Fuel Economy Calculator
Convert fuel economy between MPG (US/UK), L/100km and km/L, then estimate trip fuel cost, compare two vehicles side by side, and project annual fuel spend with a CO₂ emissions estimate.
Input
Fuel Economy Converter
Cost Settings
Prefixed to every cost below (trip, comparison and annual).
Trip Cost Calculator
Vehicle Comparison
Price per liter, applied to both vehicles.
Annual Cost & CO₂ Estimator
Output
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Guides
The Fuel Economy Calculator turns the numbers on your dashboard into answers you can actually use. It bundles four tools in one: a fuel economy converter (MPG US/UK, L/100km and km/L), a trip cost estimator, a two-vehicle cost comparison, and an annual fuel spend + CO₂ projection. Enter whatever figures you have and each section fills in on its own — no sign-up, no waiting.
How to use it
Each section works independently, so you only fill in the parts you care about.
- Fuel Economy Converter — type a value and pick its unit (MPG US, MPG UK, L/100km or km/L). The table instantly shows the equivalent in every other unit.
- Trip Cost Calculator — enter the distance, your vehicle's fuel economy, and the fuel price. You get total fuel needed, total cost, and cost per km and per mile.
- Vehicle Comparison — put two vehicles' fuel economy side by side over a shared distance and fuel price to see which is cheaper and by how much.
- Annual Cost & CO₂ Estimator — enter your yearly mileage, economy, and fuel price to project annual and monthly cost, plus an estimate of yearly carbon dioxide emissions.
Set the currency symbol once and it is applied to every cost. Use the "Try an example" option to load a full worked scenario.
What formulas does it use?
Everything is anchored to litres per 100 km as the base unit. MPG (US) = 235.2146 ÷ (L/100km) and MPG (UK) = 282.4809 ÷ (L/100km); km/L = 100 ÷ (L/100km). US gallons use 3.78541 L, UK gallons 4.54609 L, and one mile is 1.60934 km. Fuel used on a trip is (distance ÷ 100) × L/100km, multiplied by the price per litre for the cost.
How is CO₂ estimated?
Emissions use standard tailpipe factors of about 2.31 kg of CO₂ per litre of gasoline/petrol and 2.68 kg per litre of diesel, multiplied by the fuel your annual mileage consumes. These are well-established averages for combustion; real-world output varies with fuel blend, driving style, and conditions, so treat the figure as a solid ballpark rather than a certified measurement.
Is MPG (US) the same as MPG (UK)?
No — a UK (imperial) gallon is larger than a US gallon, so the same car shows a higher number in UK MPG. This calculator converts between both explicitly, which is why "30 MPG (US)" and "36 MPG (UK)" describe the same efficiency.
Which fuel economy figure should I enter?
Use your combined or average consumption for the most representative results. If you only know city or highway figures, enter those and read the output as a best/worst case.
Privacy
This calculator runs entirely in your browser. Your mileage, prices, and vehicle figures are never uploaded, logged, or sent to a server — the math happens locally on your device, and nothing is stored after you close the page.