How to Calculate Fuel Cost for Any Trip

The Fuel Cost Formula

Two equivalent forms, depending on how your car’s economy is expressed.

The middle quantity in both is the fuel you will actually burn. Work that out first — litres or gallons — then multiply by the pump price, and the whole thing stays clear even when you switch units.

The Three Ways to Measure Economy

Knowing which one your car uses prevents most errors.

• Litres per 100 km is common in Europe and much of the world. Lower is better: 6 l/100km beats 9 l/100km.
• Kilometres per litre is common in South Asia and elsewhere. Higher is better: 18 km/l beats 12 km/l.
• Miles per gallon is used in the US and UK. Higher is better — but a US gallon and a UK gallon differ, so 40 US mpg and 40 UK mpg are not the same car.

The direction flips between the first and the others, which is the trap. With l/100km you want a small number; with km/l and mpg you want a large one. Comparing a “6” against an “18” without noticing they are different units gives nonsense.

Worked Examples

In kilometres per litre

500 km, car does 12 km/l, fuel 1.50 per litre. Fuel used: 500 ÷ 12 = 41.7 litres. Cost: 41.7 × 1.50 = 62.50.

In litres per 100 km

500 km, car uses 8 l/100km, fuel 1.50 per litre. Fuel used: (500 ÷ 100) × 8 = 40 litres. Cost: 40 × 1.50 = 60.00.

In miles per gallon

300 miles, car does 40 US mpg, fuel 3.50 per US gallon. Fuel used: 300 ÷ 40 = 7.5 gallons. Cost: 7.5 × 3.50 = 26.25.

Cost per kilometre

Dividing cost by distance gives a per-km figure worth remembering for your own car. The first example works out at 12.5 per 100 km, handy for estimating any future trip in seconds.

Converting Between Units

The two that trip people up most are mpg and l/100km, which are reciprocals rather than a simple multiplication, so each needs a constant.

The two mpg constants differ because a UK imperial gallon (about 4.546 litres) is larger than a US gallon (about 3.785 litres), so the same “40 mpg” describes a thirstier car in the US than in the UK. Other useful anchors: 1 mile is 1.609 km, and to turn km/l into US mpg, multiply by 2.352.

A worked conversion: a car rated 40 US mpg is 235.215 ÷ 40 = 5.88 l/100km, which is also 17 km/l. Three numbers, one car.

Why Real Consumption Is Higher

The economy figure on the sticker is measured in ideal conditions, and your actual mileage is usually worse, which matters when the trip is long enough for the gap to add up.

City driving with constant stopping burns more than steady motorway cruising. Speed hurts at the top end, since air resistance rises sharply above about 100 km/h. Air conditioning, a roof box, a heavy load, under-inflated tyres, cold weather, and hilly terrain all cost fuel. For budgeting a real trip, adding roughly 10 to 15 percent to the official consumption gives a more honest estimate than the brochure figure.

Common Mistakes

1. Mixing distance-per-fuel and fuel-per-distance without noticing the direction flips.
2. Using US and UK gallons interchangeably; they differ by about a fifth.
3. Trusting the sticker economy for a real-world budget instead of adding a margin.
4. Forgetting to convert miles to kilometres, or the reverse, before applying a price per litre.
5. Treating mpg and l/100km as if they convert by simple multiplication rather than through a constant divided by the figure.

Frequently Asked Questions

How do I calculate fuel cost for a trip?

Work out the fuel you will use, then multiply by the price. If economy is in km/l or mpg, divide distance by economy for the fuel used. If it is in l/100km, multiply distance in hundreds of km by the consumption. Then multiply the fuel by the pump price.

How do I convert mpg to litres per 100 km?

Divide 235.215 by the figure for US mpg, or 282.481 for UK mpg. So 40 US mpg is 235.215 divided by 40, about 5.88 l/100km. The two constants differ because US and UK gallons are different sizes.

Is a US gallon the same as a UK gallon?

No. A UK imperial gallon is about 4.546 litres and a US gallon about 3.785, so the imperial gallon is roughly 20 percent larger. The same mpg number therefore describes a less efficient car in the US than in the UK, which matters when comparing figures across the two.

Which is better, a high or low fuel economy number?

It depends on the unit. For litres per 100 km, lower is better because you are burning less fuel per distance. For kilometres per litre and miles per gallon, higher is better because you are covering more distance per unit of fuel. Always check which unit you are reading.

Why is my real fuel use worse than the official figure?

Official figures come from controlled tests. City driving, high speeds, air conditioning, extra weight, roof boxes, cold weather, and hills all raise consumption. Adding 10 to 15 percent to the sticker figure gives a more realistic budget for an actual journey.

How do I work out cost per kilometre or mile?

Divide the total fuel cost by the distance. Doing this once for your own car gives a per-distance rate you can multiply by any future trip length, which is faster than recalculating fuel used each time.

Does driving style really change fuel cost much?

Meaningfully, yes. Hard acceleration, high motorway speeds, and heavy braking can raise consumption by a noticeable margin over smooth driving. Steady speeds, correct tyre pressure, and removing unnecessary weight are the simplest ways to bring real cost closer to the official figure.

The Bottom Line

Fuel cost is the fuel you will burn times the price per unit, so work out the fuel first and the rest follows in any unit system. Watch the direction: low is good for l/100km, high is good for km/l and mpg, and US and UK gallons are not the same size. For a realistic budget, add a margin to the sticker economy rather than trusting the ideal figure. Run your numbers through the fuel cost calculator, and work out your car’s cost per kilometre once so every future trip is a single multiplication.