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The quickest formula is: if your car uses litres per 100 kilometres (L/100 km), multiply that figure by your fuel price per litre and divide by 100. If it uses kilometres per litre (km/L), divide the price per litre by the km/L figure.
For example, at 7.5 L/100 km and 1.80 currency units per litre, fuel costs 0.135 currency units per kilometre. At 12 km/L and the same fuel price, the result is 0.15 currency units per kilometre. The method is simple; the accuracy depends on compatible units, a realistic fuel price and a consumption figure that reflects how you drive.
The two formulas you need
Fuel cost per kilometre is the fuel used for each kilometre multiplied by what that fuel costs. Your vehicle may express consumption in either of two common ways.
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Use this formula:
cost per km = (L/100 km × price per litre) ÷ 100
The same calculation can be made in two stages:
- Multiply L/100 km by the price per litre to find the fuel cost for 100 km.
- Divide the 100-km cost by 100.
Example: A car rated at 7.5 L/100 km, with fuel priced at 1.80 currency units per litre:
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(7.5 × 1.80) ÷ 100 = 0.135
Fuel therefore costs 0.135 currency units per kilometre, or 13.5 currency units per 100 km.
When consumption is shown as km/L
Use this formula:
cost per km = price per litre ÷ km/L
Example: At 12 km/L and 1.80 currency units per litre:
1.80 ÷ 12 = 0.15
The result is 0.15 currency units per kilometre, or 15 currency units per 100 km.
These examples illustrate the arithmetic only. They are not a current fuel-price claim or a result for a particular vehicle.
Check the units before calculating
Most incorrect answers come from mixing units rather than from difficult mathematics. Confirm each input before pressing a calculator button.
- Consumption: identify whether the vehicle figure is L/100 km or km/L.
- Fuel price: use the price you actually pay. Express it per litre when using either formula above.
- Distance: keep the distance in kilometres. Convert a trip stated in miles before applying a per-kilometre result.
- Currency: keep the price and final answer in the same currency. A result in currency units per kilometre is meaningful only when the price uses that currency.
- Fuel volume: if a station displays a price per gallon, convert it to a per-litre price first. Do not combine a per-gallon price with an L/100 km rating.
Do not confuse the two consumption formats. A lower L/100 km number means less fuel used, while a higher km/L number means more distance from each litre. They describe the same relationship in opposite directions.
Convert between L/100 km and km/L
If one source gives L/100 km and another gives km/L, convert them before comparing. The reciprocal relationship is:
km/L = 100 ÷ (L/100 km)L/100 km = 100 ÷ (km/L)
For example, 7.5 L/100 km corresponds to about 13.33 km/L, while 12 km/L corresponds to about 8.33 L/100 km. Use the unrounded conversion in a spreadsheet when possible; display rounding can otherwise create a small difference in the final cost.
These conversions do not change the vehicle’s underlying fuel use. They only put the same consumption result into the format required by your calculator, comparison table or local vehicle guide.
Calculate the cost of a trip or a driving period
From a per-kilometre result
Once you know cost per kilometre, multiply by the distance you intend to drive:
trip fuel cost = cost per km × trip distance
If the calculated cost is 0.135 currency units per kilometre and the trip is 420 km:
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The estimated fuel cost is 56.70 currency units, assuming the same consumption and fuel price throughout the trip.
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From L/100 km without first calculating cost per kilometre
You can calculate the litres needed for a distance directly:
litres needed = (L/100 km × distance) ÷ 100
Then multiply those litres by the price per litre:
trip cost = ((L/100 km × distance) ÷ 100) × price per litre
For a monthly or weekly estimate, replace trip distance with the kilometres driven during that period. This is still a fuel-only estimate; it does not include maintenance, insurance, financing, depreciation, parking or other ownership costs.
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Use the total kilometres driven, not just the outbound distance. Add the return leg, regular detours and any local driving at the destination before multiplying by cost per kilometre. If the route has substantially different city and highway portions, a combined figure that reflects that mix is more useful than a single highway or city number.
Estimate your real fuel cost from fill-ups
A published consumption rating is useful for a quick comparison, but your own full-to-full records can show what your vehicle actually used on your routes.
What to record
- Odometer reading when you fill the tank.
- Odometer reading at the next fill-up, or the distance travelled between fills.
- Litres purchased at the later fill-up.
- Price per litre paid for that purchase.
Use a notes app, spreadsheet or paper log. A dedicated logbook is optional; the arithmetic requires only the readings, litres and price. Natural Resources Canada’s fuel-consumption calculator guidance describes the same core inputs—odometer reading, distance travelled and litres purchased—and lists a kit containing a fuel log, scale and calculator as a tracking aid.
Use a full-to-full interval
Start with a full tank and refill to the usual full level at the end of the interval. Calculate the distance from the odometer readings, then use the litres bought at the refill:
L/100 km = (litres purchased ÷ kilometres driven) × 100
For example, if you drive 615 km and the refill takes 46.2 litres:
(46.2 ÷ 615) × 100 = 7.512...
