Working out what a trip costs in fuel
How to work out the fuel cost of a trip from distance, mileage and price, how km per litre, litres per 100 km and mpg convert, and why real mileage differs from the brochure.
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In short
- The fuel cost of a trip is distance ÷ mileage × price per litre; 450 km at 15 km per litre and ₹100 a litre is 30 litres and ₹3,000.
- The cost per kilometre is simply price ÷ mileage, so ₹100 petrol in a car doing 15 km per litre costs ₹6.67 a km.
- Mileage is quoted four ways. 8 litres per 100 km is 12.5 km per litre, 30 US mpg is 12.75 km per litre, and 40 UK mpg is 14.16 km per litre.
- A 10 percent drop in mileage raises the fuel bill by 11.1 percent, because fuel used rises faster than mileage falls.
- Use the mileage you measure yourself by filling the tank twice, not the figure in the brochure.
Most trips begin with the same question: what will the fuel cost? The answer needs only three figures, the distance, the mileage and the price per litre, and a short piece of arithmetic. This page sets out that arithmetic, explains the several ways mileage is quoted around the world, and shows why the number on the car's brochure is rarely the number you will see at the pump.
The formula
Fuel cost is the litres you burn multiplied by what each litre costs, and the litres are the distance divided by how far each litre takes you:
fuel cost = distance ÷ mileage × price per litre
Take a drive of 450 km in a car that gives 15 km per litre, with petrol at ₹100 a litre. The car needs 450 ÷ 15 = 30 litres. At ₹100 a litre, 30 litres cost 30 × 100 = ₹3,000. Spread over the distance, that is ₹6.67 for every kilometre.
That last figure has a shortcut worth remembering:
cost per km = price per litre ÷ mileage
At ₹100 a litre, a car doing 10 km per litre costs ₹10 a km, one doing 15 costs ₹6.67, one doing 20 costs ₹5, and one doing 25 costs ₹4. You can price any journey in your head once you know your own cost per kilometre.
| Mileage (km per litre) | Cost per km at ₹100 a litre | 250 km | 500 km | 1,000 km |
|---|---|---|---|---|
| 12 | ₹8.33 | ₹2,083 | ₹4,167 | ₹8,333 |
| 15 | ₹6.67 | ₹1,667 | ₹3,333 | ₹6,667 |
| 20 | ₹5.00 | ₹1,250 | ₹2,500 | ₹5,000 |
Every figure in the table is distance × cost per km, so you can scale it to any distance.
Four ways to quote mileage
India says kilometres per litre. Much of Europe says litres per 100 kilometres. The United States says miles per US gallon, and the United Kingdom, for its own reasons, still says miles per imperial gallon, which is a larger gallon. A calculator that serves all of them has to convert to one unit first, and the working is worth knowing:
| Quoted as | Conversion to km per litre | Example |
|---|---|---|
| km per litre | none | 15 km/l is 15 km/l |
| litres per 100 km | 100 ÷ the figure | 8 L/100 km is 12.5 km/l |
| miles per US gallon | × 0.4251 | 30 mpg is 12.75 km/l |
| miles per UK gallon | × 0.3540 | 40 mpg is 14.16 km/l |
The two multipliers come from fixed definitions. One mile is 1.609344 km, one US gallon is 3.785411784 litres, and one imperial (UK) gallon is 4.54609 litres. So one US mpg is 1.609344 ÷ 3.785411784 = 0.4251 km per litre, and one UK mpg is 1.609344 ÷ 4.54609 = 0.3540 km per litre. A car rated at 40 miles per gallon sounds far better if the quote is in imperial gallons, and it is: 40 UK mpg is 14.16 km per litre, while 40 US mpg is 17.0.
Litres per 100 km works the other way round from the rest. It is fuel per distance rather than distance per fuel, so a lower figure is better. That is why 6 litres per 100 km is the more economical car, equal to 16.67 km per litre, while 10 litres per 100 km is 10 km per litre.
When the mileage is in miles per gallon, the price is also per gallon. The calculator divides the price by the size of the gallon to get a price per litre before it does anything else, which keeps the maths consistent.
Why your mileage is not the brochure mileage
The figure on a brochure comes from a test, run on a fixed cycle in a laboratory. It is the right basis for comparing two cars and the wrong basis for predicting your own bill. Real driving differs in several ways that all push the same direction:
- Speed. Air resistance rises steeply with speed, so a highway drive at 120 km/h uses markedly more fuel per kilometre than one at 80.
