400 km at 7 L/100 km
The trip needs 28 litres. At 1.65 per litre, that comes to about 46.20 — roughly 0.1155 per kilometre.
Round trips catch people out here: driving there and back is 800 km, not 400, and doubles every figure.
Fuel cost calculator
Two everyday questions about fuel: what a trip will actually cost, and what your car really uses once traffic, hills and habits are factored in — rather than the figure on the window sticker.
Result
Fill in the fields to see your result.
How it works
Consumption and cost are separated on purpose. Whatever unit your car’s figure comes in — litres per 100 km, kilometres per litre, or miles per gallon in either the US or imperial size — it is first converted to litres per 100 km, the one form every other figure can be built from.
The three ways of stating consumption disagree about which direction is "better". Litres per 100 km falls as the car gets more efficient; kilometres per litre and miles per gallon rise. Converting between them is not a straight-line calculation — it goes through a division each time, which is why doubling the miles-per-gallon figure does not halve the fuel used.
The real-consumption mode reverses the usual calculation: instead of predicting fuel from an advertised figure, it works backwards from an actual fill-up. Divide the fuel added by the distance covered since the last full tank, and the result is closer to what the car does in practice than any number on a specification sheet.
Formulas used
Fuel used
fuel (L) = distance (km) × consumption (L/100 km) ÷ 100
Converting miles per gallon
L/100 km = 100 × litres per gallon ÷ (mpg × 1.609344)
The US gallon (3.785 L) and the imperial gallon (4.546 L) give different results for the same mpg figure.
Trip cost
cost = fuel (L) × price per litre
Worked examples
The same trip in miles per gallon
A car rated at 33.6 mpg (US) covers the same ground on the same fuel — 7 L/100 km and 33.6 mpg describe the same efficiency.
Quoting the imperial gallon instead would show about 40.4 mpg for the identical car, purely because the gallon is bigger.
Checking real consumption
A fill-up of 32 litres after 450 km driven gives 7.11 L/100 km — usually a touch higher than the advertised figure, which is measured under ideal conditions.
The same numbers convert to about 14.06 km/L or 33.1 mpg, three ways of saying exactly the same thing.
Assumptions and limits
- Consumption is constant for the whole distance.
- The fuel price entered does not change during the trip.
- No congestion, detours or extra stops are included beyond the distance entered.
Frequently asked questions
Why do US and imperial gallons give different mpg numbers?
Because the imperial gallon holds about 20% more than the US one. The same car, using the same fuel over the same distance, reports a higher mpg figure when the larger gallon is used to measure it.
The two numbers are not interchangeable, and a spec sheet rarely says which one it means — check the source before comparing.
Why is my real consumption always worse than advertised?
Official figures come from a standardised laboratory test, driven at controlled speeds with no traffic, no hills, no air conditioning and no cargo.
Real driving adds all four, plus tyre pressure and personal style, and typically adds 10 to 25% to the fuel used for the same distance.
How do I work out consumption from a fill-up?
Fill the tank completely, reset the trip counter, drive normally, then fill it completely again. The litres needed to fill it back up is what the trip actually used.
One fill-up gives one estimate; averaging several trims out short journeys and cold starts, which use more fuel than they should.
Should I use the outward distance or the round trip?
Whichever fuel you will actually burn. A day trip there and back needs the round-trip distance entered, or the cost will read half of what you will really pay.
A one-way relocation, by contrast, genuinely only needs the one-way figure.
Updated
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