How to Calculate Fuel Cost for a Trip
Fuel is the one road-trip cost you can predict to within a few dollars before leaving the driveway — if you use the right formula for your unit system. There are two, and mixing them up produces confidently wrong numbers.
Metric: L/100km measures thirst
Metric consumption states how many liters the car drinks per 100 km — lower is better. The formula:
fuel = distance ÷ 100 × L/100km
A 400 km trip at 8 L/100km: 400 ÷ 100 × 8 = 32 liters. At $1.60/L, that's $51.20.
US: MPG measures reach
Miles per gallon states how far a gallon takes you — higher is better:
fuel = distance ÷ MPG
A 300-mile trip at 30 MPG: 300 ÷ 30 = 10 gallons. At $3.50/gallon, $35.00.
Because one metric is per-distance and the other per-fuel, they're reciprocals in disguise: converting between them uses 235.2 ÷ x (so 8 L/100km ≈ 29.4 MPG). That inverse relationship is exactly why intuition fails when hopping between systems.
Use a realistic consumption figure
Official ratings are laboratory numbers. Real-world driving typically runs 10–20% thirstier, and worse with a loaded roof rack, city traffic, winter temperatures or high speeds — drag rises with the square of speed, so 120 km/h costs noticeably more than 100. Your car's trip computer average over recent months is the most honest number you have; round it up slightly for budgeting.
Splitting the cost fairly
Total fuel cost ÷ people is the obvious split — the less obvious part is agreeing on it beforethe trip. For a 3-person, $51 trip that's $17 each. Some groups add a wear-and-tear allowance for the driver; standard mileage-reimbursement rates (which bundle fuel, maintenance and depreciation) run roughly double the raw fuel cost, which is a defensible benchmark if the driver's car is doing the work.
Round trips and detours
The formulas take whatever distance you give them — just remember to double one-way distances and pad 5–10% for city legs and detours. Mapping apps give the driving distance; resist the temptation to use the straight-line one.
Calculate yours in seconds
The fuel cost calculator handles both unit systems, shows fuel needed and total cost, and splits per person. Converting km to miles or gallons to liters for a rental abroad? The unit converter covers every pair.