To estimate a trip’s fuel cost you need the distance, the car’s fuel economy and the price of fuel.
The formula in US units
Cost = gallons × price per gallon
Example: 280 miles in a car that does 30 mpg, with gas at $3.50 per gallon. Gallons = 280 ÷ 30 ≈ 9.33 gal. Cost = 9.33 × 3.50 ≈ $32.67. A return trip doubles it: $65.33. Split three ways, about $21.78 each.
The formula in metric units
Cost = litres × price per litre
450 km at 6 L/100 km is 27 litres; at 1.60 per litre, 43.20 in your currency.
Converting between mpg and L/100 km
30 mpg is about 7.84 L/100 km. The UK uses imperial gallons (4.546 L), so the factor is 282.48 instead. Our calculators use the US gallon when you choose imperial units.
Why real consumption differs
- Speed: air resistance rises quickly with speed.
- Load and roof boxes: more weight and worse aerodynamics.
- Hills and traffic: climbs and frequent stops raise consumption.
- Tyre pressure: under-inflated tyres increase rolling resistance.
The most reliable figure is the average your trip computer records on similar journeys. If you don’t have one, fill the tank, note the odometer and at the next fill-up divide miles driven by gallons added.
Don’t forget tolls and parking
On many trips tolls weigh as much as fuel. Add them in the trip fuel calculator to get the real cost per person.