How do I work out how much waterproofing I need?

Firstly, how much of the wetroom area do you want to waterproof? Just the shower area or the entire bathroom? To work out how much waterproofing you will need, work out the square meterage of the floor area and each wall that you wish to waterproof, and add them together allowing 10% extra for wastage. Allow a bit extra also to cover any overlaps if using Aquaguard.