Free Fuel Cost Calculator — Trip, Annual & EV vs Petrol
Calculate the fuel cost of any trip, your annual fuel bill, or compare petrol vs diesel vs electric running costs side by side. Works in km/L, L/100km, US MPG and UK MPG, every major currency. Includes per-trip CO₂ emissions, a session trip log with totals, and one-click Excel export. Free, no login.
Fuel Cost Calculator Tool
More than just trip cost — what makes this calculator different
Most free fuel calculators do exactly one thing — multiply distance by efficiency by price and show a number. The LazyTools fuel cost calculator does that, plus eight things that competitors either skip or charge for.
How to calculate your fuel cost
LazyTools vs other free fuel cost calculators
We benchmarked against the most popular free fuel cost calculators — fueleconomy.gov, calculator.net, omnicalculator.com, gasbuddy.com and theaa.com's fuel cost calculator. Here's how the features stack up.
| Feature | LazyTools | fueleconomy.gov | calculator.net | omnicalculator | theaa.com |
|---|---|---|---|---|---|
| Trip cost calculation | ✓ | ✓ | ✓ | ✓ | ✓ |
| Annual cost projection | ✓ | ✓ | — | ✓ | — |
| EV vs petrol comparison | ✓ | EV only | — | — | — |
| 3-vehicle side-by-side compare | ✓ | — | — | — | — |
| Per-trip CO₂ emissions | ✓ | — | — | basic | — |
| L/100km, km/L, US & UK MPG | ✓ | US only | ✓ | ✓ | UK only |
| 12 currencies | ✓ | USD | USD | ✓ | GBP |
| Save trip log to browser | ✓ | — | — | — | — |
| Excel / CSV export | ✓ | — | — | — | — |
| Live calculation (no submit button) | ✓ | — | — | ✓ | — |
| No login required | ✓ | ✓ | ✓ | ✓ | ✓ |
Where LazyTools wins: the only free calculator with three-vehicle side-by-side comparison mixing electric and combustion vehicles, the only one with per-trip CO₂ emissions calculation, the only one with a saved trip log, and the only one with one-click Excel export. Where competitors win: fueleconomy.gov (US Department of Energy) has a giant database of real-world MPG figures for every car ever sold in the US, GasBuddy has live local fuel prices through its app — both useful for sourcing accurate input data to plug into the LazyTools calculator.
Standard fuel emission factors
The CO₂ figures shown by the calculator use these standard emission factors, which come from the chemistry of fuel combustion and are widely published by government environmental agencies including the UK Department for Energy Security & Net Zero, the US EPA and the IPCC Fourth Assessment Report.
| Fuel | CO₂ per litre | CO₂ per US gallon | CO₂ per UK gallon |
|---|---|---|---|
| Petrol (gasoline) | 2.31 kg | 8.74 kg | 10.50 kg |
| Diesel | 2.68 kg | 10.14 kg | 12.18 kg |
| LPG / autogas | 1.51 kg | 5.71 kg | 6.86 kg |
| CNG (compressed natural gas) | 2.00 kg / m³ | — | — |
| Electric (global avg grid) | ~0.40 kg / kWh | — | — |
| Electric (low-carbon grid) | ~0.05 kg / kWh | — | — |
Note: EV emissions vary massively by country and time of day depending on the local electricity grid mix. Norway, France, Sweden and parts of Canada have low-carbon grids; coal-heavy grids in parts of Asia and Eastern Europe push EV emissions per kWh much higher. The default 0.4 kg/kWh used in the comparison mode is a global average — override it with your local grid factor for the most accurate result.
The Real Cost of Driving: Fuel, Efficiency & Carbon
Most people radically underestimate how much they spend on fuel. The price per litre at the pump is the visible part — the part you grumble about when it goes up — but the actual annual cost is hidden behind a fog of "I'll figure it out later" arithmetic. Twenty thousand kilometres a year times seven litres per hundred kilometres times two dollars a litre is twenty-eight hundred dollars. That's a holiday. That's three months of groceries. Knowing this number, and watching how it changes when you tweak any of the three inputs, is the first step toward making informed decisions about commuting, vehicle choice and route planning. This guide walks through the formula, the unit conversions that trip people up, the comparison with electric vehicles, and the carbon footprint that matters more every year.
