In most countries of the world, the fuel consumption is expressed in Liters required to drive 100 kms, or L/100 kms.
Many on-line converters are available on the web to convert one consumption rate into the other, but no easy method, that could be used by a simple 4-basic-function pocket calculator.
Here it is :
Just remember the "magic" number 235.
This is a one-step method that works both ways :
235 / mi./gal. = L/100kms
and as well :
235 / L/100 kms = mi./gal.
This is due to the fact that any numbers of mi./gal. and corresponding L/100 kms, when multiplied one by the other, give a constant product :
235.2141128......
Using 235 gives a result accurate to 3 decimals (actually between 1 and 2 thousanths). Enough by far for any day-to-day use.
The "magic" number is obtained by multiplying the number of liters in a gallon and the number of miles in 100 kilometers. Doing the math will show you how the result is actually the constant product revealed here.