Convert MPG and km/L both ways, with an explicit US or UK gallon choice to avoid mixing fuel economy units.
Table of contents
How to use our MPG to KM/L Converter
- Choose Convert to set the direction: MPG to km/L or km/L to MPG.
- Enter the Fuel economy value as a number only, such as 30 or 12.75.
- Set MPG gallon type to US gallon for most US vehicle labels, or UK Imperial gallon for UK-style MPG.
- Use Round answers to if you need fewer or more decimal places, then calculate.
- Sanity-check the Converted fuel economy against If the other MPG gallon type was meant; a big difference means the gallon choice matters for your comparison.
Definitions
MPG: Miles per gallon. It tells how many miles a vehicle can travel on one gallon of fuel.
km/L: Kilometers per liter. It tells how many kilometers a vehicle can travel on one liter of fuel.
US gallon: A US liquid gallon, equal to 3.785411784 liters.
UK Imperial gallon: A UK gallon, equal to 4.54609 liters, so UK MPG is not the same as US MPG for the same fuel use.
L/100 km: Liters used per 100 kilometers. Lower is better because the vehicle uses less fuel to travel the same distance.
Conversion factor: The multiplier used to change one unit into another, such as km/L per MPG.
Common mistakes and quick fixes
Mistake: Leaving Fuel economy value blank or typing words like 30 mpg.
Fix: Enter only the number in Fuel economy value, such as 30 or 12.75.
Mistake: Using MPG gallon type as US gallon when the MPG number came from a UK source.
Fix: Change MPG gallon type to UK Imperial gallon before trusting Converted fuel economy.
Mistake: Comparing Converted fuel economy without checking If the other MPG gallon type was meant.
Fix: Use If the other MPG gallon type was meant as a quick warning check for US vs UK MPG mix-ups.
Mistake: Treating Liters used per 100 km like MPG, where higher feels better.
Fix: Remember that lower Liters used per 100 km means less fuel used for the same distance.
Mistake: Setting Round answers to 0 decimal places when you need to match a spec sheet closely.
Fix: Set Round answers to 2, 3, or 4 decimal places for a closer Converted fuel economy.
Limitations & Key Assumptions / Boundary Conditions
- The calculator does unit conversion only. It does not estimate real fuel use from speed, weather, traffic, tires, driving style, or fuel blend.
- The MPG gallon type must be chosen by the user. The calculator does not guess whether an MPG number is US or UK.
- Fuel economy values must be greater than 0. Blank, zero, negative, and non-numeric entries are errors.
- Very small positive km/L values create very large L/100 km values because L/100 km equals 100 divided by km/L.
- Rounding changes only the displayed answers. Internal calculations use the unrounded value.
- The alternate gallon result is a check for possible unit mix-ups, not a second official answer for the chosen inputs.
Methodology
Core conversion
The converter uses 1 mile = 1.609344 kilometers, 1 US liquid gallon = 3.785411784 liters, and 1 UK Imperial gallon = 4.54609 liters [2]. The selected MPG gallon type decides which gallon size is used.
km_per_liter = mpg * 1.609344 / gallon_liters
mpg = km_per_liter * gallon_liters / 1.609344
For US MPG, the MPG to km/L factor is 1.609344 / 3.785411784 = 0.425143707430272. For UK Imperial MPG, the factor is 1.609344 / 4.54609 = 0.354006189934647.
Other gallon check
The alternate gallon result repeats the same conversion with the gallon type not selected. This helps catch the common mistake of treating US MPG and UK MPG as the same unit.
other_km_per_liter = entered_mpg * 1.609344 / other_gallon_liters
other_mpg = entered_km_per_liter * other_gallon_liters / 1.609344
Liters per 100 km
After the calculator has a km/L value, it also computes L/100 km. This unit moves in the opposite direction from MPG and km/L: lower L/100 km means less fuel used for the same distance.
liters_per_100km = 100 / km_per_liter
Mini-example
For 30 US MPG, the km/L result is 30 * 1.609344 / 3.785411784 = 12.75431122290816 km/L, which rounds to 12.75 km/L with two decimal places. The L/100 km value is 100 / 12.75431122290816 = 7.840486111111111, which rounds to 7.84 L/100 km.