How to calculate quantity of Cement and Sand in Plaster

How to calculate quantity of Cement and Sand in Plaster?

Ex:
   Area of plaster = 100 ft × 100 ft
   Thickness of plaster = 12mm

Step 1) find out volume of plaster:
 (Need to all units that is "ft" and "mm" convert into "m" )

                  1000mm = 1m
So that;        12mm = 0.012m

                     3.28ft = 1m
So that;        100ft = 30.49m

    Volume of plaster = 30.49 × 30.49 × 0.012
                                     = 11.15 m³

Step 2) Convert plaster volume wet to dry:
             Take 33%
    Dry vol = 11.15 × 1.33
                  = 14.83 m³

Step 3) Consider wastage:
              Take 5%
    Total dry vol = 14.83 × 1.05
                            = 15.57 m³

Step 4) find out Cement and sand required:
              Take;
                   Cement : sand
                              1 : 6

    Cement req = 1/(1+6) × 15.57
                          = 2.22 m³
            Need to cement in bags
       (density of cement 1440 kg/m³ )
                          = 2.22 × 1440
                          = 3196.8 kg               (1bag = 50 kg)
                          = 3196.8/50
                          = 63.93 bags = 64 bags

     Sand req = 6/(1+6) × 15.57
                      = 13.34 m³              (1brass = 2.83m³)
                      = 13.34/2.83
                      = 4.7 brass
                               OR
                      = 13.34 × 35.315
                      = 471.1 cft                    (1m³ = 35.315)

For proper understanding watch my video on YouTube:
https://youtu.be/dJk5Xp0y07A