How to use this calculator
Choose a unit system, then enter total trip distance, fuel consumption or economy, and fuel price. For a round trip, include the return distance. Metric uses kilometers, L/100 km and price per liter. US uses miles, US mpg and price per US gallon. UK uses miles, imperial mpg and price per liter. Changing systems restores sample numbers; currency changes display only.
The math behind it
Metric liters = distance in km × (L/100 km) ÷ 100 US gallons = distance in miles ÷ US mpg UK liters = distance in miles ÷ imperial mpg × 4.54609 Fuel cost = fuel needed × price per matching fuel unit
A worked example
A 300 km trip at 7 L/100 km needs 21 L. At 1.50 per liter, it costs 31.50. A 300 mile trip at 30 US mpg needs 10 US gallons; at 3.50 per gallon, it costs 35.00.
Assumptions & precision
Constant average fuel consumption and a user-entered price. Distance and price must be nonnegative; consumption or economy must be positive. UK mpg uses an imperial gallon of exactly 4.54609 L, verified against NIST SP 811 Appendix B.8. US mpg and US-gallon prices are used together without converting gallons. No live prices or exchange rates are fetched. Real driving, traffic, load, terrain and idling can change consumption. Fuel displays up to 4 decimals; currency uses its normal minor units. Intermediate values are not rounded. Results above 10¹⁵ or non-finite results are rejected.
A common question
Are US mpg and UK mpg the same?
No. US and imperial gallons differ. Select the system used by your vehicle’s consumption figure. The price label changes to match the selected calculation, with UK fuel priced per liter.