How to calculate the quantity of materials for mortar and Ytong stones?

Ytong is not mortared, Ytong is glued, it does not add thickness.

This has less to do specifically with the stone material but with the "Planblock" concept and applies equally to KS and others. Anyone who does not immediately understand from 249 mm stone height that a bed joint height of 1 mm is assumed still has a long way to go as a beginner mason. In that case, even a garden shed is almost a challenging practice project.