Why can't I just multiply MPG by a constant to get L/100km?

Because they measure opposite things. MPG is distance per fuel and L/100km is fuel per distance, so the relationship is a reciprocal: L/100km equals 235.21 divided by US MPG. A straight multiplication gives the wrong answer at every value except the crossover point.

What is the difference between US MPG and imperial MPG?

Only the size of the gallon. A US gallon is 3.785 litres and an imperial gallon is 4.546 litres, so the same consumption reads about 20% higher in imperial MPG. Reviews from the UK almost always quote imperial.

Which fuel economy unit is best to compare cars in?

L/100km is the easiest to reason about for costs, because fuel used scales linearly with the number — going from 10 to 9 L/100km saves the same fuel per kilometre as going from 6 to 5. MPG compresses the differences at the efficient end of the scale.

Does the Fuel Economy Converter account for real-world driving?

No. It converts whatever figure you enter between units. Manufacturer figures come from standardised test cycles, so if you want real numbers use your own measured consumption from a full tank.

Are the distances and prices I enter stored?

No. Every conversion and trip cost is computed on your device, and nothing you type is transmitted or written to an account.

Fuel Economy Converter

Convert between MPG (US), MPG (imperial), L/100km and km/L, and work out the fuel and cost for a trip — calculated in your browser.

The Fuel Economy Converter runs entirely in your browser. The economy figures, distances and prices you enter never leave your device.

Open the Fuel Cost Calculator

About Fuel Economy Converter

Fuel Economy Converter translates a single economy figure into all four units used around the world. The catch it handles for you is that the units are not the same kind of measurement: MPG and km/L are distance per unit of fuel, so higher is better, while L/100km is fuel per unit of distance, so lower is better. Converting between the two families is a reciprocal rather than a scale factor, which is why 30 MPG is 7.84 L/100km and not some fixed multiple. The US and imperial gallons differ too — 3.785 L against 4.546 L — so the same car reads about 20% higher in UK MPG. A trip section turns the figure into litres or gallons and a cost.

Features

Converts between MPG (US), MPG (imperial), L/100km and km/L at once
Handles the reciprocal relationship between the two families correctly
Distinguishes the US gallon from the larger imperial gallon
Trip calculator for fuel volume and total cost over a distance
Switches distance and volume units to match the economy unit chosen
Accepts figures with thousands separators
Copy every converted value in one click
Runs entirely on your device — no journey data is uploaded

How to use the Fuel Economy Converter

Enter your fuel economy figure and pick the unit it is quoted in
Read the equivalent value in the other three units
Optionally add a trip distance and fuel price to see volume and cost
Copy the converted figures for a spreadsheet or expense claim

Example

Input

30 MPG (US)

Output

7.84 L/100km · 36.03 MPG (UK) · 12.75 km/L

The reciprocal relationship means a higher MPG maps to a lower L/100km, not a proportionally larger one.

Common errors & troubleshooting

The L/100km figure looks far too small compared to the MPG value. — That is the expected relationship. L/100km measures consumption rather than efficiency, so an economical car has a low number — 5 L/100km is very good, while 5 MPG would be terrible.
The converted MPG does not match the figure on a UK review site. — Check which gallon is meant. An imperial gallon is 4.546 L against the US gallon's 3.785 L, so the same car reads roughly 20% higher in UK MPG.
The trip cost is asking for a price per litre when you buy by the gallon. — The trip section follows the unit family of the economy figure. Switch the economy unit to MPG and the distance and price switch to miles and gallons.
Entering zero returns no result. — Zero economy and zero consumption are both undefined rather than infinite, so the converter needs a value above zero.

Frequently asked questions

Why can't I just multiply MPG by a constant to get L/100km?: Because they measure opposite things. MPG is distance per fuel and L/100km is fuel per distance, so the relationship is a reciprocal: L/100km equals 235.21 divided by US MPG. A straight multiplication gives the wrong answer at every value except the crossover point.
What is the difference between US MPG and imperial MPG?: Only the size of the gallon. A US gallon is 3.785 litres and an imperial gallon is 4.546 litres, so the same consumption reads about 20% higher in imperial MPG. Reviews from the UK almost always quote imperial.
Which fuel economy unit is best to compare cars in?: L/100km is the easiest to reason about for costs, because fuel used scales linearly with the number — going from 10 to 9 L/100km saves the same fuel per kilometre as going from 6 to 5. MPG compresses the differences at the efficient end of the scale.
Does the Fuel Economy Converter account for real-world driving?: No. It converts whatever figure you enter between units. Manufacturer figures come from standardised test cycles, so if you want real numbers use your own measured consumption from a full tank.
Are the distances and prices I enter stored?: No. Every conversion and trip cost is computed on your device, and nothing you type is transmitted or written to an account.

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