How to calculate fuel cost for a trip
Your fuel cost for a journey depends on three things: how far you travel, how efficient your vehicle is (its fuel economy), and the price of fuel. The calculator turns your distance and fuel economy into the litres or gallons you will burn, then multiplies by the price.
What is fuel economy?
Fuel economy (the middle field) is how far your vehicle goes on a given amount of fuel, or how much fuel it uses over a set distance. It comes in a few units depending on your country, so pick the one your car uses:
- MPG (miles per gallon): how many miles you get from one gallon. Common in the US and UK. Higher is better.
- Litres per 100 km (L/100km): how many litres it takes to drive 100 km. Used across Europe, Africa and most of the world. Lower is better.
- Kilometres per litre (km/L): how many km you get from one litre. Higher is better.
Watch the direction: with MPG and km/L a bigger number means more efficient, but with litres per 100 km a smaller number means more efficient. Just choose the unit your car shows and type in that figure.
Does engine size, model or driving speed matter?
Yes, a lot, and this calculator does account for them, through the fuel economy figure you enter. A BMW 320i and a 330i use different amounts of fuel, so you simply give each its own economy number and you get a different cost. Engine size, model, and whether it is manual or automatic all show up in that single figure.
Speed and traffic matter too. Fuel economy is usually best at a steady 45 to 60 mph and gets worse in stop-go city traffic or at very high motorway speeds. If the figure you have is the official combined rating, use the Driving conditions dropdown to nudge the estimate: choose city if most of your trip is stop-go (about 15% more fuel), or highway for steady cruising (about 15% less). Leave it on combined if you entered your own real-world figure.
Estimating cost at a specific speed
Want to know the cost of cruising at, say, 120 km/h? Fill in the optional Cruising speed field. Air resistance rises with the square of speed, so fuel use climbs the faster you go above an efficient cruise of about 90 km/h (55 mph). The calculator applies that curve to estimate the extra fuel. As a rough guide, 120 km/h uses around a quarter more fuel than 90 km/h, and 140 km/h around half as much again.
This is an estimate, because the exact penalty depends on your car's shape and weight. For a precise number, note the live economy your dashboard shows while cruising at that speed and enter that as your fuel economy instead. Leave the speed field blank to ignore it and use the driving-conditions setting.
Where to find your car's fuel economy
Your dashboard trip computer usually shows it. You can also look up the manufacturer's figure in the manual or online, or measure your own: fill the tank, drive, refill, then divide the distance travelled by the fuel you added.
Worked example
A 600 km trip in a car that uses 8 litres per 100 km needs 48 litres. At a fuel price of 15 per litre, the trip costs 720.
Good to know
You can mix units freely, and to find your cost per kilometre or mile, divide the trip cost by the distance. Smooth driving and correctly inflated tyres genuinely improve real-world fuel economy.
Frequently asked questions
What is fuel economy?
It is how far your vehicle travels on a given amount of fuel, shown as MPG (miles per gallon), litres per 100 km, or km per litre. Enter it in the middle field using whichever unit your car displays.
Which fuel economy unit should I use?
The one your car shows. The US and UK use MPG; most of the world uses litres per 100 km; some show km per litre. Remember higher MPG or km/L is better, but lower litres per 100 km is better.
Where do I find my car's fuel economy?
Your dashboard trip computer usually shows it, or check the manual or manufacturer's website. You can also measure it: fill up, drive, refill, and divide the distance by the fuel added.
Does it account for my exact car, like a BMW 320i vs 330i?
Yes, through the fuel economy figure. The two engines use different amounts of fuel, so you enter each car's own economy number and get a different cost. Engine size, model and manual-vs-auto are all captured by that figure.
Does driving speed change the result?
It can. Economy is best at a steady 45 to 60 mph and worse in city stop-go or at very high speeds. Use the driving conditions dropdown, or fill the optional cruising speed field for a specific speed like 120 km/h.
How does the cruising speed estimate work?
Air resistance rises with the square of speed, so above an efficient cruise of about 90 km/h fuel use climbs. The calculator applies that curve, so 120 km/h uses roughly a quarter more fuel than 90. It is an estimate; the exact amount depends on your car's aerodynamics.
How do I calculate fuel cost for a trip?
Multiply the distance by your vehicle's fuel consumption to get the fuel needed, then multiply by the fuel price. The calculator supports metric and imperial units.
How do I work out cost per km?
Divide the total trip cost by the distance. For example, a 720 cost over 600 km is 1.20 per km.
Can I mix units?
Yes. You can enter distance in miles, economy in litres per 100 km, and price per gallon if you like. The calculator converts everything for you.
What is the difference between US and UK gallons?
A US gallon is about 3.79 litres and a UK (imperial) gallon is about 4.55 litres, so MPG figures differ between the two.
Does it cover round trips?
Enter the total distance you will travel. For there and back, use double the one way distance.