The calculation of cement & sand in 1:6 is calculated in the following manner.
For a Sample calculation, I will assume a 12mm thick plaster and a mix ratio of 1:6.
Step-1: Calculate the volume of mortar required. I am calculating it for 1 Sqm and 12 mm thickness (Assumption).
So, Volume of plaster = Area X Thickness = 1 X 0.012 = 0.012 cum
Step-2: Calculate the dry volume of mortar required.
Since wet volume is always less than the dry volume. The dry volume of motor required for plastering = 1.27 X Dry volume of plaster = 1.27 X 0.012 = 0.01524 cum
Step-3: The mix ratio of motor is 1:6, So
Cement required = 1/7 X 0.01524 = 0.00217 cum
Sand required = 6/7 X 0.01524 = 0.01306 cum
Step-4: To get the weights of materials required in multiply it with its density
Cement required = 0.00217 cum X 1440 kg/cum = 3.12 Kgs
Since Sand is usually measured in cft = 0.01306 X 35.29 = 0.46 cft
You can calculate the cement and Sand requirement for other Thickness of plasters and design mix also in the similar fashion