Cement for Plastering: Bags per 100 sq ft for 12 mm, 15 mm and 20 mm
Plaster is one of the biggest cement consumers in a house. Here are the bag counts per 100 sq ft for the common thicknesses and mixes.
For 100 sq ft of plaster (using a dry factor of 1.27): 12 mm thick needs about 0.82 bags of cement in 1:4 mortar or 0.58 bags in 1:6; 15 mm needs about 1.0 bag (1:4) or 0.73 bags (1:6); 20 mm needs about 1.36 bags (1:4) or 0.97 bags (1:6). Internal walls usually take 12 mm in 1:6, external walls 20 mm in 1:4, and ceilings 6 mm in 1:3 or 1:4.
Quick table: cement per 100 sq ft
| Thickness | 1:4 | 1:5 | 1:6 |
|---|---|---|---|
| 12 mm | ~0.82 bag | ~0.68 bag | ~0.58 bag |
| 15 mm | ~1.02 bags | ~0.85 bag | ~0.73 bag |
| 20 mm | ~1.36 bags | ~1.13 bags | ~0.97 bag |
Sand for 12 mm in 1:6 is about 4.3 cft per 100 sq ft; in 1:4 about 4.0 cft.
Which thickness and ratio where?
| Surface | Typical thickness | Typical mix |
|---|---|---|
| Internal brick walls | 12 mm | 1:6 |
| Rough internal surfaces | 15 mm | 1:6 / 1:5 |
| External walls | 20 mm (often two coats) | 1:4 / 1:5 |
| Ceiling | 6 mm | 1:3 / 1:4 |
Follow your specification; local practice varies.
The method, step by step (12 mm, 1:6)
- Area = 100 sq ft = 9.29 m²
- Wet volume = 9.29 × 0.012 = 0.1115 m³
- Dry volume = 0.1115 × 1.27 = 0.1416 m³ (accounts for voids, uneven surfaces and joints filling)
- Sum of ratio = 1 + 6 = 7
- Cement = 0.1416 ÷ 7 = 0.0202 m³ × 1,440 = 29 kg ≈ 0.58 bag
- Sand = 0.0202 × 6 = 0.121 m³ ≈ 4.3 cft
Some engineers use a factor of 1.33 or add a separate 20% for uneven surfaces. On rough brickwork, actual consumption can exceed these figures.
Ceiling plaster (6 mm, 1:3)
- Wet volume = 9.29 × 0.006 = 0.0557 m³
- Dry = 0.0708 m³
- Cement = 0.0708 ÷ 4 = 0.0177 m³ × 1,440 = 25.5 kg ≈ 0.51 bag per 100 sq ft
Which cement is best for plaster?
PPC is widely preferred for plaster: it is workable, gives a smooth finish and releases heat slowly, which helps reduce cracking. OPC 43 also works. Avoid OPC 53 for plaster — the faster heat release increases the chance of fine cracks. See OPC 53 vs OPC 43.
Tips for crack-free plaster
- Wet the wall before plastering so bricks do not suck water out of the mortar.
- Rake masonry joints for a better key.
- Use chicken mesh at junctions of concrete and brickwork.
- Use clean, well-graded plastering sand, free from silt. See sand silt test.
- Do not over-trowel — it brings fine particles to the surface and causes crazing.
- Cure for at least 7 days (10 for PPC), keeping the surface damp.
More in cracks in concrete and plaster.
Key takeaways
- 12 mm internal plaster in 1:6 uses about 0.6 bag per 100 sq ft.
- 20 mm external plaster in 1:4 uses about 1.4 bags per 100 sq ft.
- Use a dry factor of about 1.27 plus extra for rough surfaces.
- PPC gives smoother, less crack-prone plaster.
Frequently asked questions
How much cement is required for 100 sq ft of plaster?
For 12 mm plaster, about 0.58 bag in 1:6 mortar or 0.82 bag in 1:4 mortar, before extra allowance for uneven surfaces.
What is the ratio for internal plaster?
Internal wall plaster is commonly 12 mm thick in 1:6 cement-sand mortar; rougher surfaces may need 15 mm.
What is the ratio for external plaster?
External plaster is commonly 20 mm thick in 1:4 or 1:5 mortar, often applied in two coats.
Which cement is best for plastering?
PPC is widely preferred for plaster because of its workability, smooth finish and lower heat release, which helps reduce cracking.
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This guide is general information for planning and buying. Quantities, grades and methods for your structure must follow your approved drawings, your engineer’s specification and the applicable Indian Standards.
