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Fuel Economy Converter

Convert fuel economy between L/100 km, mpg (US), mpg (UK), km/L and gallons per 100 miles, and estimate the fuel and cost of a trip.

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Formula:

≈ means the result was rounded to the chosen number of significant figures.

L/100 km and gal/100 mi measure fuel used per distance (lower is better); mpg and km/L measure distance per fuel (higher is better). A US gallon is 3.785 L, an imperial (UK) gallon 4.546 L.

Trip fuel cost

Fuel needed
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Trip cost
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Cost per 100 km
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Cost per mile
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Same currency as the price you enter. Real consumption depends on speed, load, weather and traffic.

{value} in every unit

UnitResult

Quick conversion table: {from} to {to}

FromTo

Common values

ReferenceValueResult

Formula

UnitFormula

How to use the fuel economy converter

  1. Type a fuel economy in Value and pick its unit in From, for example 7 L/100 km or 35 mpg (US).
  2. Choose the unit you want in To. The result appears as you type, with the Formula for the pair and the calculation using your number.
  3. Use Swap units (⇄) to reverse the conversion, Significant figures for the precision and Copy to copy the result line.
  4. In Trip fuel cost, enter a Distance (km or miles), a Fuel economy in any unit and a Fuel price per litre, per US gallon or per imperial gallon. The tool shows the Fuel needed, the Trip cost, the Cost per 100 km and the Cost per mile.

The page also lists your value in every unit, a quick table for common consumptions, typical figures for a few kinds of vehicles, and the formula from your starting unit to each of the others.

Why L/100 km is the “inverse” of mpg

The two families of units measure opposite ratios:

  • L/100 km and gal/100 mi measure fuel per distance. Lower is better.
  • mpg and km/L measure distance per fuel. Higher is better.

Converting from one family to the other is therefore not a multiplication but a division by the value, a reciprocal. With the exact definitions (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 imperial gallon = 4.54609 L):

  • L/100 km = 235.214583 ÷ mpg (US)
  • L/100 km = 282.480936 ÷ mpg (UK)
  • L/100 km = 100 ÷ km/L
  • mpg (US) = mpg (UK) × 0.832674

A side effect surprises many drivers: gains in mpg are not linear. Going from 10 to 20 mpg saves about 11.8 L/100 km, while going from 40 to 50 mpg saves only about 1.2 L/100 km. When you compare cars, fuel-per-distance units show the real savings; that is why US window stickers also give gallons per 100 miles.

US and UK mpg are different

An imperial gallon is about 20% larger than a US gallon, so the same car shows a higher mpg figure in the UK: 30 mpg (US) is about 36 mpg (UK). Always check which gallon a figure uses before comparing.

The trip cost helper

The helper first converts your fuel economy to L/100 km, then computes:

  • fuel (L) = distance (km) × L/100 km ÷ 100
  • cost = fuel × price per litre (a price per gallon is divided by the size of that gallon)

Example: 500 km at 7 L/100 km needs 35 L; at 1.80 per litre the trip costs 63.00. The result uses the currency of the price you type.

Frequently asked questions

How do I convert mpg to L/100 km in my head?

Divide 235 by the US mpg figure (or 282 by the UK figure). 30 mpg (US) gives about 7.8 L/100 km.

Why does 0 give no result?

A consumption of 0 L/100 km would mean infinite miles per gallon, and 0 mpg means the car never moves. Neither has an equivalent, so the tool says so instead of showing infinity.

What does gal/100 mi mean?

US gallons used to drive 100 miles, the fuel-per-distance measure printed on US fuel economy labels. 1 gal/100 mi equals about 2.35 L/100 km.

Are the typical vehicle figures official?

No. They are rough typical values for comparison. Official ratings (WLTP in Europe, EPA in the US) come from standard test cycles, and real-world consumption is often higher.

Is the trip cost exact?

It is an estimate. Speed, traffic, load, weather, air conditioning and tyre pressure all change real consumption.

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