The formula with 50 substituted
km/L = 50 × 0.425144 = 21.26
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The MPG to km/L Converter explains where each comes from and gives a longer table.
The same 50 mpg (US) is 60.05 mpg on the imperial gallon, 4.70 L/100km and 21.26 km/L. If your 50 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 17.70 km/L instead.
What 50 mpg (US) means in practice
50 mpg (US) converts to 21.26 km/L. Small hybrids reach this on the EPA cycle; on Japan's WLTC the same cars are rated higher, at 25 to 30 km/L, because of the cycle difference.