Key Information Before You Start

Before you can calculate the materials, you need to know three things:

1. Plaster Area: The total area of the wall or ceiling you plan to plaster (in square meters or square feet).
2. Plaster Thickness: The thickness of the plaster layer.
3. Mix Ratio: The ratio of cement to sand.

Here are the standard values you’ll need:

How to Calculate Cement and Sand: A Step-by-Step Guide

Follow these 5 steps to find the exact quantities you need.

Step 1: Calculate the Total Plaster Area

Measure the length and height of your wall.

• Formula: Area = Length × Height
• Example: A wall 5 meters long and 3 meters high. Area = 5 m × 3 m = 15 m²

Step 2: Calculate the Wet Volume of Mortar

This is the volume of the plaster after it’s mixed with water and applied to the wall.

• Formula: Wet Volume = Area × Plaster Thickness
• Example (using 12 mm thickness):
  • First, convert thickness to meters: 12 mm = 0.012 m
  • Wet Volume = 15 m² × 0.012 m = 0.18 m³

Step 3: Calculate the Dry Volume of Mortar

This is the most important step. The volume of dry materials (cement + sand) is always more than the final wet volume. This is because the fine cement particles fill the voids (empty spaces) in the sand, and we must also account for waste.

• Rule of Thumb: To find the dry volume, multiply the wet volume by 1.33 to 1.35. We’ll use a factor of 1.33.
• Formula: Dry Volume = Wet Volume × 1.33

• Example:
  • Dry Volume = 0.18 m³ × 1.33 = 0.2394 m³
  • This is the total volume of dry cement and sand you need.

Step 4: Calculate the Quantity of Cement

Now, we use the mix ratio to find out how much of the dry volume is cement.

• Mix Ratio: Let’s use 1:4 (1 part cement + 4 parts sand)
• Sum of Ratios: 1 + 4 = 5
• Formula: Cement Volume = (Dry Volume × Cement Part) / Sum of Ratios
• Example:
  • Cement Volume = (0.2394 m³ × 1) / 5 = 0.04788 m³

To find the number of cement bags: The standard volume of one 50kg bag of cement is 0.0347 m³.

• Formula: Bags of Cement = Cement Volume / 0.0347
• Example: 0.04788 m³ / 0.0347 m³ = 1.38 bags
• Result: You should buy 2 bags of cement.

Step 5: Calculate the Quantity of Sand

Finally, we calculate the amount of sand needed.

• Formula: Sand Volume = (Dry Volume × Sand Part) / Sum of Ratios
• Example (using 1:4 ratio):
  • Sand Volume = (0.2394 m³ × 4) / 5 = 0.19152 m³

Note on Sand Units: Sand is often sold in cubic feet (cft) or tonnes.

• To convert to Cubic Feet (cft): 1 m³ = 35.3147 cft
• Example: 0.19152 m³ × 35.3147 = 6.76 cft
• Result: You will need 0.19 m³ or 6.76 cft of sand.

Example Summary (For a 15 m² Wall)

Here are the final materials for plastering a 15 m² wall at 12 mm thickness with a 1:4 mix ratio:

• Cement: 2 bags (50kg each)
• Sand: 0.19 m³ (or 6.76 cft)

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