First, Calculate The Dry Volume Of Mortar.

The required mortar volume for our plasterwork is 4 cubic feet (We calculated this above).

But that is the wet volume of mortar.

We need to calculate the dry volume of that to get the cement quantity.

And, the dry volume is,

= 4 × 1.33

= 5.32 cubic feet.

Next, Calculate The Volume Of Cement In The Mortar

The formula is,

= (Dry volume of mortar ÷ Sum of ratio) × Cement ratio

= (5.32 ÷ 5) × 1

= 1.06 cubic feet.

[NOTE: The common cement-sand ratio for plastering mortar is 1:4 or 1:6. I used a 1:4 cement-sand ratio for the sake of our calculation]

Now, Convert Cement Volume Into Bags.

The formula is,

= Volume of the required cement ÷ Volume of one bag of cement.

= 1.06 ÷ 1.25 (One bag of cement is equal to 1.25 cubic feet)

= 0.85 bag.

If you wish to plaster with concrete mix, then one part cement and 2-part concreting sand are important. But if you like to use mortar mix for the walls, then the perfect cement mortar ratio for plastering would be one part of cement and for to five parts of building sand.