Fuel Cost Calculator

How the Fuel Cost Calculator Works

Three numbers drive the calculation: how far you're going, how efficiently your vehicle uses fuel, and what fuel costs at the pump right now. Enter all three and the calculator works out how many litres (or gallons) the trip needs, multiplies by the fuel price, and shows you the total.

For round trips, the distance doubles before the calculation runs. For commutes, enter how many trips you make per month and the calculator scales everything up to weekly, monthly, and annual totals. The extra costs section lets you add tolls, parking, and a maintenance allocation per trip so the result reflects what the journey actually costs, not just the petrol.

The Formula

Unit Conversions

The calculator accepts three efficiency formats and converts them internally before running:

• L/100km to km/L: divide 100 by your L/100km figure
• US MPG to km/L: multiply MPG by 0.4251
• UK MPG to km/L: multiply MPG by 0.3540 (UK gallon is 4.546 L, US gallon is 3.785 L)

You don't need to convert anything manually. Select the unit that matches your vehicle's displayed efficiency and the calculator handles the rest.

The Annual Fuel Bill Most Drivers Have Never Actually Seen

Most people refuel without thinking about what it adds up to over a year. The transaction feels small each time. It isn't small.

A driver covering 15,000 km per year in a car averaging 13 km/L at a fuel price of around $1.60 per litre (typical across much of Europe and Australia in 2025 and 2026) spends roughly $1,846 on fuel annually. At 12,000 miles per year in the US with a car getting 28 MPG and fuel at $3.40 per gallon, the annual bill is around $1,457. In the UK at 40 MPG and 165p per litre, it comes to about £1,530.

None of those numbers are shocking in isolation. What makes them land differently is comparison. The average person in each of those markets spends less per year on their electricity bill than on car fuel. The fuel cost exceeds what most households spend on clothing, streaming, gym memberships, and dining out combined.

The reason it doesn't feel that way: you pay in small chunks, six or eight times a month, and each transaction disappears into normal life. The calculator's monthly and annual outputs are often the first time someone has seen the total written down. Use that number. It's more useful than the trip cost.

What a 2 km/L Improvement Is Actually Worth

Upgrading from 12 km/L to 14 km/L on 15,000 km per year at $1.60/L saves:

Over five years that's $1,430 in fuel, ignoring any price increases. That's the number to keep in mind when a salesperson quotes the difference in purchase price between two models.

Speed Costs More Than Most Drivers Realise

Aerodynamic drag increases with the square of speed. That's not a vague warning to drive slower. It's physics that produces a specific, measurable cost at highway speeds.

A typical family car tested at 80 km/h (50 mph) might achieve 15 km/L. The same car at 120 km/h (75 mph) on the same road will typically return somewhere between 10 and 11 km/L. Not because the engine is working slightly harder. Because drag at 120 km/h is 2.25 times the drag at 80 km/h, and overcoming that drag is where the fuel goes.

On a 400 km motorway trip, that difference works out like this:

At $1.60/L that's $18.24 extra for the same journey. You also arrive about 83 minutes earlier. Whether that trade-off is worth it depends on the trip, but most drivers making it haven't done the arithmetic first.

The Optimal Speed Range

For most petrol and diesel vehicles, peak fuel efficiency occurs somewhere between 60 and 90 km/h (37 to 56 mph). Below that range, the engine runs less efficiently per unit of distance. Above it, aerodynamic drag dominates. The exact sweet spot varies by vehicle shape, weight, and engine size. Hatchbacks tend to hit their optimum around 70 km/h. Large SUVs and pickup trucks often peak closer to 60 km/h because their frontal area is larger and drag hits harder at higher speeds.

Cruise control on long highway stretches typically improves fuel efficiency by 7 to 14% compared to manual speed maintenance, purely by eliminating the small speed variations most drivers don't notice they're making.

When Does a More Fuel-Efficient Car Actually Pay for Itself?

This is the question behind almost every car buying decision involving fuel economy, and the answer is rarely what the dealership implies.

Say you're choosing between two cars. Car A costs $28,000 and averages 11 km/L. Car B costs $31,500 and averages 16 km/L. The $3,500 price difference feels significant. Here's how to work out whether it actually is.

Five years for a driver covering 15,000 km annually. For someone doing 25,000 km a year, break-even arrives in about three years. For someone doing 8,000 km a year, it takes closer to nine years, which may be longer than they plan to keep the car.

