Trip fuel cost calculator

Divide the distance by the fuel economy to get the fuel used, then multiply by the price. 300 km at 7 L/100 km uses 300 × 7 ÷ 100 = 21 L, which costs 36.75 at 1.75 a litre. In US units, 500 miles at 30 mpg uses 16.7 gallons, or $58.33 at $3.50 a gallon. Add tolls and parking, then double everything for a round trip.

Divide 235.21 by US mpg, or 282.48 by UK mpg; the same division converts L/100 km back to mpg. The constants follow from the US gallon (231 cubic inches, 3.785411784 L), the imperial gallon (4.54609 L) and the mile (1.609344 km), as listed in NIST SP 811. So 7 L/100 km is 33.6 US mpg or 40.4 UK mpg.

The imperial gallon holds 4.54609 L and the US gallon 3.785 L, so a UK gallon is 20% larger and the same car travels 20% further on it. Multiply UK mpg by 0.8327 to get US mpg: 45 UK mpg is 37.5 US mpg. Reading a UK figure as US mpg understates the fuel used by a sixth.

Add fuel, tolls and parking and divide by the number of people sharing: the default 46.75 trip is 23.38 each for two. That covers only out-of-pocket costs. To include wear, insurance and depreciation, some drivers charge the IRS standard mileage rate instead, 76 cents a mile for business driving from 1 July 2026 (IR-2026-29); fuel alone at 30 mpg and $3.50 a gallon is 11.7 cents a mile.

Driving style and weather move real consumption away from the label. The US Department of Energy's fueleconomy.gov says aggressive driving can cut gas mileage by 15–30% at highway speeds and 10–40% in stop-and-go traffic, and a petrol car's city mileage is roughly 15% lower at 20 °F (−7 °C) than at 77 °F (25 °C). Economy measured from your own fill-ups gives the closest trip cost.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “300 km at 7 L/100 km, 1.75 per litre, 10 in tolls, 2 people” is checked against Hand calculation: 300 × 7/100 = 21 L × 1.75 = 36.75 + 10 = 46.75; ÷ 2 = 23.375.

US DOE / EPA fueleconomy.gov — trip cost calculator; NIST Handbook 44 Appendix C — gallon and litre definitions; NIST SP 811 Appendix B.9 — conversion factors for gallon (US and imperial) and mile.