How to Calculate the Fuel Cost of Any Trip
I built this fuel cost calculator to make it easy to estimate what a drive will cost before you leave. Whether you're budgeting for a road trip, comparing the cost of driving versus flying, or calculating reimbursements for work travel, the formula is the same: divide trip distance by fuel efficiency, then multiply by the fuel price. This calculator handles both imperial (miles per gallon) and metric (litres per 100 km) systems so you can work in whatever unit feels natural.
For the most accurate result, use your car's actual real-world fuel consumption rather than the EPA or manufacturer estimate. Most vehicles achieve 10–20% less than their official rating in everyday driving, particularly in city conditions. You can find your actual consumption by filling the tank, driving until you need to refuel, noting the miles driven, and dividing by the gallons used to refill.
Factors That Affect Real-World Fuel Economy
Fuel economy varies significantly based on conditions. Here are the main factors that reduce efficiency from your car's rated figure:
- City driving with frequent stops: typically 20–30% worse than highway economy.
- Air conditioning: adds roughly 5–25% to fuel consumption in hot weather.
- Highway speed above 55 mph: aerodynamic drag increases as the square of speed — driving at 75 mph uses about 20% more fuel than 55 mph.
- Roof racks and cargo carriers: even empty roof racks can reduce MPG by 2–8%.
- Cold weather: engines burn more fuel in temperatures below 40°F until they reach operating temperature.
How to Reduce Your Fuel Costs
Tyre pressure is the easiest free improvement — tyres underinflated by 10 PSI reduce fuel economy by about 1% per tyre. Check and inflate to the recommended pressure (found on the driver's door jamb sticker, not on the tyre sidewall) once a month. Fuel economy improves quickly once tyres are correctly inflated.
Smooth driving — anticipating traffic and braking gradually rather than hard-stopping — can improve real-world fuel economy by 10–30% compared to aggressive driving. Accelerating at a moderate pace, maintaining a consistent speed, and coasting to red lights are simple habits that add up to meaningful savings over a year of driving.
Frequently Asked Questions
How do I convert between MPG and L/100km?
To convert US MPG to L/100km, divide 235.21 by the MPG figure. So 30 MPG = 235.21 ÷ 30 = 7.84 L/100km. To go the other way, divide 235.21 by the L/100km figure. A lower L/100km number means better efficiency; a higher MPG number means better efficiency — this calculator handles the conversion automatically so you always see the cost in both systems.
Is it worth driving out of the way to find cheaper fuel?
It depends on how much cheaper and how far out of the way. If the cheaper station saves $0.10 per gallon and your tank holds 15 gallons, you save $1.50. Driving an extra 5 miles round-trip in a 30 MPG car uses about 0.17 gallons — at $3.50/gallon that detour costs about $0.58, so you still come out ahead. But a 15-mile detour for the same saving would cost more in extra fuel than you save.
How do I calculate fuel cost for a work trip reimbursement?
The IRS standard mileage rate (2024: 67 cents per mile) is the simplest method for reimbursements — it covers fuel, depreciation, and maintenance in one figure. For actual fuel cost only, use this calculator with your vehicle's real-world MPG and the current fuel price. Many employers accept either method, but the standard mileage rate is simpler to document and often more generous than fuel cost alone.
