Fuel cost calculator for car trips

What does a car journey cost in fuel?

This fuel cost calculator works out how much fuel a trip needs and what it costs, from three figures: the distance, your car's average consumption and the price of fuel. The tool does not supply any price and shows no currency symbol: you type the price you see at the pump, and the result is in that currency. Everything is calculated in your browser.

How to use the three tabs

1. Under "Units", pick "UK (miles, mpg, price per litre)", "US (miles, mpg, price per gallon)" or "Metric (km, L/100 km)". UK is selected by default.
2. "Trip cost": fill in "Distance", "Average consumption" and "Fuel price". Tick "Return trip" to double the distance. In the optional box, "Number of people" gives the cost per person, "Tolls (for the whole trip)" and "Parking" are added to the total, and "Trips per week" with "Weeks per year" gives the cost per month and per year.
3. "My real consumption": enter the "Distance between two fills" and the "Fuel added at the fill". Add the price to get the fuel cost per mile.
4. "Converter": fill in one box out of L/100 km, km/L, mpg US and mpg UK, and the other three are worked out.

Tick "Electric vehicle" to switch from fuel to electricity: miles per kWh in UK units, kWh per 100 miles in US units, kWh/100 km in metric. "How it is worked out" shows each formula with your numbers.

The formulas, with an example

In UK units: litres = miles ÷ mpg × 4.54609, then cost = litres × price per litre. A 180-mile drive at 45 mpg with petrol at 1.45 a litre needs 180 ÷ 45 × 4.54609 = 18.18 litres and costs 26.37. As a return trip it comes to 52.73, or 13.18 each with four people in the car.

In US units: gallons = miles ÷ mpg, and cost = gallons × price per gallon. A 300-mile trip at 30 mpg with gas at 3.50 a gallon uses 10 gallons and costs 35.00.

For a commute of 12 miles each way, as a return trip, 5 trips a week over 46 weeks, at 45 mpg and 1.45 a litre: 24 × 5 × 46 = 5,520 miles a year, costing 808.60 a year or 67.38 a month.

Measuring your real mpg from fill to fill

Brim the tank until the nozzle clicks off and reset the trip meter. Drive as usual. At the next fill, note the miles covered and the litres added. If you put in 32 litres after 310 miles, that is 310 ÷ (32 ÷ 4.54609) = 44 mpg, or 6.41 L/100 km. Averaging three or four fills gives a steadier figure.

Your result will often be worse than the official figure. That figure comes from the WLTP test, a standard cycle that cannot reproduce every use. Driving style, motorway speed, a heavy load, a roof box, soft tyres, air conditioning and short trips on a cold engine generally push consumption up.

What fuel cost leaves out

Fuel is only part of what a car costs. Insurance, servicing, tyres, wear and depreciation are not in this calculation, so the true cost per mile is higher. In the UK, HMRC publishes approved mileage allowance rates that are meant to cover those running costs when you use your own car for work.

Car sharing. Splitting costs should not leave the driver with a profit. The general rule: add up fuel and tolls, then divide by everyone in the car, driver included.

Petrol or electric: a worked example

Over 100 miles, a petrol car at 45 mpg with fuel at 1.45 a litre costs 14.65. An electric car doing 3.5 miles per kWh, charged at 0.28 per kWh, uses 28.57 kWh and costs 8.00. These prices are examples, not current rates: public rapid chargers usually cost far more per kWh than charging at home. Put in your own.

Using less fuel

• Hold a steady speed and ease off a little on the motorway.
• Read the road ahead: lift off early rather than brake late.
• Check tyre pressures once a month.
• Take out heavy items and remove roof bars or boxes you are not using.
• Switch the engine off when stopped for more than a short wait.

mpg or L/100 km?

With mpg, a higher number is better. With litres per 100 km, used across Europe, a lower number is better. UK and US mpg are not the same, because the imperial gallon (4.546 litres) is larger than the US gallon (3.785 litres). So 40 mpg UK = 33.3 mpg US = 7.06 L/100 km. The exact formulas: L/100 km = 282.480936 ÷ mpg UK, and L/100 km = 235.214583 ÷ mpg US.

Frequently asked questions

Where do I find my car's average consumption?

On the trip computer, or more reliably with the "My real consumption" tab, which measures it from fill to fill.

Does it work for diesel?

Yes. The formula is the same for any fuel sold by volume: enter your car's mpg on that fuel and its pump price.

Are tolls counted one way or both ways?

The amounts typed in "Tolls (for the whole trip)" and "Parking" are added to the total as they are. For a return trip, enter the sum for both directions.

Do my figures change when I switch units?

No. Switching between UK, US and metric changes the labels and the formulas, not what you typed.