Plastering covers brick walls with a thin layer of cement-sand mortar to create a smooth surface ready for painting. The calculator gives you cement bags and sand needed for any wall area, mix ratio, and thickness.
What is Plaster?
Plaster serves two purposes: it smooths out the rough surface of brick walls (so paint sits evenly) and protects the brick from weather/water damage. Internal walls use thinner plaster (12 mm) and a leaner mix (1:6). External walls need thicker plaster (15-20 mm) and stronger mix (1:4) for water resistance.
The math: wall area × thickness gives the wet plaster volume. Multiply by 1.33 (dry volume factor) and add wastage. Then split between cement and sand based on the mix ratio.
Hand-applied plaster has high wastage — 15-20% is normal because mortar drops to the floor as the mason works.
Wall area × thickness × dry-factor
- Area
- Wall area—Length × Height in m²
- thickness
- Plaster layer—12 mm internal, 15-20 mm external
- c, s
- Mix ratio—1:6 = c=1, s=6
How to use this calculator
Wall dimensions, plaster thickness, mix ratio, and wastage.
Enter wall length and height
In metres. Calculate per wall, sum across the room.
Pick plaster thickness
12 mm for internal walls (smooth), 15 mm for external, 20 mm only for very uneven external.
Pick mix ratio
1:6 for internal, 1:4 for external (water resistance), 1:3 for ceilings.
Set wastage
15% standard, up to 20-25% for ceilings (more falls during overhead work).
Plastering applications
Internal wall plastering
12mm × 1:6 — most common. Smooth finish before painting.
External wall plastering
15mm × 1:4 — water-resistant, weather-protective.
Ceiling plastering
12mm × 1:3 — richer mix for adhesion against gravity. Higher wastage (20%+).
Skim coat / finish plaster
2-5 mm of fine plaster over base plaster for ultra-smooth surface. Very low cement usage.
Common mistakes to avoid
Ignoring wastage and uneven-surface thickness
Brick walls are never perfectly plumb — actual plaster thickness averages 15-20% more than the nominal 12 mm. Add 15-20% material margin on top of the calculated quantity.
Using the internal 1:6 mix for external walls
External plaster needs the richer 1:4 mix for weather resistance. Using 1:6 outside invites dampness and peeling within a couple of monsoons.
Not deducting door and window openings
Subtract the full area of each opening from the wall area (standard practice deducts 100% of openings for plaster). On a typical room, this saves nearly a bag of cement.
Glossary
- Internal plaster
- Inside walls. 12mm thickness, 1:6 mix typically.
- External plaster
- Outside walls. 15-20mm thickness, 1:4 mix for water resistance.
- Skim coat
- Final fine layer (2-5mm) for ultra-smooth finish.
- Dry volume factor
- 1.33× — accounts for voids and shrinkage when water is added to the dry mix.