Below is the step by step process of calculating quantity of mortar required for cementing.
 Area of the surface multiply with the thickness gives the quantity of mortar for uniform thickness.
 This quantity is increased by 30% for filling the joints and to make up uniform surface of wall. It will give wet mixed mortar.

This mortar quantity is further increased by 25% to get the dry volume of the ingredients.

Quantity of each material of the mortar may be found by dividing the dry volume of mortar by the sum of numerals of the proportions and multiplying the answer with the individual numeral.
For example
Materials for 12 mm or half inch thick plastering in wall for 100 square meter
 First of all, we have to multiply 100 square meter surface with 12 mm.
 100 square meter x 0.012 m = 1.2 cubic meter. (12 mm can also be written as 0.012)
 1.2 cubic meter is wet mixed mortar for uniform thickness.
 Add 30% in this value to fill up joints, uneven surfaces, etc., the quantity of mortar comes out 1.2 + 0.36 = 1.56 cubic meter.
 Increasing by 25% the total dry volume will be 1.2+0.36+0.39 = 1.95 cubic meter or 2 cubic meter.
 For cement sand mortar, cement = dry volume / ( sum of ratios) x numeral of cement.
 For 1:4 cement sand mortar, cement will be 2/5 x 1 = 0.4 cubic meter.
 For 1:4 cement sand mortar, sand will be 2/5 x 4 = 1.6 cubic meter.
 In this way you can calculate the dry volume of any ratio of mortar ingredients.
Jaspal Singh says
In the case of sand, how it becomes 1.6cub m
If we divide 2 by 20 it results 0.1 cub m.
Jaspal Singh says
Yups got the point
Thanks
Hannu says
The quantity of plastering material needed in the above example is 2 cubic meters (m3). When it is made with a cement/sand ratio of 1/4 there will be 1 part of cement, 4 parts of sand, so 5 parts in total. Therefore, the amount of sand needed is 4/5 x 2 m3 = 1.6 m3. The rest (2.01.6) = 0.4 m3 is cement. You will also need water, of course, but its quantity doesn’t affect this calculation.