While it is a nice concept to think that the recommendations included on a bag of thinset mortar or container of adhesive are accurate in regards to the square footage per container, the reality is that those numbers are estimations, not fact. Given the intricacies of tile installations, there are several variables that can alter the thickness and thus the amount of tile adhesive per square meter.
Notch of the Trowel
When you read the back of a container of tile adhesive you will see estimations given by the manufacturer in regards to how much coverage you will get with that particular container using various sizes of trowels. The trowel is the primary determining factor in how much adhesive per square meter is used. However, these estimations are only accurate if every element of the installation is perfect. If there is one thing that is true in home construction, it is that nothing is every perfect.
Floor Versus Wall
Because wall tile installs differently than floor tile, even though you might have a rough idea how much adhesive you need based on the size of your notched trowel you will sometimes need more mud on wall installations than on floors. This is to help the tiles with gravity. For example, with walls you sometimes want to skim coat the wall as well as the backs of each individual tile before installing, which will use up more adhesive.
Inconsistencies in the Surface
Back to the fact that no construction project is ever perfect, and the reality of construction inconsistencies. In the case of the installation surface, there are always issues such as low spots, high spots, walls that need to be built out for plumb and voids that need to be filled in. All of these areas require the use of additional adhesive. This is one of the reasons 15 to 20 percent extra material is always ordered when working on home improvement projects.
If you start working with rough stone you can throw out the recommendations on the adhesive container, because stones always require far more than what a notched trowel can provide. Instead, the adhesive needs to be applied based upon each individual piece of natural stone or rock, with between 1/2 inch and 3/4 inch joints left behind as well. Your best option in this case is to use a mortar calculator to try and estimate how much you might need.