Fuel Cost Calculator

Enter your distance, how much fuel the car uses and the price at the pump, and the calculator returns the litres or gallons burned, the total fuel cost, and the cost for each kilometre or mile. Pick the unit system that matches your dashboard: metric litres per 100 km, US miles per gallon, or UK miles per gallon on the larger imperial gallon. The price field carries your local currency symbol, so the total comes back in the money you actually hand over.

How the cost is worked out

Each mode runs a short two-step calculation: work out the fuel quantity, then multiply by the price.

Metric: litres = distance × L/100km ÷ 100,   cost = litres × price

US: gallons = distance ÷ mpg,   cost = gallons × price

The UK path is the same as the US one but converts imperial gallons to litres with the factor 4.54609 before pricing, because British pumps sell fuel by the litre while cars are still rated in mpg.

Take the metric defaults: 450 km at 7.2 L/100km. That burns 450 × 7.2 ÷ 100 = 32.4 litres. At 1.85 a litre the trip costs 32.4 × 1.85 = 59.94, which works out to 0.13 per kilometre. Switch to the US defaults and 280 miles at 32 mpg needs 8.75 gallons, or 30.19 at 3.45 a gallon. Nothing in the arithmetic is harder than a multiplication and a division, but the order matters: fuel economy has to become a fuel quantity before any money enters the sum, and that first step is where the three unit systems part company.

Two ways to measure the same thing

L/100km and mpg describe identical behaviour from opposite ends. One is volume per fixed distance, the other distance per fixed volume, which makes them reciprocals rather than a simple rescaling of each other. That inversion has a practical edge: you cannot meaningfully average mpg figures or add them, and equal-looking mpg gains do not save equal amounts of fuel. Over 10,000 miles, lifting a thirsty car from 15 to 20 mpg saves about 167 gallons, while lifting an efficient one from 40 to 50 mpg saves only 50, even though the second jump looks larger. Consumption stated as L/100km shows that saving directly, which is part of why continental Europe settled on it.

The conversion constants fall out of the unit definitions. To turn US mpg into L/100km you divide 235.21 by the mpg figure; for UK mpg the constant is 282.48, larger because the imperial gallon is larger. That same gallon makes UK mpg read roughly 20 percent better than US mpg for an identical car, the single most common mix-up when comparing models across the Atlantic.

Read across a row and all three cells describe the same car. The metric default of 7.2, the US default of 32 mpg, which is 7.35 L/100km, and the UK default of 40 mpg, which is 7.06 L/100km, all land inside a narrow band, so the three modes price a broadly similar vehicle.

Where the two gallons came from

The gap between the American and British gallon is a piece of eighteenth-century administration that never got tidied up. England once kept several gallons at once: a wine gallon for wine and spirits, an ale gallon of 282 cubic inches for beer, and a corn or Winchester gallon of about 268.8 cubic inches for grain. The wine gallon was fixed at 231 cubic inches by an Act of Parliament passed under Queen Anne in 1706 and in force from 1707, mainly so customs officers could assess duty on imported wine. When the American colonies separated in 1776 they carried that Queen Anne wine gallon with them, and it survives as the US liquid gallon, still defined as exactly 231 cubic inches, or 3.785411784 litres.

Britain then changed course. The Weights and Measures Act of 1824 swept away the competing gallons and defined a single imperial gallon as the volume of ten pounds of distilled water at 62 degrees Fahrenheit with the barometer at 30 inches of mercury, which worked out at 277.274 cubic inches. That measure took legal effect on 1 January 1826, and the exact modern figure of 4.54609 litres, about 277.42 cubic inches, was settled only after the litre itself was redefined in 1964. The United States, already independent, had no reason to adopt the new standard, so the two systems parted permanently. A British gallon has been about a fifth larger than an American one ever since, which is why the same efficiency yields a bigger mpg number in Britain and why this tool keeps the two modes separate.

