Fuel Cost Calculator
A fuel cost calculator turns a distance, a consumption figure and a fuel price into litres, money and CO2. In metric it uses litres = distance × consumption ÷ 100, so 100 km at 8 L/100 km is 8 litres — €14.40 at €1.80 a litre, and 18.48 kg of CO2. Tick Round trip and every figure doubles.
Total for round trip:
About Fuel Cost Calculator
Choose Metric (km, L) or Imperial (mi, gal) at the top, because the two modes take different inputs. Metric wants distance in kilometres, consumption in litres per 100 km and price per litre; imperial wants miles, miles per gallon and price per gallon. Five Quick trips buttons fill the distance for you — commute 30, city trip 100, weekend 300, road trip 800 and cross country 1500 — and every field recalculates as you type.
Four results appear together: Fuel needed, Total cost, Cost per km and CO2 emissions. The metric maths is litres = distance × consumption ÷ 100 and cost = litres × price. Imperial divides instead: gallons = distance ÷ MPG, because MPG counts distance per unit of fuel rather than fuel per unit of distance. The Round trip switch doubles the distance before any of that runs, so the fuel, the cost and the emissions all double with it.
The CO2 line uses a fixed factor per unit of fuel. In imperial that factor is 8.887 kg per gallon, the figure the US Environmental Protection Agency publishes for petrol in its note on greenhouse gas emissions from a typical passenger vehicle; the same page gives 10,180 grams per gallon for diesel. The metric mode uses 2.31 kg per litre, which is about 1.6 per cent below the EPA figure converted (8.887 ÷ 3.785 = 2.35), so the two modes will not agree to the last decimal on the same journey.
Limits: it is a single-leg estimate for one fuel type at one steady consumption. There is no diesel or LPG factor, no allowance for a cold engine, a roof box, traffic or altitude, no tolls or parking, and no electric-vehicle mode, since kWh per 100 km and a tariff per kWh are a different calculation. Treat the result as a budget figure, not a fuel gauge.
Use Cases
How to use
Pick Metric (km, L) or Imperial (mi, gal) at the top; the field labels change to match.
Set the distance by typing it or by pressing one of the Quick trips buttons.
Enter your consumption — litres per 100 km in metric, miles per gallon in imperial — and the fuel price per litre or per gallon.
Read the four results: Fuel needed, Total cost, Cost per km and CO2 emissions.
Tick Round trip if you are coming back; the distance doubles and every result doubles with it.
Pro Tips
- Switch the unit toggle before typing: metric expects L/100 km, where lower is better, and imperial expects MPG, where higher is better, so the same number means the opposite thing in the two modes.
- Round trip doubles the distance before anything else is worked out, so the fuel, the cost, the cost per kilometre and the CO2 already include the way home — do not double the distance yourself as well.
- The Quick trips buttons only set the distance; consumption and price stay where you left them, which is what makes them useful for comparing several trips with one car.
- Take the consumption figure from your own trip computer over a few tankfuls rather than the manufacturer's number; the difference between a claimed 5.5 and a real 7 L/100 km is the difference between a useful estimate and a wrong one.
- The CO2 factor is for petrol. Diesel releases more per gallon — the EPA gives 10,180 grams against 8,887 — so add roughly 15 per cent to the emissions line if you are running on diesel.
Troubleshooting
All the results show zero.
One of the three fields is empty or not a number, and an unreadable value is treated as zero, which makes the whole product zero. Check that distance, consumption and price all contain figures — the decimal separator has to be a dot, not a comma.
The cost looks far too high after switching units.
The unit toggle changes what the fields mean but not what is in them. A consumption of 8 is 8 L/100 km in metric and 8 MPG in imperial, which is a very thirsty vehicle, and a price of 1.80 is per litre in one mode and per gallon in the other. Retype the three values after switching.
The distance is twice what I entered.
Round trip is on. The switch doubles the distance before the calculation, and the line underneath it shows the doubled figure. Turn it off for a one-way estimate.
A Quick trips button did not change my cost much.
Those buttons set only the distance. If the cost barely moved, the consumption or the price is the dominant term — try changing one of those instead, since cost per kilometre does not depend on the distance at all.
My real fuel use is nothing like the estimate.
The calculation assumes one steady consumption figure for the whole trip. Cold starts, town traffic, a loaded boot, a roof box, headwinds and hills all push real use above a cruising average. Feed in the figure your own trip computer reports over several tankfuls rather than a manufacturer's number.
Frequently Asked Questions
Multiply the distance by your consumption, divide by 100 to get litres, and multiply by the price. This page does exactly that: 100 km at 8 L/100 km needs 8 litres, which is €14.40 at €1.80 a litre. In imperial the first step is different — gallons are distance divided by MPG.
L/100 km is litres used divided by kilometres driven, times 100: 40 litres over 500 km is 8 L/100 km. MPG is the other way up, miles divided by gallons. That is why the metric mode multiplies by consumption and the imperial mode divides by it — they describe the same thing from opposite ends.
Enter the three numbers and read Total cost. The result is only as good as the consumption figure: use what your car actually returns rather than the brochure value, and remember that a fully loaded car, a roof box or town traffic will push real consumption above your usual average.
The tool divides the total cost by the distance travelled and shows it as Cost per km. At 8 L/100 km and €1.80 a litre that is about €0.144 per kilometre, whatever the length of the trip — which is what makes it the right figure for mileage claims and for comparing cars.
Yes. Tick Round trip and the distance is doubled before the calculation runs, so all four results already cover both directions. The note under the switch shows the doubled distance so you can check it against what you entered.
By multiplying the fuel used by a fixed emission factor. In imperial mode that factor is 8.887 kg of CO2 per gallon of petrol, the number the US Environmental Protection Agency publishes for a typical passenger vehicle. Metric mode uses 2.31 kg per litre, so the two modes differ by about 1.6 per cent on the same journey.
No. The emission factor built in is for petrol, and diesel produces more per gallon — the EPA's figure is 10,180 grams against 8,887. An electric car needs kWh per 100 km against a price per kWh, which is a different sum entirely and is not offered here.
Because the two modes use slightly different constants. 8.887 kg per gallon converts to 2.35 kg per litre, while the metric mode is set to 2.31. On a 100 km trip using 8 litres that is 18.48 kg against 18.78 kg — enough to notice if you convert your figures and compare, and not enough to change a decision.
The efficiency bar under the results calls anything in the lowest third of its scale Excellent, the middle third Average and the top third Poor, using 15 L/100 km as the far end in metric and 50 MPG in imperial. It is a rough visual guide, not a rating: a loaded van and a city hatchback are not comparable on one scale.
No. The calculation runs in the page as you type and nothing is sent to a server or kept after you close the tab. Reloading the page returns it to its defaults of 100 km, 8 L/100 km and 1.80 per litre.