Why use this calculator
Fuel is the running cost people notice because they pay it at the pump every week, and it is the one that decides whether a road trip beats flying, whether to take the small car or the four-wheel drive, and what to ask passengers to chip in.
The arithmetic is simple but easy to fumble, especially across units: L/100km and mpg move in opposite directions, so a car doing 40 mpg is more economical than one doing 30 mpg, while 6 L/100km beats 8 L/100km. A UK driver quotes mpg on the imperial gallon but buys fuel by the litre, and a US gallon is smaller than a UK one.
This calculator handles all three systems, doubles for a return trip, lifts an official label figure to something closer to real driving and gives a per-person split. It also puts the answer in context: fuel is usually not the largest cost of owning a car.
How to use it
- Pick your units: metric, US or UK.
- Enter the one-way trip distance from a maps app or the odometer.
- Choose whether the trip is one way or return.
- Enter your fuel economy, ideally your own measured figure from filling the tank twice and dividing, in L/100km or mpg to match the units.
- Say whether that figure is your own measurement or the official label, since label figures get a 15 per cent real-world uplift.
- Enter the fuel price per litre, or per US gallon in US mode, where you expect to fill up.
- Finally enter the number of people sharing the cost.
What each result means
- Total fuel cost
- What the trip costs at the pump at the economy and price you entered, one way or both ways as chosen.
- Cost per person
- The total fuel cost split evenly between everyone sharing, a fair basis for passengers chipping in for petrol.
- Fuel used
- Litres, or US gallons in US mode, the trip will burn. Compare it with the tank size to see how many fills you need.
- Running cost per distance
- Fuel cost for each kilometre or mile. It is only the fuel part of running a car, not the full cost per kilometre.
- Distance driven
- Total distance including the return leg if you chose a return trip, in kilometres or miles.
- Fuel economy used
- The economy figure the calculation actually used, including the 15 per cent uplift when you entered a label figure.
The formula
Economy in
L/100km = the figure entered in metric, 235.215 / US mpg, or 282.481 / UK mpg, multiplied by 1.15 when it is an official label figure
Kilometres driven = distance (x 1.609344 for miles) x 2 for a return trip
Litres used = L/100km / 100 x kilometres
Cost = litres x price per litre
Cost per person = cost / people sharing
Running cost per distance = cost / kilometres driven, shown per mile outside metric mode
How this number is used in practice
Official fuel figures come from laboratory drive cycles, and real driving nearly always uses more. US EPA ratings have been adjusted since 2008 to be closer to real use, but Australian and older European label figures, based on shorter and gentler lab cycles, are commonly beaten by 10 to 30 per cent in independent real-world testing.
Towing, a roof box, a full load, air conditioning in the heat and speeds above about 100 km/h or 65 mph all push consumption higher. The most reliable figure is your own: fill the tank, reset the trip meter, drive normally, fill again and divide the litres by the distance.
Fuel is rarely the largest cost of running a car. Motoring clubs such as the NRMA and RACV in Australia and AAA in the US publish annual running cost surveys, and for a new car depreciation is usually the biggest single cost, often more than fuel, insurance and servicing combined over the first five years.
That is why a cheap-to-run car can still be expensive to own, and why mileage allowances such as the ATO cents per kilometre rate or the IRS standard mileage rate are several times the fuel cost per kilometre.
When splitting costs with passengers, fuel is the usual basis because it rises directly with the trip. Tolls and parking are easy to add on top. Some drivers ask for a share of wear and tear as well, which is fair on long trips, but the fuel figure is the one everyone can check.
Worked examples
An 800 km drive at 8 L/100km with petrol at $1.90 a litre, one way
64.0 L of fuel for $121.60, which is $0.15 per km
A US 500 mile return trip in a 28 mpg car with gas at $3.50, three people sharing
$125.00 in total, $41.67 each, using 35.71 gallons
A UK 200 mile run using a 50 mpg label figure, petrol at £1.45 a litre
The label is lifted to 43.5 mpg, using 20.9 L for £30.32
When people use it
- Budgeting a holiday road trip or a long weekend away
- Splitting petrol fairly between friends on a shared drive
- Comparing the fuel cost of driving against flights or trains
- Choosing which of two household cars to take on a long trip
- Checking fuel reimbursement for a work trip
Tips
- Measure your own economy over a few tanks. It beats any label figure.
- Fill up before highways and remote stretches, where prices are usually higher.
- In Australian capital cities, petrol prices follow a weekly cycle. An app shows where the cycle is.
- Keep tyres at the placard pressure. Under-inflated tyres cost a few per cent in fuel.
- Take off roof racks and boxes when not in use. They add drag at highway speed.
- Cruising at 100 km/h instead of 110 km/h usually saves around 10 per cent in fuel.
What it assumes, and where it stops being accurate
- Fuel economy is constant for the whole trip. Hills, headwinds, city sections and speed changes all move it.
- The pump price is the same for every fill. Prices vary between cities, highways and remote areas.
- Label figures are uplifted by a flat 15 per cent. The real gap varies by car, test cycle and driving style.
- A return trip is assumed to be the same distance and conditions in both directions.
- Only fuel is costed. Tolls, parking, depreciation, tyres, servicing and insurance are not included.
- Electric vehicles are not modelled. Use a per kWh comparison for those.
Terms used on this page
- L/100km
- Litres of fuel used to travel 100 kilometres. Lower is more economical.
- mpg
- Miles per gallon. Higher is more economical. US and UK gallons differ in size.
- Label figure
- The official fuel economy from a standard lab test, shown on the window sticker or fuel label.
- Depreciation
- The fall in a car's value over time, usually its largest ownership cost.
Questions people ask
- How do I convert mpg to L/100km?
- Divide 235.215 by a US mpg figure, or 282.481 by a UK mpg figure. So 28 US mpg is 8.4 L/100km and 40 UK mpg is 7.1 L/100km. The numbers move in opposite directions: higher mpg is better, while lower L/100km is better.
- Why is my real fuel use higher than the label?
- Label figures come from a standard laboratory test that is gentler than real driving. Higher speeds, short trips with a cold engine, air conditioning, load, hills and traffic all add fuel. Real-world use 10 to 30 per cent above an older lab figure is common.
- Is fuel the biggest cost of running a car?
- Usually not for a newer car. Motoring club running cost surveys consistently find depreciation is the largest single cost, followed by finance, insurance and fuel. Fuel is simply the most visible because you pay it every week at the pump.
- How should passengers split fuel costs?
- The simplest fair split is total fuel divided by the number of people, including the driver. On long trips some groups also add tolls and parking, and occasionally a share of wear and tear. Agree before leaving so nobody is surprised.
- Why is a UK gallon different from a US gallon?
- The UK imperial gallon is 4.546 litres and the US gallon is 3.785 litres. A car doing 40 mpg in UK figures does only about 33 mpg in US figures, even though it uses exactly the same fuel, which is why the two cannot be compared directly.
Use this with
Fuel Cost Calculator answers one part of the question. These pick up where it stops: Fuel Efficiency Calculator, Road Trip Planner Calculator, Car Running Cost Calculator and EV vs Petrol Cost Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- US Department of Energy and EPA: fueleconomy.gov. US fuel economy ratings and real-world economy guidance
- Australian Government Green Vehicle Guide. Australian fuel consumption label figures
- Australian Automobile Association. Transport affordability and running cost research
Fuel Cost Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.