Fuel Economy Converter (MPG, L/100km, km/L)

How to use the fuel economy converter

Choose the unit you have in the “From unit” dropdown (for example, US MPG or L/100km).
Type your numeric value in the value box.
All other fields update instantly with the equivalent fuel economy.
Use the Swap US MPG ↔ L/100km button for the most common conversion.

The tool supports:

Miles per gallon (US)
Miles per gallon (UK / Imperial)
Kilometers per liter (km/L)
Liters per 100 km (L/100km)
Liters per km (L/km)
US gallons per 100 miles
UK gallons per 100 miles

Fuel economy conversion formulas

Fuel economy can be expressed as either:

Distance per volume – e.g. miles per gallon (MPG), km/L. Higher is better.
Volume per distance – e.g. L/100km, gal/100mi. Lower is better.

Key constants

1 US gallon = 3.785411784 liters
1 UK gallon = 4.54609 liters
1 mile = 1.609344 kilometers

US MPG ↔ L/100km

To convert between US miles per gallon and liters per 100 km:

From US MPG to L/100km

Because 1 US gallon = 3.785411784 L and 1 mile = 1.609344 km:

1 US MPG ≈ 235.214583 / (L/100km)

So:

\[ \text{L/100km} = \frac{235.214583}{\text{US MPG}} \]

From L/100km to US MPG

\[ \text{US MPG} = \frac{235.214583}{\text{L/100km}} \]

UK MPG ↔ L/100km

Using the UK (Imperial) gallon:

1 UK gallon = 4.54609 L

1 mile = 1.609344 km

1 UK MPG ≈ 282.480936 / (L/100km)

\[ \text{L/100km} = \frac{282.480936}{\text{UK MPG}} \]

\[ \text{UK MPG} = \frac{282.480936}{\text{L/100km}} \]

km/L ↔ L/100km

1 km/L means 1 liter to travel 1 km.

\[ \text{L/100km} = \frac{100}{\text{km/L}} \]

\[ \text{km/L} = \frac{100}{\text{L/100km}} \]

US MPG ↔ km/L

Start from the definitions:

1 mile = 1.609344 km
1 US gallon = 3.785411784 L

\[ \text{km/L} = \text{US MPG} \times \frac{1.609344\ \text{km}}{3.785411784\ \text{L}} \approx \text{US MPG} \times 0.425143707 \]

\[ \text{US MPG} = \text{km/L} \times \frac{3.785411784}{1.609344} \approx \text{km/L} \times 2.35214583 \]

Worked examples

Example 1 – Convert 30 US MPG to L/100km

Given: 30 US MPG.

Use the formula:

\[ \text{L/100km} = \frac{235.214583}{\text{US MPG}} \]

\[ \text{L/100km} = \frac{235.214583}{30} \approx 7.84\ \text{L/100km} \]

A car that does 30 US MPG uses about 7.8 L/100km.

Example 2 – Convert 5.5 L/100km to US MPG and km/L

Given: 5.5 L/100km.

US MPG:

\[ \text{US MPG} = \frac{235.214583}{5.5} \approx 42.77\ \text{MPG} \]

km/L:

\[ \text{km/L} = \frac{100}{5.5} \approx 18.18\ \text{km/L} \]

So 5.5 L/100km ≈ 42.8 US MPG and 18.2 km/L.

Example 3 – Convert 50 UK MPG to US MPG

Step 1 – Convert UK MPG to L/100km:

\[ \text{L/100km} = \frac{282.480936}{50} \approx 5.65\ \text{L/100km} \]

Step 2 – Convert that L/100km to US MPG:

\[ \text{US MPG} = \frac{235.214583}{5.65} \approx 41.63\ \text{MPG} \]

So 50 UK MPG ≈ 41.6 US MPG.

US MPG vs UK MPG – what’s the difference?

MPG can be confusing because the gallon is not the same everywhere:

US gallon ≈ 3.785 L
UK (Imperial) gallon ≈ 4.546 L

For the same car, the UK MPG number will always be higher than the US MPG number, simply because the UK gallon is larger.

Always check whether a value is in US MPG or UK MPG before comparing fuel economy figures.

FAQ

Is higher MPG always better?

Yes. For distance-per-volume units (MPG, km/L), a higher number means you travel further on the same amount of fuel, so fuel economy is better.

Is lower L/100km always better?

Yes. For volume-per-distance units (L/100km, gal/100mi), a lower number means you use less fuel to travel a given distance.

How accurate is this converter?

The calculator uses precise SI conversion factors and performs calculations in double precision. Results are rounded for display, but the underlying math is accurate enough for engineering and everyday use.

Can I paste values like “35 mpg” or “7.2 l/100km”?

Yes. The input box accepts values with units; the script extracts the numeric part and you can then select the correct unit from the dropdown.