Fuel Consumption Calculator (fuel needed & trip cost)

Understanding the fuel cost calculator — concept and principle

This calculator works out how much fuel a trip needs and what it costs, from the distance, your car's fuel economy and the fuel price, in any common unit. It splits the cost between passengers and shows the cost per 100 km. Use it to budget a road trip, compare driving with the train, or agree on fuel money with friends.

Distance × consumption × price: with the default inputs the result is $44.63 (one way · $12.75 per 100 km).

How it is calculated

Distance is converted to km (miles × 1.609344) and doubled for a return trip.
Fuel economy is converted to litres per 100 km: km/L → 100 ÷ value; US mpg → 235.215 ÷ value; UK mpg → 282.481 ÷ value.
Fuel needed (L) = km × L/100 km ÷ 100; US gallons = litres ÷ 3.785411784.
Price is converted to price per litre (per US gallon ÷ 3.785411784, per UK gallon ÷ 4.54609).
Cost = litres × price per litre; per person = cost ÷ people; per 100 km = price per litre × L/100 km.

The principle

Fuel use is proportional to distance for a given consumption, so the calculation is a chain of multiplications. The only care needed is with units. Litres per 100 km measures fuel per distance, so lower is better, while mpg and km/L measure distance per fuel, so higher is better; converting them needs a reciprocal. The constants 235.215 and 282.481 come from the exact definitions of the mile and the US and UK gallons, which are different sizes.

Worked example

Default inputs: 350 km one way, 7.5 L/100 km, $1.70 per litre, 1 person.

Distance = 350 km; economy = 7.5 L/100 km.
Fuel = 350 × 7.5 ÷ 100 = 26.25 litres = 26.25 ÷ 3.785 = 6.93 US gallons.
Cost = 26.25 × 1.70 = $44.63; per person $44.63.
Cost per 100 km = 1.70 × 7.5 = $12.75.

Each point is the calculator re-run with only “Trip distance” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Trip distance	Fuel needed (litres)
87.5	6.56
175	13.13
262	19.69
350	26.25
438	32.81
525	39.38
700	52.5

How to read the result

Real consumption often differs from the official figure: motorway speed, air conditioning, roof boxes, load and cold weather all raise it. If you know your car's real average from fill-ups, use that. For a fair split among friends, agree beforehand whether tolls and parking are shared too, since they are not included here.

Common mistakes

Mixing US and UK gallons; a UK gallon is about 20% larger.
Entering mpg into the L/100 km field.
Forgetting the return journey.
Using the official figure for a heavily loaded car.

Keep in mind: The result covers fuel only, not tolls, parking, wear or depreciation. It is a budgeting estimate, not financial advice.

Frequently asked questions

How do I calculate fuel cost for a trip?

Multiply the distance by the consumption to get litres, then by the price per litre.

How do I convert mpg to L/100 km?

Divide 235.215 by US mpg, or 282.481 by UK mpg.

What is a good fuel economy?

It depends on the car type; compare with the official figure for your model and with your own fill-up records.

Sources: NIST Special Publication 811 (2008), Appendix B (1 mile = 1.609344 km; 1 US gallon = 3.785411784 L; 1 UK gallon = 4.54609 L)

Checked 9 Nov 2026. Exact unit definitions from NIST SP 811 (2008), Appendix B: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L.

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