The mileage you actually drive is the variable that matters most in this calculation, not the fuel price. A 10% rise in fuel prices moves the break-even point by less than six months. Driving twice the assumed annual distance halves the break-even period.

The Hybrid and EV Version of the Same Question

Hybrid and electric vehicles typically carry a purchase premium of $3,000 to $8,000 over equivalent conventional models. The fuel (or electricity) saving per kilometre is larger, but the premium is also larger. The break-even calculation works the same way. Divide the price premium by the annual fuel saving. The result tells you how many years of driving at your typical annual distance are needed to recover the extra cost through running expenses alone.

Depreciation, insurance, and servicing cost differences are separate variables that can shift the conclusion significantly in either direction depending on the specific models being compared.

Your Driving Habits Have More Impact Than the Fuel Price

Fuel price gets attention because it's visible. The pump displays it in large numbers. Your driving style is invisible, which is why most people underestimate how much it matters. Research from multiple transport authorities consistently shows that the difference between an aggressive driver and a smooth driver in the same vehicle, on the same route, ranges from 20 to 40% in fuel consumption.

That's a bigger range than the price difference between the cheapest and most expensive fuel stations in most cities.

Cold Starts

A petrol engine's fuel consumption during the first five minutes after a cold start is roughly two to three times its normal rate. The engine management system runs a rich fuel mixture until the engine reaches operating temperature, which typically takes 5 to 8 minutes in moderate weather and longer in cold climates. Short trips of under 5 km never fully warm the engine up, meaning the vehicle operates in this inefficient state for most of the journey on every single trip.

This is why a driver making four 3 km trips per day uses disproportionately more fuel than a driver covering the same 12 km in one continuous trip.

Acceleration and Braking

Every time you accelerate hard and then brake to a stop, you're converting fuel into kinetic energy and then throwing that energy away as heat in the brake pads. Smooth acceleration and coasting to stops rather than braking hard can reduce fuel consumption on urban routes by 15 to 20%. This isn't a marginal improvement. It's the single highest-impact habit change available to a driver who can't change their vehicle or their route.

Loads and Accessories

Roof carriers and roof boxes add aerodynamic drag even when empty. An empty roof box at 100 km/h can reduce fuel efficiency by 10 to 25% depending on its size and shape. A loaded roof box is worse. Carrying 100 kg of extra weight in the car adds roughly 6 to 8% to fuel consumption on mixed routes.

Air conditioning at full output in slow city traffic increases fuel consumption by 8 to 15% in most petrol vehicles, and by more in smaller-engined cars where the AC compressor represents a larger fraction of total engine load.

What Your Cost Per Km Is Telling You About Your Car

The cost per km output from this calculator is more useful than most people treat it. Most drivers glance at the total trip cost and ignore the per-km figure. That's backwards. The total changes with every trip because the distance changes. The cost per km should stay roughly stable for a given vehicle unless something has changed.

If your cost per km has crept up over the past few months without a corresponding fuel price rise, the vehicle is trying to tell you something.

What Specific Changes Usually Mean

A sudden 15 to 20% drop in fuel efficiency, appearing over a week or two, usually points to one of three things: a clogged fuel injector, a failing oxygen sensor, or an air filter so blocked that the engine is running rich to compensate. Any of these is a reasonably quick workshop fix and typically costs far less than the ongoing fuel waste if left alone.

A gradual decline over six months or more, rather than a sudden drop, usually reflects accumulated wear: tyre pressure slowly falling, piston rings beginning to wear, or timing drift in older engines. Tyre pressure is worth checking first because it's free and faster than you'd expect: running 6 PSI below the recommended pressure increases rolling resistance enough to drop efficiency by 3 to 5% in most vehicles.

City mileage that's 40% worse than highway mileage, when the normal gap is 15 to 25%, often points to brake drag: a stuck caliper or handbrake cable that isn't fully releasing. The vehicle is fighting itself every metre it travels.

A Reference Range by Vehicle Type

If your vehicle's cost per km sits significantly above the upper end of its category range, the car either needs attention or the official efficiency rating was optimistic for your specific driving conditions. Both are worth investigating.

Calculator Category

This tool belongs to Travel Calculators. Browse similar tools for planning trips, fuel efficiency, and journey costs.