Quick answer
Petrol fuel cost per kilometre equals petrol price per litre divided by the car's mileage in kilometres per litre. At an assumed ₹100/litre and 15 km/l, the result is about ₹6.67/km. This is fuel cost only, not the complete cost of owning the car.
For a useful estimate, enter your local fuel price and realistic efficiency for your routes. Multiply the result by monthly kilometres for the fuel budget. Keep insurance, servicing, parking and finance separate so the calculation stays clear.
The formula needs only two inputs
Write down the price of petrol in rupees per litre and the mileage in km/l. Divide the first by the second. The litres cancel, leaving rupees per kilometre. If either input is wrong, a precise-looking answer will still be misleading.
For example, ₹100 divided by 15 equals ₹6.6667. Rounding to ₹6.67/km makes the result easy to read. For a longer budget calculation, keep more decimal places until the final amount so that repeated rounding does not create an avoidable difference.
Do not divide the car's purchase price by mileage or use the tank capacity as the efficiency figure. Tank capacity tells you how much fuel can be stored, not how far the vehicle travels on each litre.
Use a local price with a date
Petrol prices can differ by location and outlet. IndianOil's official price page provides a method for checking indicative local prices using the outlet's dealer code. It also notes that prices can vary between outlets in a sales area.
Use the amount on a recent receipt or a current official outlet check. Record the date and place next to the calculation. A metro-city figure copied from an old article should not silently become the cost assumption for a different town.
The examples below deliberately use round assumed prices, not a claim about today's national petrol rate. Replace them before using the result for a purchase decision. Refresh the calculation when your local price changes materially.
Choose mileage that matches your driving
A certified fuel-efficiency figure is measured under specified test conditions. It should not be treated as a guarantee for your own traffic, air-conditioning use and journey pattern. Use a sensible range until you have records from the vehicle.
For an existing car, several refill cycles are more useful than one short dashboard reading. A single fast highway run may not represent a month dominated by short trips. Keep city and highway records separate if they serve different parts of your routine.
For a car you have not bought, obtain the official specification for the exact engine and transmission, then make a cautious planning estimate. Label it as an assumption. Do not claim a model delivers a particular real-world result unless you have suitable evidence.
Worked examples at one assumed fuel price
The table holds petrol at an assumed ₹100/litre and changes only mileage. It is a sensitivity check, not a comparison of named cars or a current fuel-price report.
| Assumed mileage | Fuel cost per km | Fuel for 1,000 km | Cost for 1,000 km |
|---|---|---|---|
| 10 km/l | ₹10.00 | 100 litres | ₹10,000 |
| 12 km/l | ₹8.33 | About 83.33 litres | About ₹8,333 |
| 15 km/l | ₹6.67 | About 66.67 litres | About ₹6,667 |
| 18 km/l | ₹5.56 | About 55.56 litres | About ₹5,556 |
| 20 km/l | ₹5.00 | 50 litres | ₹5,000 |
The cost for 1,000 km is calculated from the unrounded value. That is why multiplying the displayed two-decimal figure may differ slightly. Do not interpret the rows as a promise that changing driving style alone can move any car between these results.
Turn cost per kilometre into a monthly budget
Monthly petrol spending equals monthly distance divided by mileage, multiplied by price. At 1,200 km, an assumed 15 km/l and ₹100/litre, the car uses 80 litres and costs ₹8,000 in petrol for the month.
At 600 km with the same assumptions, fuel cost is ₹4,000. At 1,800 km, it is ₹12,000. Distance matters as much as the efficiency figure, so count expected journeys before deciding whether a fuel type suits your budget.
Add separate monthly amounts for parking and other routine expenses if you want a cash budget. Keep the fuel subtotal visible. A combined monthly number without its components makes it harder to understand whether a change came from fuel use, distance or another bill.
Combine city and highway use properly
Do not simply average two mileage figures unless their relationship makes that appropriate. Calculate litres used for each distance segment, add the litres and divide total distance by total litres. This respects how fuel is actually consumed.
Consider an assumed month with 600 km at 12 km/l and another 600 km at 18 km/l. The first segment uses 50 litres and the second about 33.33 litres. Total use is about 83.33 litres for 1,200 km.
The combined efficiency is therefore 14.4 km/l, not 15 km/l. At ₹100/litre, the monthly fuel bill is about ₹8,333 and the combined cost is about ₹6.94/km. The example uses assumptions to show the method, not measured results for a vehicle.