How fuel economy became a number worth watching

Fuel economy began as a sporting statistic rather than a regulatory one. From 1936 to 1968, with a break for the Second World War, the Mobil Economy Run sent showroom-standard cars across the United States on public roads under the eye of United States Auto Club observers, and from 1959 it ranked them plainly on miles per gallon. For decades, though, the number stayed a curiosity rather than a constraint: American petrol was cheap, and cars grew steadily heavier and more powerful.

That changed in the autumn of 1973. The Organization of Arab Petroleum Exporting Countries declared an embargo that October against the United States and other states seen as backing Israel during the Yom Kippur War, and crude rose from roughly 3 dollars a barrel to nearly 12 by the time it was lifted in March 1974. Pump prices jumped and stations rationed supply. In December 1975 Congress responded with the Energy Policy and Conservation Act, which created the Corporate Average Fuel Economy standards. Those rules set a passenger-car fleet average of 18 mpg for the 1978 model year rising to 27.5 mpg for 1985, roughly double the new-car average of the mid-1970s, with the National Highway Traffic Safety Administration setting the targets and the Environmental Protection Agency running the tests behind the window-sticker figure.

Europe reached for a different measuring stick. Rather than distance per gallon, regulators framed efficiency as the fuel used over a standard 100-kilometre journey, a figure that ties straight to running cost and carbon output. The test cycles have since been tightened: the old New European Driving Cycle gave way to the Worldwide Harmonised Light Vehicle Test Procedure, required for new EU type approvals from September 2017 and for every new registration from September 2019, precisely because laboratory numbers had drifted so far from what drivers saw on the road.

What the estimate leaves out

The result is pure fuel. It ignores tolls, parking, tyres, servicing and depreciation, which together often cost more per mile than the petrol does. It also assumes a steady real-world economy, so enter the figure your trip computer shows over several tanks rather than the optimistic brochure number, which comes from a controlled laboratory cycle that rarely matches motorway speeds, cold starts or a loaded roof rack.

Pump conventions differ by market, and the mode you choose should match your dashboard rather than your passport. The US prices per US gallon and rates cars in mpg. Canada sells fuel by the litre and rates economy in L/100km despite older mpg habits, so metric mode fits. The UK sells by the litre but quotes mpg, which is exactly what UK mode handles. The eurozone and Australia both use litres and L/100km throughout, so metric mode is the natural pick there too.

Frequently asked questions

How do I convert mpg to L/100km?

Divide a fixed constant by the mpg figure. For US mpg the constant is 235.21, so 32 US mpg is 235.21 ÷ 32 = 7.35 L/100km. For UK mpg the constant is 282.48 because the imperial gallon is larger, so 40 UK mpg is 282.48 ÷ 40 = 7.06 L/100km. The relationship is inverse, which is why a small mpg gain matters more on a thirsty car than an efficient one.

Why does a UK car get more miles per gallon than a US one?

The gallons are different sizes. A US gallon is 3.785 litres while a UK imperial gallon is 4.546 litres, about 20 percent bigger, so the same fuel efficiency produces a bigger mpg number in the UK. A car rated 40 mpg in Britain is only about 33 mpg by the US measure. When comparing cars across the Atlantic, convert both to L/100km first.

How much does it cost to drive 100 miles?

At 32 US mpg you burn 100 ÷ 32 = 3.125 gallons, and at 3.45 a gallon that is 10.78 for the 100 miles, or about 0.11 per mile. In the UK at 40 mpg and 1.85 a litre the same 100 miles uses 11.37 litres and costs 21.03. Halving the price or doubling the economy halves the total in either system.

Does the calculator include tolls, wear or an electric range?

No. It prices only the liquid fuel for the distance you enter, so tolls, parking, tyres, servicing and depreciation sit outside the figure. For an electric car the fuel side does not apply at all; use an electricity cost calculator with your kWh per 100 km and your tariff instead.

Why is the pump number higher than the manufacturer's mpg claim?

Lab economy figures come from controlled test cycles that rarely match real driving. Cold starts, motorway speeds, air conditioning, roof boxes and heavy traffic all push real consumption 10 to 30 percent worse than the sticker. Enter the economy you actually see on the trip computer over a few tanks for a realistic trip cost.