Measure the wall or ceiling area in square feet or square meters
Subtract openings such as doors and windows
Determine the plaster thickness required
Convert thickness to the same unit as the area
Calculate wet plaster volume using Area × Thickness
Add wastage allowance, usually 5% to 10%
Convert wet volume to dry volume by multiplying by a factor of about 1.33 to 1.5
Determine the mix ratio of cement, sand, and other materials if needed
Calculate the quantity of each material from the dry volume
Convert material quantities into bags, kilograms, or cubic feet as required