Rounded appropriately, that interval used about 7.5 L/100 km. If the fuel cost 1.80 currency units per litre, calculate the interval’s fuel cost per kilometre directly:
cost per km = (litres purchased × price per litre) ÷ kilometres driven
(46.2 × 1.80) ÷ 615 ≈ 0.135
The direct fill-up formula is useful because it uses the fuel actually purchased and the distance actually driven, rather than a brochure figure.
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Track several intervals
One short interval can be distorted by a partial fill, a different pump shut-off point, unusual traffic or a trip dominated by hills. Record several full-to-full intervals that include the driving you normally do. You can calculate each interval separately, or add all litres and all kilometres over the same set of intervals before applying the formula. The latter approach reduces the effect of one unusual tank.
When combining records, add litres first and kilometres first, then divide the totals. Do not average the individual L/100 km figures without considering the distance represented by each tank; a short interval should not have the same influence as a much longer one. Keep fuel receipts if you want to check that the litres and price entered in the log match the pump transaction.
Why your result can differ from the vehicle rating
Official consumption figures are comparison measures produced under controlled laboratory conditions. They do not promise that every driver will obtain the same number on the road. The Australian Government’s Green Vehicle Guide states that fuel consumption achieved on the road depends on how the vehicle is used and may vary from the figures reported on its website.
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Your personal result can change when any of these conditions changes:
- City-heavy driving with frequent stops versus longer highway runs.
- Different daily distances or trip lengths.
- Changes in how the vehicle is used, loaded or driven.
- A different local fuel price or a price that changes between fill-ups.
The U.S. Department of Energy’s Alternative Fuels Data Center uses daily distance and the share of city and highway driving in its calculator inputs, and allows fuel prices to be changed to reflect local conditions. That illustrates why a calculator result is only as representative as the assumptions entered.
Weather, traffic and route conditions can make a particular interval atypical even when the vehicle and driver have not changed. Treat the official number as a consistent comparison point and your multi-fill result as evidence of your own operating pattern.
How to choose a consumption figure
For a quick estimate
Use the vehicle’s published consumption figure in the format supplied by the manufacturer or an official vehicle guide. Keep the test basis with the number when you write down your estimate, because a combined, city or highway figure represents a different driving pattern.
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For a personal budget
Use your own full-to-full average when you have enough records. If you are planning a route you have not driven before, choose the figure that best resembles that route rather than automatically using the lowest published number.
For comparing two vehicles
Apply exactly the same fuel price, distance and driving assumptions to both vehicles. Otherwise, the apparent difference may come from the inputs rather than the vehicles.
| Use case | Best consumption input | Why |
|---|---|---|
| Fast approximate calculation | Published L/100 km or km/L figure | Available before you have personal records |
| Household fuel budget | Average from several full-to-full intervals | Reflects your routes and driving style |
| Planned trip | Figure matching the expected city/highway mix | Avoids treating an unrelated test condition as your route |
| Vehicle comparison | Same price and driving assumptions for every vehicle | Keeps the comparison fair |
Making a fair vehicle comparison
Fuel-only cost per kilometre answers one narrow question: how much fuel money is associated with each kilometre under stated assumptions. It is not total ownership cost. A broader ownership comparison may also include purchase price, maintenance, insurance and other expenses.
For a fuel comparison, document these inputs beside every result:
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- Fuel type and price per litre.
- Date and location of the price, since prices vary over time and place.
- Consumption basis: published, city, highway, combined or personal full-to-full average.
- Distance used for the comparison.
- Driving mix, such as mostly urban, mostly highway or a defined split.
Natural Resources Canada says its combined EnerGuide rating represents 55% city and 45% highway driving. Its annual fuel-cost estimate uses that combined rating, 20,000 km driven and the indicated fuel price. Those are label assumptions, not a prediction that every driver travels 20,000 km or drives in that exact mix. If your driving differs, calculate with your own distance and a more suitable consumption figure.
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Use price and date assumptions transparently
A result without a price date is difficult to reproduce. Write the fuel price beside the answer, along with the currency, station location and date. If you are preparing a yearly budget, use a price assumption you can explain and revisit it when local prices change. For a trip, the price at departure may be a reasonable starting point, but the actual bill can differ if you refuel elsewhere.
When comparing vehicles, change only one input at a time if you want to understand its effect. Holding price, distance and driving mix constant isolates the consumption difference. Changing the price and the consumption basis together can hide which assumption caused the result to move.
Spreadsheet and calculator setup
A spreadsheet makes repeated estimates easier and leaves an audit trail for each input.
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Set up cells for:
A2: consumption in L/100 kmB2: fuel price per litreC2: distance in kilometres
Use these formulas:
Cost per km: =(A2*B2)/100Trip cost: =((A2*C2)/100)*B2Litres for trip: =(A2*C2)/100
Basic km/L worksheet
Set up:
A2: efficiency in km/LB2: fuel price per litreC2: distance in kilometres
Use:
Cost per km: =B2/A2Trip cost: =(B2/A2)*C2
Format the result as currency or retain additional decimal places for short distances. Rounding 0.135 to 0.14 may be convenient for display, but keep the unrounded value when multiplying by a long trip distance.
