Mileage Converter

About this calculator

Fuel economy is quoted four ways, and two of them run backwards relative to the other two. km/L and mpg count distance per unit of fuel, so bigger is better. L/100 km counts fuel per unit of distance, so bigger is worse.

Crossing between the families is a reciprocal, not a multiplication. 20 km/L is 5 L/100 km — not 2,000, and not 0.2. This converter does all four at once and says plainly when the direction of "better" has flipped.

How to use this calculator

1. Enter the figure you have.
2. Say which unit it is in.
3. Pick the unit you want it in.

All four are shown regardless, so you only need to be right about what you started with.

The formula

Everything is converted through km/L:

  from km/L        x 1
  from mpg (US)    x 1.609344 / 3.785411784  = x 0.425144
  from mpg (imp)   x 1.609344 / 4.54609      = x 0.354006
  from L/100 km    100 / value               (reciprocal)

and back out the same way.

Using km/L as the pivot means no pair of units needs its own factor and no two pairs can disagree with each other. The reciprocal is handled explicitly rather than folded into a factor, because there is no factor that could express it.

Worked example

20 km/L, written every way:

km/L           20
L/100 km       5
mpg (US)       47.04
mpg (imperial) 56.50

Four numbers, one car. Note that the L/100 km figure is the smallest and the best.

Notes and limits

• The reciprocal has a practical consequence: equal steps in one unit are not equal steps in the other. Going from 5 to 6 km/L saves far more fuel over a year than going from 20 to 21, even though both look like a one-unit improvement.
• That asymmetry is why consumption units are preferred for regulation and economy units for marketing.
• The two mpg differ by about 20%. Any figure quoted as plain "mpg" needs its gallon identified before it means anything.
• CNG economy in km/kg converts to nothing on this page — a kilogram of gas is not a litre of liquid, and there is no fixed relation between them.

What this calculation assumes

• The conversion factors are exact by definition, not measured, so the conversion is exact.
• The result describes the figure you entered. It is not a claim about any vehicle.