The Simple Formula
Fuel cost = (Distance ÷ Fuel efficiency) × Price per unit
That is it. The only complexity is getting the units right.
In miles and gallons (US):
A 300-mile trip in a car that gets 25 miles per gallon, with gas at $3.50 per gallon:
300 ÷ 25 = 12 gallons × $3.50 = $42.00
In kilometers and liters:
A 500-kilometer trip in a car that uses 7 liters per 100 km, with fuel at €1.80 per liter:
(500 ÷ 100) × 7 = 35 liters × €1.80 = €63.00
Understanding Fuel Efficiency Ratings
Different countries express fuel efficiency differently:
Miles per gallon (MPG) — Used in the US and UK. Higher is better. A car rated at 35 MPG travels 35 miles on one gallon of fuel. Note that US and UK gallons are different sizes (US gallon = 3.785 liters, UK gallon = 4.546 liters).
Liters per 100 kilometers (L/100km) — Used in most of Europe, Canada, and Australia. Lower is better. A car rated at 6 L/100km uses 6 liters of fuel for every 100 kilometers driven.
Kilometers per liter (km/L) — Used in parts of Asia and South America. Higher is better.
To convert: MPG (US) = 235.21 ÷ L/100km. So 7 L/100km ≈ 33.6 MPG.
Factors That Affect Real-World Fuel Consumption
Official fuel efficiency ratings are measured under controlled conditions. Real-world consumption is usually 10–20% higher. Factors that increase fuel use:
Speed. Aerodynamic drag increases with the square of speed. Driving at 80 mph (130 km/h) uses significantly more fuel than driving at 60 mph (100 km/h).
Terrain. Hill driving, mountain passes, and stop-and-go city driving all increase consumption compared to steady highway cruising.
Climate. Air conditioning adds 5–15% to fuel consumption. Cold weather increases consumption due to longer warm-up periods and higher-viscosity oil.
Load. More weight means the engine works harder. A car loaded with passengers and luggage uses more fuel than the same car driven solo.
Driving style. Aggressive acceleration and hard braking waste fuel. Smooth, steady driving improves efficiency by 10–30%.
Planning a Road Trip Budget
For a road trip, calculate the fuel cost for each leg of the journey:
- Look up the distance for each segment (city to city)
- Use your vehicle's real-world fuel efficiency (check your last few fill-ups for actual numbers rather than the manufacturer's rating)
- Look up current fuel prices along the route (prices vary significantly by region)
- Multiply and add up the segments
Remember to budget for the return trip — unless fuel prices are the same in both directions, calculate each way separately.
How to Use the Toobits Fuel Cost Calculator
Enter your trip distance, your vehicle's fuel efficiency, and the current fuel price. The calculator instantly shows total fuel needed and total cost. Switch between miles/gallons and kilometers/liters as needed. Use it for trip planning, commute cost comparison, or vehicle efficiency analysis. Everything runs in your browser.