The fundamental formula
Trip fuel cost is one equation: cost = (distance ÷ efficiency) × price. The complication is that "efficiency" can be measured in four different ways depending on where you live, and "distance" and "price" both depend on which units you're using. In the metric world, efficiency is usually given in litres per 100 kilometres (L/100km), where lower is better — 5 L/100km is excellent and 12 L/100km is thirsty. The trip formula in metric units is straightforward: litres needed = distance in km × (L/100km ÷ 100), then multiply by the price per litre. So a 400 km trip in a car at 7 L/100km uses 28 litres, and if petrol costs $1.85 per litre that's $51.80. The math is identical for diesel — the fuel cost calculation doesn't care about the fuel type, only about how much you burn and how much it costs per unit.
In the US, efficiency is measured in miles per gallon (MPG), where higher is better — 30 MPG is average, 40+ is efficient, 50+ is hybrid territory. The formula is the same shape: gallons needed = distance in miles ÷ MPG, then multiply by the price per gallon. A 250-mile trip in a 30 MPG car uses 8.33 gallons, and at $3.50 per gallon that's $29.17. The UK uses MPG too but with the imperial gallon (4.546 L) instead of the US gallon (3.785 L), so the same physical car has a higher MPG number in the UK than in the US. This is one of the great sources of cross-Atlantic confusion — a 35 MPG American car is roughly a 42 MPG British car, even though the actual fuel efficiency hasn't changed.
Converting between L/100km and MPG
The math is uglier than it should be because L/100km and MPG are reciprocals — one measures volume per distance, the other measures distance per volume. The conversion is L/100km = 235.215 ÷ MPG (US) or L/100km = 282.481 ÷ MPG (UK), where 235.215 = 100 × 3.785 ÷ 1.609 (one US gallon in litres divided by one mile in km, scaled to per-100km) and 282.481 = 100 × 4.546 ÷ 1.609 (the same with the UK gallon). To convert the other way: MPG (US) = 235.215 ÷ L/100km, MPG (UK) = 282.481 ÷ L/100km. So 7 L/100km is about 33.6 US MPG or 40.4 UK MPG. The calculator does these conversions automatically — pick whichever unit your car's manual or trip computer uses, and the result comes out in that same unit's currency.
Real-world vs published efficiency
Manufacturer-published fuel efficiency figures come from standardised lab tests (the WLTP test in Europe, the EPA test in the US) that simulate driving conditions in a controlled environment. Real-world efficiency is almost always worse — typically 10 to 20 percent worse for petrol cars, 5 to 15 percent worse for diesel, and the gap is biggest for hybrids and small turbocharged engines whose tuning is optimised for the test cycle. So if your car is rated at 6 L/100km but in the real world you actually average 7.2 L/100km, the calculation should use 7.2, not 6. The most accurate source for real-world efficiency is your own car's trip computer averaged over a full tank, or a site like fueleconomy.gov (for US cars) or honestjohn.co.uk (for UK cars) that aggregates owner-reported figures. For trip planning where you don't have personal data, add a 15 percent buffer to the manufacturer's figure as a safe estimate.
Annual fuel cost — the number that matters
Trip cost is useful for planning a single journey, but the number that actually affects your finances is annual fuel cost — total spent on fuel over a year. This is the same formula scaled up: annual cost = (annual distance ÷ efficiency) × price. The average UK driver covers about 7,400 miles per year (~12,000 km); the average US driver covers about 14,000 miles per year (~22,500 km). At an average efficiency of 8 L/100km and a fuel price of $1.85/L, that's an annual fuel bill of about $1,776 for the UK driver and $3,330 for the US driver. The number scales linearly with each input — a 10% drop in fuel price saves you 10%, a 10% improvement in efficiency saves you 10%, a 10% drop in mileage saves you 10%. This makes the annual cost calculator a useful sensitivity tool: try changing one number at a time and see how much it moves the total. You'll quickly discover that small changes in efficiency — switching from a 9 L/100km SUV to a 6 L/100km hatchback — save more money over a year than you'd guess from the per-kilometre numbers.
