Plastering is one of the most important work needs to be done to protect the surface of wall or roof ceiling. is the method of wrapping the blocks walls and ceiling with a smooth finish with different plaster materials (a blend of Cement and sand with a specific ratio 1:1/5,1:2,1:3 etc. according to the work requirements).
Let's learn together how to know calcul quantity of cement and sand required for 100  sq.ft plastering wall (12 mm thickness). After that you will be able to calculate as per your desired area.
To make my calculation easier I will convert feet unit in meter unit.
Area of plaster = 100 sq.ft = 9.29 m²
Thickness of plaster = 12 mm = 0.012 m
Mix ratio of plaster = cement : sand = 1: 4
Volume of plaster = Area of plaster x Thickness of plaster = 9.29 x 0.012 = 0.11 m³
Now we need to add extra 20% for joint filling, depression and wastage.
Volume of plaster = 0.11 b+ (20×0.11/100) = 0.13 m³
This is wet volume, we need find out dry volume. To get dry volume multiply wet volume with 1.35
Dry volume = wet volume x 1.35 = 0.17 m³
QUANTITY OF CEMENT:
Cement = ( Dry vol of plaster x cement ratio)/ Total sum of ratio
= 0.17 x 1/5 = 0.03 m³
We know density of cement = 1440kg/m³
Weight of cement = 0.03 x 1440 = 48.96 = 50 kg
1 bag cement contains 50 kg of cement.

QUANTITY OF SAND:
Sand = Quantity of cement x 4 = 0.3 x4/100 = 0.12 m³
1 m3 = 35.3147 cubic feet = or cft
Sand required = 0.12 x 35.3147 = 4.23 cft

SUMMERY:
Cement Required =  1 bag
Sand required = 4.23 cft