Fuel cost per mile is one of the most practically useful numbers a driver can know, yet most people can only tell you their car's fuel economy rating (MPG or L/100km) rather than what it actually costs them to drive a mile or kilometre. The formula is simple, but the inputs are often guessed rather than measured.

The Basic Formula

For US drivers using miles per gallon:

Cost per mile = Fuel price (per gallon) ÷ MPG

Example: If fuel costs $3.50 per gallon and your car achieves 28 MPG, your fuel cost per mile is $3.50 ÷ 28 = $0.125, or 12.5 cents per mile. Over a 40-mile round-trip commute, that is $5.00 in fuel per day, $25 per week, and approximately $1,250 per year at five days per week.

The European Equivalent (L/100km)

In countries using litres per 100 kilometres, the formula is:

Cost per km = (Fuel price per litre × L/100km) ÷ 100

Example: At £1.55 per litre and 7.0 L/100km, the cost per kilometre is (1.55 × 7.0) ÷ 100 = £0.1085, or about 10.9 pence per kilometre.

Why Your Real-World Fuel Economy Differs from the Official Figure

Official MPG or L/100km ratings are produced under standardised laboratory test conditions (the EPA cycle in the US, the WLTP cycle in the EU and UK). Real-world driving consistently diverges from test results due to: highway versus urban split (stop-start driving burns significantly more fuel per mile than steady highway cruising), climate (cold engines and air conditioning increase consumption by 10–25%), tyre pressure (under-inflated tyres increase rolling resistance), and load (a fully loaded car with roof luggage can add 10–15% to fuel consumption). The most accurate cost-per-mile figure comes from your own fill-up records rather than the manufacturer specification.

How to Measure Your Actual MPG

Fill your tank to full. Reset your trip odometer to zero. Drive until you need fuel. Fill again, noting the exact litres or gallons added. Your fuel economy = miles driven ÷ gallons added (or 100 × litres added ÷ kilometres driven for L/100km). Doing this across three or four fill-up cycles and averaging the results gives you a reliable real-world baseline for cost calculations.

Use the USECALC Fuel Cost Calculator to calculate your cost per mile or kilometre, total trip cost, and annual fuel spend from your vehicle's fuel economy and your local fuel price.