- Stop and go. City traffic means repeated acceleration from rest, which is the thing engines are least efficient at.
- Air conditioning, load and roof racks. Each adds work for the engine. A fully loaded car on a hill uses more fuel than the same car with one passenger on a flat road.
- Tyre pressure and condition. Soft tyres roll less easily.
- Driving style. Gentle acceleration and early lifting off the pedal save fuel on every drive.
The honest way to find your own number takes two fill-ups. Fill the tank to the brim, note the odometer, drive as you normally do, then fill to the brim again and note how many litres went in. Divide the kilometres by the litres. A car that took 38 litres for 520 km is doing 520 ÷ 38 = 13.7 km per litre, whatever the brochure says. Do it over a few tanks and average the results; one tank can be skewed by a long highway run.
Why mileage hurts more than it looks
Fuel used is distance ÷ mileage, so it changes in proportion to the inverse of mileage, and that is not symmetrical. If a car's mileage drops by 10 percent, from 15 to 13.5 km per litre, the 450 km trip no longer needs 30 litres but 450 ÷ 13.5 = 33.3 litres. That is 11.1 percent more fuel for a 10 percent fall in mileage.
The practical lesson is that the last few kilometres per litre are worth more than they seem. Going from 12 to 15 km per litre trims a 1,000 km bill from ₹8,333 to ₹6,667, a saving of ₹1,667. Going from 20 to 23 saves about ₹652 over the same distance, even though both are an improvement of three kilometres per litre.
When the price changes
Fuel prices move, and the cost is directly proportional to the price: a ₹5 rise on a ₹100 litre is a 5 percent rise in the bill. On a trip needing 30 litres it adds 30 × 5 = ₹150. If you are budgeting a long road trip, run the calculator twice, once at today's price and once a few rupees higher, and carry the higher number. Current fuel prices for each city are published by the oil companies and tracked by the Petroleum Planning and Analysis Cell.
Diesel and petrol are priced differently and the cars deliver different mileage, so enter the figures for the car you are actually driving. CNG vehicles quote mileage per kilogram rather than per litre; the same formula works if you enter the mileage per kilogram and the price per kilogram, treating the kilogram as the unit.
Sharing the cost, and the costs that are not fuel
If a group of friends is travelling in one car, divide the fuel cost by the number of people, which the calculator does when you enter the headcount. A ₹3,000 fuel bill among four is ₹750 each. For a longer trip with several payers and several kinds of expense, the trip expense splitter works out who owes whom in the fewest transfers.
Fuel is the largest running cost of a drive, but not the only one. This calculator does not include:
- Tolls, which on some highways add up to a meaningful share of the fuel bill.
- Parking, permits and entry fees.
- Wear and tear, tyres, servicing and depreciation, which rise with the kilometres driven. Some people add a fixed per-kilometre allowance for these.
- The cost of the return journey, unless you choose the round-trip option, which doubles the distance.
If you also want to check how a change of tyre size would affect the speedometer and the distance it records, the tyre size calculator shows the error. Both fuel use and the odometer depend on the wheel's circumference, which is why a taller tyre can quietly change what a trip seems to cost.
Worked example, step by step
A family drives from one city to another, 620 km each way, and returns the same day. Their car is a diesel hatchback that real-world testing shows at 18 km per litre. Diesel costs ₹92 a litre. Four people are in the car.
- Choose a round trip, so the distance counted is 620 × 2 = 1,240 km.
- Fuel needed is 1,240 ÷ 18 = 68.9 litres.
- At ₹92 a litre, the fuel costs 68.9 × 92 = about ₹6,338.
- The cost per kilometre is 92 ÷ 18 = ₹5.11.
- Shared between four, each person's fuel share is about ₹1,584.
Change any input and the rest follows. Try the same journey at 15 km per litre and the fuel needed jumps to 82.7 litres and about ₹7,605, which is ₹1,267 more, just from a car that is three kilometres per litre less efficient.
What this calculator assumes
- Mileage is constant for the whole trip. Real mileage varies with speed, load and traffic, so treat the answer as an estimate and keep a margin.
- The fuel price is the same throughout. On a long highway trip prices can differ from state to state.
- One kilometre is 0.621371 miles, one US gallon is 3.785411784 litres and one UK gallon is 4.54609 litres, all exact definitions.
- The result covers fuel only, and not tolls, parking, servicing or depreciation.