Fuel Cost Calculator

How to use the fuel cost calculator

1. Pick your unit system — US (miles, mpg, gallons), UK (miles, imperial mpg, litres) or metric (km, L/100 km, litres). This sets every input at once, and you can still change any single unit afterwards.
2. Enter the one-way distance of the trip.
3. Enter your car’s fuel efficiency in whichever unit you know it: mpg (US), mpg (imperial), L/100 km or km/L.
4. Enter the fuel price per gallon or per litre, and pick your currency.
5. Tick round trip if you are driving back, and set how many people are splitting the cost.
6. Optionally turn on compare two vehicles and add an annual distance to see what a more efficient car would save in a year.

Everything updates as you type, and nothing you enter leaves your browser.

How it works

The calculator converts every input to litres and kilometres, then does one multiplication.

consumption (L/100 km) = 235.215 / mpg (US)
                       = 282.481 / mpg (imperial)
                       = 100 / (km per litre)
litres used = consumption × distance (km) / 100
cost        = litres used × price per litre

Two conversions catch people out. First, mpg and L/100 km run in opposite directions, so you cannot average or compare them without converting. Second, US and imperial gallons differ, which is why the same car carries two very different mpg ratings on either side of the Atlantic.

The cost per mile or per kilometre is simply the total cost divided by the distance actually driven, so it doubles the distance — not the rate — when round trip is ticked.

Worked examples

A weekend drive. 300 miles at 30 mpg (US) with fuel at $3.50 a gallon uses 10 gallons and costs $35.00, which is 11.7 cents a mile. Make it a round trip and it is 20 gallons and $70.00.

Sharing the cost. That same $35 trip split between four people is $8.75 each — a figure worth agreeing before setting off rather than at the services.

A European road trip. 500 km at 7.0 L/100 km with fuel at 1.75 per litre uses 35 litres and costs 61.25, or 0.12 per kilometre.

A UK commute. 200 miles at 50 mpg (imperial) uses 4 imperial gallons, which is 18.18 litres. At £1.45 a litre that is £26.37.

Buying a more efficient car. Driving 12,000 miles a year at $3.50 a gallon, a 30 mpg car burns 400 gallons and costs $1,400 a year. A 40 mpg car burns 300 gallons and costs $1,050 — a saving of $350 a year, 25% of the fuel bill, or $1,750 over five years.

Notice how the saving shrinks as efficiency rises. Going from 15 to 20 mpg over those 12,000 miles saves $700 a year, while going from 40 to 50 mpg saves only $210. Fuel used is the reciprocal of mpg, so improvements at the thirsty end of the scale are worth far more than improvements at the frugal end.

Tips and common mistakes

• Use your real-world efficiency, not the brochure figure. Fill the tank, drive normally, fill it again and divide the distance by the fuel added.
• Check which gallon you are using. A US gallon is about 20% smaller than an imperial one, and mixing them up moves the cost by a fifth.
• Remember the return leg. Half of all “the trip cost more than I thought” surprises are a forgotten round trip.
• Motorway speed matters. Aerodynamic drag rises with the square of speed, so the same car is measurably thirstier at 80 mph than at 65.
• Weight and drag add up. A loaded roof box, low tyre pressures or a boot full of luggage can add several percent to consumption.
• Fuel is only part of the cost of driving. Depreciation, insurance, tax, tyres and servicing often cost more per mile than the fuel does — compare full running costs before changing car.

The figures here are fuel-cost estimates based on the numbers you enter, not a quote or financial advice. Real consumption varies with driving conditions, so add a margin when budgeting a long trip.

Glossary

• mpg – miles per gallon; higher is better. Always check whether the gallon is US or imperial.
• L/100 km – litres of fuel used to drive 100 km; lower is better.
• km/L – kilometres travelled per litre, common in Japan, Korea and parts of South America.
• Cost per mile – total fuel cost divided by miles driven, the usual basis for mileage sharing.
• Real-world economy – the efficiency you actually achieve, typically worse than the official test figure.

Last reviewed: September 21, 2026