1 liter = 0.264172052 US gallons
you can probably figure out the rest from there...
but, i belive the footprint of your tank is most important. so what i do is figure 3" deep uniform across the bottom of the tank. of course you will have some high and low spots but if you figure 3", you are generally fine and adding wood and rocks can always take up more volume for you leaving you with extra substrate.
so figure out the dimensions and get it in cubic inches or cubic feet, whatever you like...then convert that to liters on google...