How to Calculate Fuel Cost for a Trip or a Month

The Formula in Both Unit Systems

Make sure the units match: MPG with gallons of the same type (US or UK), L/100 km with litres. Mixing them is the most common error. If your car reports one unit and your fuel price uses another, MPG vs L/100 km shows how to convert.

Worked Example: A Single Trip

Trip: 240 miles, car averages 32 MPG (US), fuel costs $3.60 per US gallon.

1. Fuel used: 240 ÷ 32 = 7.5 gallons
2. Cost: 7.5 × $3.60 = $27.00

For a return trip, double the distance before you start, or the result.

Metric version: 380 km, car uses 6.5 L/100 km, fuel costs 1.75 per litre.

1. Fuel used: 380 × 6.5 ÷ 100 = 24.7 litres
2. Cost: 24.7 × 1.75 = 43.23 in your currency

Worked Example: A Monthly Commute

Commute: 18 miles each way, 21 working days a month.

1. Monthly distance: 18 × 2 × 21 = 756 miles
2. Fuel used at 30 MPG: 756 ÷ 30 = 25.2 gallons
3. Cost at $3.50: 25.2 × $3.50 = $88.20 a month, or about $1,058 a year

Add weekend and errand driving if you want your full monthly fuel bill, not just commuting.

Cost per Mile or per Kilometre

Cost per unit of distance is handy for comparing cars or deciding whether to drive:

• Per mile = price per gallon ÷ MPG. At $3.50 and 30 MPG, that's about $0.117 per mile.
• Per km = L/100 km × price per litre ÷ 100. At 6.5 L/100 km and 1.75 per litre, about 0.114 per km.

Multiply by any distance for an instant estimate.

Splitting Fuel Costs for a Shared Trip

For car-pooling or road trips with friends, calculate the total fuel cost for the whole journey, then divide by the number of people. Some groups exclude the driver or add a share for wear and tear; agree that in advance. Remember fuel is only part of the cost of running a car; insurance, maintenance and depreciation usually add more per mile.

Getting the Inputs Right

Your answer is only as good as the numbers you put in:

• Fuel economy: official figures are measured in controlled test cycles and are often better than real-world results. Your car's own trip computer can be optimistic too. Measuring with the fill-up method is more reliable; how to measure your car's real fuel economy explains it.
• Driving conditions: city driving, heavy traffic, cold weather, roof boxes, heavy loads and high motorway speeds all increase fuel use.
• Fuel price: use what you actually pay, and update it when prices move.
• Distance: map apps give accurate route distances; odometer readings are better for your typical month.

Doing It Automatically

The fuel savings calculator runs this calculation for a monthly distance or a monthly fuel spend, in miles or kilometres, with MPG (US or UK), L/100 km or km/L, and a price per litre or gallon. It also shows what you'd save over a year by using a set percentage less fuel, and the fuel economy you'd need to average to get there. It runs in your browser with nothing to sign up for.

If you want to reduce the number, our guide on how to save money on fuel covers the habits that make the biggest difference.

Summary

Fuel used is distance divided by economy; cost is fuel used times price. Keep units consistent, use realistic fuel economy, and the same formula answers trip, monthly and yearly questions.