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Plaster volume = area × thickness; multiply by 1.27 for dry volume, then split by the mix. 100 m² of 12 mm plaster in 1:4 is 1.2 m³ wet, 1.524 m³ dry, so cement is 1.524 ÷ 5 = 0.3048 m³ = 8.78 bags of 50 kg, and sand 1.22 m³.

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Surface
m²
mm
Mix
Plaster 1:4 on 100 m²Plaster 1:4 on 100 m²wall plaster 12 mmmyconstructioncalculator.comNot to scale
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Results

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Cement (50 kg bags)
8.78bags
Sand1.22 m³
Wet plaster volume1.200 m³
Cement weight438.9 kg
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Working
  1. Wet volume = area × thickness = 100.0 m² × 0.0120 m = 1.2000 m³
  2. Dry volume = 1.2000 × 1.27 = 1.5240 m³
  3. Cement = 1.5240 × 1 ÷ 5 = 0.3048 m³ = 8.78 bags
  4. Sand = 1.5240 × 4 ÷ 5 = 1.2192 m³
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
Formula


Wet volume   = Area × Thickness
Dry volume   = Wet volume × 1.27
Cement (1:n) = Dry volume ÷ (1 + n)        → ÷ 0.0347 m³ per 50 kg bag
Sand         = Dry volume × n ÷ (1 + n)


Worked example


100 m² of internal wall plaster, 12 mm thick, 1:4 mix:


    

  1. Wet volume = 100 × 0.012 = 1.200 m³
    2. 

  2. Dry volume = 1.200 × 1.27 = 1.524 m³
    3. 

  3. Cement = 1.524 ÷ 5 = 0.3048 m³ → ÷ 0.0347 = 8.78 bags (about 439 kg)
    4. 

  4. Sand = 1.524 × 4 ÷ 5 = 1.22 m³
    5. 



Cement and sand per 100 m² at 12 mm






Mix
Cement (50 kg bags)
Sand (m³)






1 : 3
10.97
1.143




1 : 4
8.78
1.219




1 : 5
7.32
1.270




1 : 6
6.27
1.306






For other thicknesses, scale in proportion: 15 mm is 1.25 × these figures, 20 mm is 1.67 ×.


Assumptions


    

  • Area is the net plastered area after deducting doors and windows (some measurement rules deduct only openings over a set size).
    • 

  • Cement loose density 1,440 kg/m³ (50 kg bag = 0.0347 m³).
    • 

  • Wastage is included in the 1.27 factor; raise the factor for very uneven walls.
    • 



Worked Examples
Frequently Asked Questions
How much cement is needed for 100 m² of plaster?
At 12 mm thick, about 8.8 bags (50 kg) for 1:4, 7.3 bags for 1:5 and 6.3 bags for 1:6, using a dry factor of 1.27.
Why multiply plaster volume by 1.27?
The wet volume is increased by about 20% to fill joints and hollows in the wall surface, and the dry mix is larger than the wet volume because cement fills voids in the sand. Many site engineers use 1.27; some use 1.30–1.35.
What plaster thickness should I use?
Commonly 12 mm on internal brick or block walls, 15–20 mm in two coats on external walls, and 6 mm on concrete ceilings.
Which mix ratio for plastering?
1:4 for external walls and wet areas, 1:5 or 1:6 for internal walls, and 1:3 for ceilings and very exposed surfaces. Follow your specification.
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