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Fuel Economy Converter (MPG ↔ L/100km)

Convert fuel economy between US MPG and L/100km using exact NIST unit constants, and estimate cost per mile from a fuel price.

How this calculator works

Enter a fuel economy figure in either miles per gallon or liters per 100 kilometers, and pick the direction you want to convert. The calculator applies the exact US-gallon-to-liter and mile-to-kilometer constants from NIST to convert between them. A fuel price per gallon is always available too, so you can see an estimated cost per mile, computed from the full-precision MPG value no matter which direction you entered.

This calculator uses the US liquid gallon (3.785411784 liters), the unit US vehicle MPG ratings are based on. The UK/Imperial gallon (4.54609 liters) is a different, larger unit and is explicitly out of scope for this calculator.

The formula

L/100km = 235.214583 / MPG

Inverse:
MPG = 235.214583 / (L/100km)

Cost per mile = price per gallon / MPG

Where 235.214583 = 100 x 3.785411784 (US gallon in liters) / 1.609344 (mile in km)

Worked example

Converting 30 MPG to L/100km:

  • Conversion: 235.214583 / 30 = 7.84048…, rounded to 7.8 L/100km
  • At $3.50 per gallon: cost per mile = 3.50 / 30 = $0.117/mile

Frequently asked questions (FAQ)

How do I convert MPG to L/100km by hand?

Divide 235.214583 by the MPG figure. For example, 30 MPG gives 235.214583 / 30 = 7.84, which rounds to 7.8 L/100km. The same constant works in reverse: divide 235.214583 by the L/100km figure to get MPG back.

Why does a higher MPG mean a lower L/100km, instead of the two numbers moving together?

MPG measures distance per unit of fuel, so a bigger number means better efficiency. L/100km measures fuel used per unit of distance, so a smaller number means better efficiency. They move in opposite directions because one is essentially the inverse of the other.

Does this calculator use the US gallon or the UK (Imperial) gallon?

The US liquid gallon (3.785411784 liters), the definition used for MPG ratings on US vehicles. The UK/Imperial gallon (4.54609 liters) is a different, larger unit and is not supported here. Using the wrong gallon size would understate MPG by about 20 percent.

How is cost per mile calculated, and how accurate is it?

Cost per mile is your price per gallon divided by MPG, using the full-precision fuel economy figure rather than the rounded display value. It's an estimate: real-world cost also depends on driving conditions, terrain, and how fuel economy varies trip to trip.

Why is 235.214583 the conversion constant between MPG and L/100km?

It comes from 100 times the US gallon in liters (3.785411784), divided by a mile in kilometers (1.609344), both exact NIST definitions. That combination converts miles-per-gallon into the equivalent liters used to travel 100 kilometers, and back again.

Can I use this to compare a US car's window-sticker MPG to a European car's fuel consumption rating?

Yes, that's exactly what it's for. US listings show MPG while most other countries show L/100km, so converting one figure into the other lets you compare efficiency directly, as long as both figures were measured under similar test conditions.

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