Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: This trip fuel cost calculator multiplies the fuel your car will burn by the price per gallon or litre. Enter distance, your car's economy in mpg or L/100 km, and the local fuel price. A 300-mile drive in a 30 mpg car uses 10 gallons, so at $3.50 a gallon it costs about $35.
TL;DR
- Fuel used = distance ÷ mpg, or distance × L/100 km ÷ 100.
- Cost = fuel used × price.
- Supports US gallons, UK gallons and litres.
- Split the total between passengers.
- Real economy is often lower than the label.
Trip fuel cost calculator formulas
- US or UK mpg: fuel = miles ÷ mpg; cost = fuel × price per gallon. Note that a UK (imperial) gallon is 4.546 litres and a US gallon 3.785 litres, so the same car shows a higher mpg figure in UK units.
- L/100 km: fuel = km × (L/100 km) ÷ 100; cost = litres × price per litre. The calculator handles all three and converts between them.
Getting the inputs right
Use the trip distance from a map or from Speedometer PRO's trip meter after a previous drive. For economy, your car's trip computer or a fill-to-fill calculation beats the official label, which is measured in test conditions. For price, the U.S. Energy Information Administration publishes weekly average gasoline and diesel prices; local prices vary.
What changes fuel use
FuelEconomy.gov notes that aggressive driving, speeding, excess weight and idling all reduce mpg. Fuel use per mile generally rises at higher motorway speeds because aerodynamic drag grows quickly with speed. Roof boxes and under-inflated tyres also hurt. Driving smoothly is the cheapest upgrade.
Example costs
| Distance | Economy | Price | Cost |
|---|---|---|---|
| 100 mi | 25 mpg (US) | $3.50/gal | $14.00 |
| 300 mi | 30 mpg (US) | $3.50/gal | $35.00 |
| 500 km | 6 L/100 km | €1.80/L | €54.00 |
| 200 mi | 45 mpg (UK) | £1.45/L | £29.30 |
Splitting costs fairly
Enter the number of travellers to split the fuel cost. For a fairer share, include tolls and parking in the extra-costs field. For business mileage, many employers and tax authorities use a per-mile rate instead; see mileage tracking for taxes.
Related: Free speed tools · Road trip tracker: log every leg · Travel time calculator · Trip meter: duration, distance, average and max speed · Mileage tracking for taxes · Speed distance time calculator · Online speedometer: your speed in the browser · How fast am I going right now?
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Driving more efficiently (FuelEconomy.gov): fueleconomy.gov (accessed 26 October 2026)
- Gasoline and Diesel Fuel Update (U.S. EIA): eia.gov (accessed 26 October 2026)
- Fuel economy in automobiles (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
How do I calculate fuel cost for a trip?
Divide distance by fuel economy to get fuel used, then multiply by the price per unit. For example, 240 miles at 30 mpg uses 8 gallons, which costs $28 at $3.50 a gallon.
What is the difference between US and UK mpg?
A UK gallon is about 20% larger than a US gallon, so the same car shows higher mpg in UK units. The calculator lets you pick which gallon you mean.
How do I convert mpg to L/100 km?
Divide 235.21 by US mpg, or 282.48 by UK mpg. So 30 US mpg is about 7.8 L/100 km.
Does driving faster cost more?
Generally yes at motorway speeds, because aerodynamic drag rises quickly with speed. FuelEconomy.gov lists speeding among the habits that lower mpg.
Does it work for electric cars?
Enter kWh per 100 miles or km and your electricity price per kWh in the EV mode.