Plaster sand is usually mixed in the ratio 1:6. That is, one part of cement to six parts of sand. Calculating the amount of these materials allows 25% bulking of sand. There is not much difference between the densities of cement and sand so the density of sand will be indirectly used to determine mass of sand required. Recall that a standard bag of cement weighs 50kg and it has a volume of 0.033m³

Example;

Let’s say you want to determine the amount of cement and sand necessary to plaster a wall of area 900m² and the thickness of plaster is 15mm thick.  This is how you do it.

first determine the volume of plaster which is

AREA x THICKNESS

= 900×0.015= 13.5m³

since its in ratio 1:6, you use this information to find the volume of cement and that of concrete.

volume of cement is;

(1/7 )× 13.5= 1.93m³

therefore volume of sand is 13.5-1.93= 11.57m³

Increasing the sand volume by 25% to cater for bulking we have: 

1.25 x 11.57= 14.46m³

The volume of cement previously calculated is 1.93. Dividing this volume by the volume of one 50kg bag of cement we get:

1.93/ 0.033= 58.48 bags, approximately 59 bags.

let’s indirectly use the density of cement to approximately determine the mass of sand required

recall that a 50kg bag of cement has a volume of 0.033m³

so dividing the volume of sand previously calculated by the volume of one 50kg bag of cement ,we have;

14.46/ 0.033=439 (50kg bags).

therefore mass of sand is;

439 x 50= 21950 kg or 21.95 tons

Our calculation so far shows that 59 bags of cement and 21.95tons of plaster sand is required to plaster a wall of 900m² with plaster thickness of 15mm. ( mass of sand increased by 25% to cater for bulking).

However, plastering usually results in considerable amount of waste. Coupled with wall irregularities such as holes and joints that need filling. Let’s increase the amount of cement and sand needed by 30% to cater for this;

i.e 1.3×59 bags of cement = approximately 77bags

and 1.3 x 21.95 tons of sand = 28.54tons.