Convert MPG and L/100 km using US or UK gallons, and estimate trip fuel when you enter a distance.
Advanced options
Table of contents
How to use our MPG to L/100km Converter
- Choose Convert from to match the value you already have: MPG to L/100 km or L/100 km to MPG.
- Set MPG gallon type to US gallon or UK gallon before entering Fuel economy (MPG) or Fuel use (L/100 km).
- Use Decimal places if you want a shorter or longer displayed answer; the calculator still uses full precision in the math.
- Open Advanced options only if you want a trip estimate, then enter Trip distance and choose Trip distance unit.
- Sanity-check the result: higher MPG, higher Same value in km/L, lower L/100 km, and lower Fuel used per 100 miles all mean less fuel used.
Definitions
MPG: Miles per gallon. It tells how many miles a vehicle can travel on one selected gallon of fuel.
L/100 km: Liters per 100 kilometers. It tells how many liters of fuel a vehicle uses to drive 100 kilometers.
US gallon: A gallon size used in US MPG ratings. It is smaller than a UK imperial gallon.
UK gallon: The imperial gallon used in UK MPG ratings. The same MPG number converts differently if this gallon type is selected.
km/L: Kilometers per liter. Like MPG, a higher km/L value means the vehicle goes farther on the same fuel.
Fuel used per 100 miles: Selected gallons needed to drive 100 miles. Like L/100 km, a lower value means less fuel used.
Common mistakes and quick fixes
Mistake: Picking the wrong MPG gallon type when your MPG number comes from a UK or Canadian source.
Fix: Set MPG gallon type to US gallon or UK gallon before reading Converted value.
Mistake: Typing Fuel economy (MPG) while Convert from is set to L/100 km to MPG, then wondering why the answer did not use it.
Fix: Match Convert from to the value you entered, or put the number in Fuel use (L/100 km).
Mistake: Entering 0 or a negative number for Fuel economy (MPG) or Fuel use (L/100 km).
Fix: Use a positive number greater than 0 for the visible fuel input.
Mistake: Reading Converted value backward and thinking a higher L/100 km number is better.
Fix: Use What this means: higher MPG is better, but lower L/100 km is better.
Mistake: Entering Trip distance in miles while Trip distance unit is set to kilometers.
Fix: Check Trip distance unit before using Trip fuel used.
Limitations & Key Assumptions / Boundary Conditions
- The converter assumes one steady fuel economy value. Real fuel use can change with speed, traffic, weather, tires, hills, cargo, and driving style.
- US MPG and UK MPG are not interchangeable. The MPG gallon type setting changes the conversion factor.
- Trip fuel used is an estimate based only on Trip distance and the converted fuel rate. It does not include idling, detours, warm-up, or fuel left in the tank.
- Decimal places changes only the displayed rounding. Internal calculations use the full conversion factor.
- Fuel economy (MPG), Fuel use (L/100 km), and Trip distance must be positive when used. Blank trip distance is allowed and simply hides trip fuel outputs.
- Very tiny positive fuel economy or fuel use inputs can create very large converted values. The math is valid, but the input may be unrealistic.
Methodology
Core conversion
The calculator first chooses the gallon type. For US gallons, it uses 3.785411784 liters per gallon. For UK imperial gallons, it uses 4.54609 liters per gallon. One mile is treated as 1.609344 kilometers.
US factor = 100 * 3.785411784 / 1.609344 = 235.2145833333333
UK factor = 100 * 4.54609 / 1.609344 = 282.48093633182214
If Convert from is MPG to L/100 km, the selected factor is divided by Fuel economy (MPG).
L/100 km = factor / MPG
If Convert from is L/100 km to MPG, the selected factor is divided by Fuel use (L/100 km).
MPG = factor / L/100 km
Extra outputs
Same value in km/L is found from the L/100 km rate. Fuel used per 100 miles is found from the selected MPG value.
km/L = 100 / L/100 km
selected gallons per 100 miles = 100 / MPG
For the optional trip estimate, miles are converted to kilometers when needed, then the L/100 km rate is applied. The gallon result uses the selected gallon size.
trip km = trip miles * 1.609344
trip liters = trip km * L/100 km / 100
trip selected gallons = trip liters / liters per selected gallon
Mini-example
For 30 US MPG, the converter uses 235.2145833333333 divided by 30, giving 7.840486111 L/100 km. The same value is 100 divided by 7.840486111, or about 12.75 km/L. For a 250 mile trip, the distance is 402.336 km, so the estimated fuel is 402.336 * 7.840486111 / 100 = 31.545 liters, which is 8.333 US gallons.