Fill-up worksheet
For a log, use one row per interval with columns for starting odometer, ending odometer, kilometres, litres purchased, price per litre and cost per kilometre. Set kilometres to ending odometer minus starting odometer. Then use:
Interval cost per km = (litres purchased × price per litre) ÷ kilometres
At the bottom, total the litres, kilometres and fuel cost across several complete intervals. The total fuel cost divided by total kilometres gives the period’s observed cost per kilometre.
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Using the wrong formula for the display format
Dividing price by consumption is correct for km/L, not for L/100 km. For L/100 km, multiply consumption by price and divide by 100.
Mixing gallons and litres
Convert a per-gallon price to a per-litre price before using an L/100 km or km/L figure. Also confirm whether the source uses a US or imperial gallon; they are different volumes.
Using a price from the wrong place or date
A fuel cost estimate is only as current as its price input. Enter the price you expect to pay, record the date and location, and update it when prices change.
Calling fuel-only cost the cost of driving
Fuel cost per kilometre excludes non-fuel expenses. Label the result clearly so it is not mistaken for total running cost or total ownership cost.
Treating one tank as a permanent average
Use consistent full-to-full records and several intervals. A single short or unusual interval is not a reliable long-term baseline.
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Comparing vehicles with different assumptions
Use the same fuel price, distance and driving pattern for each vehicle. If one result uses a city rating and another uses a combined rating, the comparison is not like-for-like.
Rounding too early
Keep several decimal places during the calculation and round the displayed answer at the end. Early rounding can become noticeable when a small per-kilometre amount is multiplied by hundreds or thousands of kilometres.
A practical workflow
- Identify the consumption unit. Write down the vehicle’s L/100 km or km/L figure and whether it is a city, highway, combined or personal result.
- Record the fuel price. Use the price per litre you actually pay, with the currency, date and location.
- Convert units. Convert gallons to litres and miles to kilometres before calculating.
- Apply the matching formula. Use multiplication and division by 100 for L/100 km, or price divided by km/L for km/L.
- Scale to your distance. Multiply cost per kilometre by the trip, weekly, monthly or annual kilometres you want to budget.
- Validate with fill-ups. Compare the estimate with several full-to-full intervals and replace the published figure with your own average when you have enough records.
- Document assumptions. Keep the price, date, distance and driving mix alongside the answer.
What the number tells you—and what it does not
The result lets you answer practical questions such as “How much fuel will this trip cost?” or “Which of these two vehicles uses less fuel under the same conditions?” It also provides a consistent unit for a household budget or reimbursement calculation.
It does not guarantee a pump bill. Fuel prices can change, and real consumption changes with use. A controlled test rating is useful for comparison, while a multi-fill personal average is better for forecasting your own spending. State the assumptions every time you share the result so another person can reproduce it.
Frequently Asked Questions
How do I calculate fuel cost per kilometre from L/100 km?
Multiply the consumption figure by the fuel price per litre, then divide by 100: cost per km = (L/100 km × price per litre) ÷ 100. At 7.5 L/100 km and 1.80 currency units per litre, the result is 0.135 currency units per kilometre.
How do I calculate fuel cost per kilometre from km/L?
Divide the price per litre by the kilometres-per-litre figure: cost per km = price per litre ÷ km/L. At 12 km/L and 1.80 per litre, the result is 0.15 per kilometre.
How do I work out the fuel cost of a trip?
Multiply cost per kilometre by the trip distance in kilometres. Alternatively, calculate litres needed with (L/100 km × distance) ÷ 100, then multiply those litres by the price per litre.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchCan I use my own fuel receipts instead of the published rating?
Yes. Start with a full tank, record the odometer, refill to the usual full level, and record the kilometres and litres purchased. Use (litres purchased × price per litre) ÷ kilometres driven. Several full-to-full intervals provide a more useful personal average than one tank.
Why is my calculated cost higher than the vehicle’s advertised figure?
Published figures come from controlled tests and are intended for comparison. Your city/highway mix, daily distance, traffic, route and vehicle use can produce different on-road consumption. The fuel price may also differ from the assumption used in the published estimate.
Should I include maintenance and insurance in fuel cost per kilometre?
No. Fuel cost per kilometre covers fuel only. Maintenance, insurance, financing, depreciation, parking and other expenses belong in a separate running-cost or total-ownership calculation.
What fuel price should I enter?
Use the price you actually pay, expressed per litre, and note its currency, date and location. For planning, use a clearly stated local price assumption and update it when prices change.
How does Natural Resources Canada calculate its annual fuel-cost example?
Its EnerGuide guidance uses a combined rating based on 55% city and 45% highway driving, 20,000 km, and the indicated fuel price. Those are label assumptions and may not match your annual distance or driving mix.
The Bottom Line
Use (L/100 km × price per litre) ÷ 100 or price per litre ÷ km/L. Multiply the result by your trip distance, keep units and price dates consistent, and replace the published figure with a multi-fill personal average when you need a realistic household budget.
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