Comparing Vehicle Fuel Costs: How to Calculate Fuel Cost per Kilometer or Mile

The short answer: the fuel cost formula

Every fuel cost calculator, whether it works in kilometers or miles, runs the same arithmetic. It multiplies the fuel a vehicle burns over a distance by the price you pay for each unit of that fuel. The only complication is the unit the vehicle's consumption figure is quoted in, because that decides whether you multiply or divide.

Consumption in L/100 km:   cost = (distance_km ÷ 100) × L_per_100km × price_per_litre
Economy in km/L:           cost = (distance_km ÷ km_per_litre) × price_per_litre
Economy in mpg:            cost = (distance_miles ÷ mpg) × price_per_gallon
Electric, kWh/100 km:      cost = (distance_km ÷ 100) × kWh_per_100km × price_per_kWh
Electric, kWh/100 mi:      cost = (distance_miles ÷ 100) × kWh_per_100mi × price_per_kWh

A few terms are worth defining. Fuel consumption, such as L/100 km, measures how much fuel the vehicle uses to cover a fixed distance, so a lower number is better. Fuel economy, such as km/L or mpg (miles per gallon), measures how far the vehicle travels on a fixed amount of fuel, so a higher number is better. For electric vehicles, energy use is usually given in kilowatt-hours (kWh) per 100 km or per 100 miles, and a kWh is the unit electricity is billed in.

Two conditions change the result more than anything else: the consumption figure you use, and the price you enter. This guide does not supply live fuel or electricity prices or consumption figures for specific models. Use the price on your own receipts or utility bill, and either the official rating for your vehicle's model year and market or your own measured consumption.

Worked examples

The figures below are hypothetical, chosen only to show the arithmetic. They are not real prices or real model ratings, and no currency is implied.

Example 1, a single journey in km/L: a 900 km trip in a car that averages 15 km/L uses 900 ÷ 15 = 60 litres. At a price of 1.50 per litre, the journey costs 60 × 1.50 = 90. For a return trip, double the distance before you calculate.

Example 2, comparing two cars over a year in L/100 km: suppose you drive 15,000 km a year. Car A uses 6.0 L/100 km and Car B uses 8.0 L/100 km. Car A needs 150 × 6.0 = 900 litres. Car B needs 150 × 8.0 = 1,200 litres. At 1.50 per litre, that is 1,350 against 1,800, a difference of 450 a year.

Example 3, comparing two cars in mpg: over 12,000 miles, a 30 mpg car needs 12,000 ÷ 30 = 400 gallons and a 25 mpg car needs 12,000 ÷ 25 = 480 gallons. The gap is 80 gallons a year. Multiply 80 by your local price per gallon to get the yearly cost difference. Working out the fuel volume before you apply a price keeps the comparison valid as prices change.

Converting between units

Only compare vehicles once their figures are in the same unit. The conversions below come from the exact definitions of the mile (1.609344 km), the US gallon (3.785411784 litres) and the imperial gallon (4.54609 litres). The US and UK both use the term "mpg" but mean different gallons, so a UK mpg figure is not interchangeable with a US one.

km/L          = 100 ÷ (L/100 km)
L/100 km      = 100 ÷ (km/L)
L/100 km      ≈ 235.215 ÷ US mpg
L/100 km      ≈ 282.481 ÷ UK (imperial) mpg
US mpg        ≈ 2.352 × km/L
kWh/100 km    = kWh/100 mi ÷ 1.609344
price/litre   = price per US gallon ÷ 3.785411784

Because consumption and economy are reciprocals, equal-looking improvements in mpg or km/L are not worth the same amount of fuel. Moving from 15 to 20 mpg saves more fuel over a given distance than moving from 35 to 40 mpg. Converting to fuel used per distance, as in Examples 2 and 3, shows the real difference.

Comparing vehicles and powertrains fairly

If you are choosing between vehicles, the calculation is only as good as the inputs you feed it. Gasoline, diesel, and electricity each have their own price per unit, so calculate each vehicle with the price of the energy it actually uses. Do not convert electricity into a "fuel equivalent" for a cost comparison. Calculate the electric vehicle's cost directly from kWh and your electricity price, and then compare the cost per kilometer or per mile.

Criterion | What to check | Why it matters

Measurement basis | All figures come from the same test procedure or the same real-world method | Official tests in different markets (for example, US EPA ratings and the European WLTP procedure) use different test procedures, so their figures are not directly comparable

Version | Model year, engine, transmission, drivetrain and wheel size match the actual vehicles | Ratings for one trim or model year may not apply to another

Unit | Every figure is converted to the same unit | Mixing mpg with L/100 km, or US mpg with imperial mpg, produces false gaps

Price per unit | Your own current price for each fuel type and charging method | Gasoline, diesel, home charging and public charging are all priced separately

Distance | Your realistic annual or trip distance | The absolute cost gap grows in proportion to distance

Electric energy figure | Whether kWh use is measured at the wall or from the battery | Charging losses mean that figures measured at the wall and from the battery are not equivalent

Once your inputs are compatible, the trade-offs depend on how you use the vehicle. Annual distance is the biggest lever. Someone who drives a lot will see a consumption difference translate into a large yearly sum, while someone who drives little may find that the difference barely offsets a higher purchase price. Where you refuel or charge matters too. An electric vehicle's running cost depends heavily on whether it is charged mostly at home or at public chargers, which can be priced very differently, so run the calculation with the mix that matches your situation.

Vans and other load-carrying vehicles need extra care. Consumption depends on payload, towing, and body type, so a rating measured in a lighter configuration may underestimate what a loaded van uses. For these vehicles, your own logged figures under typical loads are more useful than a published figure.

Estimating payback on a more efficient vehicle

To estimate how long a more expensive but more efficient vehicle takes to recover its extra purchase cost through fuel savings, divide the price difference by the annual fuel or energy saving. Using the hypothetical Example 2, a price difference of 1,800 divided by a saving of 450 a year gives four years. This is a simple payback figure covering fuel only. It leaves out insurance, maintenance, depreciation, financing, taxes, and incentives, any of which can move the answer in either direction, and it assumes prices and your annual distance stay the same.

Measuring your own vehicle's consumption

Official ratings come from standardized tests and may differ from what you get in your own driving. The full-to-full (tank-to-tank) method gives a figure based on your real use:

• Fill the tank completely and reset the trip meter.
• Drive normally, ideally over several hundred kilometers or miles of your usual mix of roads.
• Fill the tank completely again and note the litres or gallons the pump shows.
• Divide that volume by the distance and multiply by 100 to get L/100 km, or divide the distance by the volume to get km/L or mpg.
• Repeat over several fill-ups and average the results, because a single tank can mislead.

For an electric vehicle charged at home, a dedicated charger or meter that records the energy delivered gives a wall-to-wheels figure. The car's own energy display may report a different value, depending on how the manufacturer measures it.

What this guide cannot tell you

The material available for this guide does not include current fuel or electricity prices, consumption ratings for specific makes and models, or regional tariffs. It also cannot tell you how particular online calculators handle unit conversions or charging losses. Before relying on any result for a purchase decision, check the official rating for the exact vehicle version in your market, confirm your local energy prices, and check that every vehicle you are comparing uses the same measurement basis.