Measure your own mileage without unsafe refuelling habits
Use ordinary refuelling records and follow the vehicle and fuel-station instructions. Note the odometer at a consistent refill point and record the litres added at the next comparable refill. Do not force extra fuel into the tank to produce an apparently exact measurement.
If the refill method leaves the tank at roughly the same level, distance travelled divided by litres added provides an estimate. For example, an assumed 450 km and 30 litres gives 15 km/l. Repeating the process reduces the influence of one unusual fill.
Record conditions that substantially change use, such as long idling or a different route. Do not take photographs or enter records while driving. Keep the task simple enough to maintain safely over several weeks.
A dashboard number and a receipt answer different questions
The dashboard estimate can be useful for observing trends, but its measurement and reset period need to be understood. It may cover a different distance from the one you are using for the budget. Read the manual before treating a trip display as a monthly total.
Fuel purchased in a calendar month is also not always fuel consumed in that month. You may finish with more fuel in the tank than you started with. For a short period, that difference can make spending divided by distance look unusually high or low.
Use comparable start and end conditions when measuring efficiency. For household cash planning, record the actual purchase payments too, but label them as spending rather than assuming they precisely measure consumption over the same dates.
Test a change in fuel price separately
At an assumed 15 km/l, raising petrol from ₹100 to ₹105/litre changes fuel cost from about ₹6.67 to ₹7.00/km. For 1,200 km, the bill rises from ₹8,000 to ₹8,400, assuming efficiency and distance stay unchanged.
This is a ₹400 monthly difference under those inputs. It is not a forecast of a price rise. A simple sensitivity check is more useful than a confident annual prediction about a price you do not control.
If distance also changes, show that as a separate case. Otherwise, it is easy to blame the entire increase on fuel prices when extra journeys account for part of it. Clear inputs make the calculation easier to update later.
Fuel cost is not full running cost
Insurance, service, tyres, repairs, parking and tolls are not included in the petrol formula. Decide whether you want a fuel-only comparison or a broader ownership calculation. Name the result clearly so readers do not mistake one for the other.
For example, adding ₹12,000 of annual parking to a 12,000 km budget contributes ₹1/km if you choose to express that fixed bill per kilometre. At only 6,000 km, the same parking cost contributes ₹2/km. The petrol efficiency has not changed.
Loan principal and depreciation need care in a full ownership view. Monthly EMI is useful for affordability, while depreciation reflects value loss. Do not add the whole EMI and depreciation together as if they were independent operating costs.
Use the calculation when comparing petrol with another fuel
Compare the same expected distance and realistic conditions. A CNG calculation needs km/kg and rupees per kilogram, plus any petrol used. An EV calculation needs billed energy and charging assumptions. Do not compare fuel quantities with different units directly.
Then consider any purchase premium and differences in other ownership expenses. A lower fuel cost per kilometre does not prove that the more expensive vehicle pays back within your intended ownership period. Calculate the savings and the time needed to recover the extra amount.
Keep comfort, availability and your route in the decision. A cheaper fuel that requires a long detour may not produce the expected saving. The calculator should inform a suitable car choice rather than replace the practical checks.
Questions about petrol cost calculations
What is the fuel cost at 15 km per litre
Divide your local petrol price by 15. At an assumed ₹100/litre the result is about ₹6.67/km. At ₹105/litre it is ₹7/km. These are calculations, not a national price statement.
How much petrol money do I need for 100 km
Use 100 divided by mileage, multiplied by price. At assumed 15 km/l and ₹100/litre, the cost is about ₹667. Actual use depends on conditions, so allow for uncertainty on an unfamiliar journey.
Can I use the advertised mileage for budgeting
You can use it as a clearly labelled test-condition input, but should not assume your car will match it. Use a cautious range and later replace it with records from your own driving.
Should I average city and highway mileage
Calculate fuel used for each segment instead. Add the distance and litres, then divide total distance by total litres. A simple average of km/l figures can give the wrong combined result.
Does servicing guarantee better mileage
Follow the required maintenance schedule, but do not assume a fixed improvement. If consumption changes unexpectedly, review records and conditions and seek qualified diagnosis where needed. Avoid buying an unverified additive based on a promised percentage saving.
Calculate your own petrol budget
Write down your local price, expected monthly kilometres and a realistic mileage range. Calculate a normal and a higher-cost case, then keep other ownership bills separate. After several refill cycles, update the assumptions with your own records before comparing a different fuel or vehicle.