning. Cold starts, short trips, motorway speeds above 70 mph, roof racks, air conditioning and a full load all raise consumption. A 15–25% gap between the label and real driving is normal.
Does the calculator handle a round trip?
Enter the total distance, not one leg. For a return journey, double the one-way distance before entering it — the fuel burned on the way back is the same as the way out.
How is the cost per person calculated?
The total fuel cost is divided by the number of people sharing. It does not include tolls, parking or wear, which are separate costs and sometimes split differently.
Should I use US or imperial gallons?
This calculator uses US gallons, which is what "mpg" means in the United States and in most manufacturer specifications. An imperial gallon is about 20% larger, so imperial mpg figures look better for the same real consumption. If your car quotes imperial mpg, convert before entering it.
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