Fuel Economy Converter Calculator

Fuel economy conversion is different from ordinary unit conversion. Speed, length, and mass usually convert with a fixed linear factor: double the input and the output doubles. MPG and L/100km do not behave that way. Miles per gallon measures distance traveled per unit of fuel, while liters per 100 kilometers measures fuel consumed for a fixed distance. One rewards a larger number; the other rewards a smaller number. This calculator preserves that inverse relationship so a vehicle rating is not accidentally translated as if it were a simple speed or distance unit.

The converter accepts three input units: miles per US gallon (MPG), kilometers per liter (km/L), and liters per 100 kilometers (L/100km). It always reports miles per US gallon as the primary result and also lists km/L and L/100km. Use it when comparing U.S. window stickers with international specifications, reading owner reports from another country, or translating fuel logs before estimating costs with the fuel cost calculator. For the distance and volume pieces behind the constants, see the miles to kilometers calculator and gallons to liters calculator.

Unit definitions

MPG means miles per US gallon. It is a distance-per-fuel measure. Higher is better because the vehicle travels farther on each gallon.

km/L means kilometers per liter. It is also distance per fuel. Higher is better for the same reason: more kilometers from each liter.

L/100km means liters per 100 kilometers. It is fuel per fixed distance. Lower is better because the vehicle burns fewer liters to travel the same 100 km reference trip.

That last unit changes the math. A vehicle that improves from 10 L/100km to 5 L/100km has doubled its distance-per-fuel efficiency, but the numeric value has been cut in half. Treating L/100km as if it increased with efficiency is the most common mistake.

Formulas used by the calculator

The constants are derived from the exact international mile (1.609344 km) and exact US gallon (3.785411784 L). Calculations retain those defining values and round only the shown result.

For an MPG input:

$\text{km/L} = \text{MPG} \times 0.425143707430$

$\text{L/100km} = \frac{235.214583333333}{\text{MPG}}$

For a km/L input:

$\text{MPG} = \text{km/L} \times 2.352145833333$

$\text{L/100km} = \frac{100}{\text{km/L}}$

For an L/100km input:

$\text{MPG} = \frac{235.214583333333}{\text{L/100km}}$

$\text{km/L} = \frac{100}{\text{L/100km}}$

The divisions are not optional. They are the sign that distance-per-fuel and fuel-per-distance are reciprocal descriptions of the same efficiency.

Fuel Economy example

With the default settings, the input unit is MPG and the value is 25. The calculation first keeps the MPG value as 25, then calculates km/L:

$25 \times 0.425143707430 = 10.6286$

It calculates fuel consumption by dividing, not multiplying:

$\frac{235.214583333333}{25} = 9.4086$

The converter rounds each displayed number to two decimals, so the result is 25.00 mpg, 10.63 km/L, and 9.41 L/100km. If the same vehicle were entered as 9.41 L/100km after rounding, the reverse MPG value would be close but not perfectly identical because rounding has already removed detail.

Reference table

Notice the shape of the table. MPG rises by equal steps, but L/100km does not fall by equal steps. The improvement from 10 MPG to 20 MPG saves far more fuel per 100 km than the improvement from 50 MPG to 60 MPG. That is another reason L/100km can be clearer for fuel-savings comparisons: it reports consumption directly.

Domains and use cases

In driving, MPG is familiar in the United States, while L/100km is common in Canada, Europe, Australia, and many vehicle specifications. In fleet management, L/100km can make fuel budgeting clearer because multiplying by distance gives liters used. In travel planning, a converted MPG value helps compare a rental car rating with a home-country expectation. In vehicle shopping, converting both ratings into the same unit prevents a high MPG number from being compared incorrectly with a high L/100km number. For cost planning, fuel consumed per distance can be multiplied by trip distance and fuel price after all values are put in compatible units.

Pitfalls

• Do not multiply MPG by a fixed factor to get L/100km. The calculator divides 235.214583333333 by MPG.
• Do not assume higher is always better. Higher is better for MPG and km/L; lower is better for L/100km.
• Do not mix US and Imperial gallons. This form is explicitly miles per US gallon.
• Do not round before converting back. Reciprocal conversions magnify small rounding changes.
• Do not compare fuel economy with speed converters. A vehicle traveling faster is not automatically more fuel efficient; speed and fuel consumption measure different things.

Sources

• NIST, Handbook 44, Appendix C: General Tables of Units of Measurement — exact international-mile and US-gallon definitions used to derive every conversion factor.
• FuelEconomy.gov, Compare Side-by-Side — U.S. fuel economy comparison data and MPG context.
• FuelEconomy.gov, Gasoline vehicle label — consumer label context for MPG and fuel cost comparisons.