Electric vs petrol — the comparison most people get wrong
Comparing the running cost of an electric vehicle to a petrol or diesel car looks complicated because the units are different — petrol is sold per litre, electricity per kilowatt-hour. But the math is the same shape. For petrol: cost per 100 km = (L/100km) × price per litre. For electric: cost per 100 km = (kWh/100km) × price per kWh. Plug in realistic numbers — a typical petrol car at 7 L/100km × $1.85/L = $12.95 per 100 km — and a typical EV at 17 kWh/100km × $0.30/kWh = $5.10 per 100 km. The EV is roughly 60% cheaper to run per kilometre, which over 20,000 km per year is a saving of about $1,570. Multiply by ten years of ownership and you have $15,700 — comfortably more than the price difference between equivalent petrol and electric models in many segments.
The catch is that the comparison is sensitive to local prices. If you live somewhere with cheap petrol and expensive electricity (parts of the US Gulf Coast), the gap closes dramatically. If you live somewhere with expensive petrol and cheap off-peak home charging (much of Europe), the gap widens. The compare mode in this calculator lets you plug in your own local prices for both fuels and see the real number for your situation, instead of trusting a generic comparison from a manufacturer brochure. It's also worth noting that running cost is only one part of total cost of ownership — depreciation, insurance, maintenance, charging infrastructure investment and resale value all matter, and in some markets they tilt the balance the other way.
CO₂ emissions — why every fuel calculator should show them
The chemistry of burning a fossil fuel is fixed: every litre of petrol releases approximately 2.31 kg of CO₂ when burned, every litre of diesel releases approximately 2.68 kg. These figures come directly from the carbon content of the fuel and the stoichiometry of complete combustion, and they're the same whether you burn the fuel in a Formula 1 car or a lawnmower. They're widely published — by the UK Department for Energy Security and Net Zero, the US EPA, and the IPCC — and they're used as the standard inputs for every carbon footprint calculator in the world. Multiplying the litres your trip uses by these factors gives you a per-trip CO₂ figure that's accurate to within a few percent.
The same calculation for an electric vehicle is more complicated because EVs don't burn anything directly — the emissions happen at the power station instead, and the per-kWh emission factor depends entirely on how that power station generates electricity. Norway is mostly hydro, so an EV in Norway emits roughly 0.02 kg CO₂ per kWh. Coal-heavy grids in parts of India and China can push the same number above 0.8 kg/kWh, at which point a small efficient petrol car can actually have a lower carbon footprint per kilometre than an EV running on coal-fired electricity. The global average for grid electricity is around 0.4 kg CO₂/kWh, which is what the comparison mode uses by default — but you should override it with your local grid factor for an honest answer. The International Energy Agency publishes country-by-country grid emission factors annually if you want the exact number.
Saving fuel — the boring but effective list
Once you know your real fuel cost, the next question is how to lower it. The boring answer is the same set of recommendations every fuel-economy guide has been making for fifty years, because they actually work: drive smoothly (gentle acceleration and braking, no rapid lane changes); keep your tyres properly inflated (under-inflated tyres can cost 3 to 5 percent fuel economy); remove unnecessary weight from the boot; remove roof racks when you're not using them (a roof box can cost 10 percent on long drives); use cruise control on motorway journeys; and don't idle the engine for more than 30 seconds. None of these are dramatic individually, but stacked together they typically deliver 10 to 20 percent better real-world efficiency than the manufacturer's figure, which the annual cost calculator will translate directly into hundreds of dollars per year of savings. Try plugging your current efficiency and a 15-percent-better figure into the annual mode to see exactly what better driving habits would be worth.
Turning trips into expense claims
For business travel, the reason to track fuel cost is usually to claim it back. Many countries and companies use a per-mile or per-kilometre standard mileage rate (for example, the UK HMRC rate is 45p per mile for the first 10,000 business miles), and the rate is meant to cover fuel, wear and tear, insurance and depreciation in one number. If you're claiming actual fuel cost instead of the standard rate (which is sometimes more advantageous, especially for thirstier vehicles), you need a record of every trip with the date, distance, fuel used and total cost. The trip log in this calculator does exactly that — save each trip as you make it, then export the whole log to Excel at the end of the month and either paste it directly into your expense form or feed it into the LazyTools Expense Claim Generator which builds a printable claim form ready to email to your accounts team.