How to calculate gas cost for a trip
To estimate the gas cost of a trip, divide the distance by your car's fuel economy to get the fuel used, then multiply by the price per gallon or litre. A 300-mile drive in a car that does 30 mpg, with gas at $3.50 a gallon, costs $35 one way. Work out your own trip, including round trips and splitting the cost, with the fuel cost calculator.
The formula (US: miles and gallons)
gas cost = miles ÷ mpg × price per gallon
300 miles ÷ 30 mpg = 10 gallons × $3.50 = $35.00. For a round trip, double the distance: $70. Split between three people, that is $23.33 each.
UK: miles per gallon, price per litre
UK cars are rated in miles per imperial gallon, but fuel is sold by the litre. An imperial gallon is 4.546 litres (a US gallon is 3.785 litres), so:
fuel cost = miles ÷ mpg × 4.546 × price per litre
Example: 200 miles at 45 mpg = 4.44 gallons × 4.546 = 20.2 litres. At £1.40 a litre, that is £28.29.
Don't mix up US and UK mpg: 45 UK mpg is only 37.5 US mpg, because the US gallon is smaller.
Metric: litres per 100 km
fuel cost = km × L/100 km ÷ 100 × price per litre
Example: 500 km in a car using 6 L/100 km = 30 litres. At €1.95 a litre, the trip costs €58.50.
Note that L/100 km works the other way round from mpg: a lower number means a more economical car.
Example trips
| Trip | Distance | Economy | Fuel price | Cost |
|---|---|---|---|---|
| Weekend away (US) | 300 mi | 30 mpg | $3.50/gal | $35.00 |
| Same trip, round trip | 600 mi | 30 mpg | $3.50/gal | $70.00 |
| UK motorway run | 200 mi | 45 mpg | £1.40/L | £28.29 |
| European holiday leg | 500 km | 6 L/100 km | €1.95/L | €58.50 |
The fuel prices above are examples; use the current price at the pump where you plan to fill up.
Getting a realistic fuel economy figure
Official ratings are often optimistic. Real-world economy depends on:
- Speed: most cars use noticeably more fuel above 60 to 65 mph (100 to 105 km/h), because air resistance rises sharply.
- Load and roof boxes: a loaded car with a roof box can use 10 to 20% more.
- Air conditioning, cold weather and short trips: all increase consumption.
- Driving style: smooth acceleration and anticipating traffic save fuel.
The most accurate figure is your own: fill the tank, reset the trip meter, and at the next fill-up divide the miles driven by the gallons added (or litres × 100 ÷ km for L/100 km).
Cost per mile
Divide the trip cost by the distance. At 30 mpg and $3.50 a gallon, fuel costs $0.117 per mile. This is fuel only; it does not include wear, tires, insurance or depreciation, which matter when you compare driving with flying or the train, or claim mileage.
Frequently asked questions
How do I calculate gas money for a road trip?
Total distance ÷ mpg × gas price, then divide by the number of people sharing. For 600 miles at 30 mpg and $3.50, that is $70, or $23.33 each for three people.
How many gallons will I need for my trip?
Distance ÷ mpg. A 450-mile trip at 25 mpg needs 18 gallons.
How do I convert mpg to L/100 km?
For US mpg: 235.2 ÷ mpg. For UK mpg: 282.5 ÷ mpg. So 30 US mpg is 7.84 L/100 km.
Does the calculator work in kilometres?
Yes. Choose kilometres and L/100 km, and enter the price per litre in your currency.