Plaster covers every wall and ceiling twice over — inside and out — so it adds up to a surprising share of a house's cement. A quick, consistent method lets you estimate it from the drawings.
The Method
- Area: wall or ceiling area to be plastered, in m² (deduct large openings).
- Wet volume: area × thickness (in metres).
- Dry volume: wet volume × 1.27.
- Allowance: + 20% for joints, hollows and unevenness.
- Split by ratio: cement = dry volume × (1 ÷ sum of parts); sand = dry volume × (sand parts ÷ sum).
- Cement in bags: m³ × 1,440 kg ÷ 50.
Worked Examples (per 100 sq ft = 9.29 m²)
| Plaster | Thickness | Mix | Wet vol. | Dry + 20% | Cement | Sand |
|---|---|---|---|---|---|---|
| Internal wall | 12 mm | 1:6 | 0.111 m³ | 0.170 m³ | 35 kg ≈ 0.7 bag | 0.146 m³ ≈ 5.2 cft |
| External wall | 20 mm | 1:4 | 0.186 m³ | 0.283 m³ | 82 kg ≈ 1.6 bags | 0.226 m³ ≈ 8.0 cft |
| Ceiling | 6 mm | 1:3 | 0.056 m³ | 0.085 m³ | 31 kg ≈ 0.6 bag | 0.064 m³ ≈ 2.3 cft |
A Whole-Room Example
A 12 × 12 ft room with 10 ft walls, one door (3 × 7 ft) and one window (4 × 4 ft):
- Wall area: 4 × 12 × 10 = 480 sq ft − 21 − 16 = 443 sq ft
- Internal plaster (12 mm, 1:6): 4.43 × 0.7 = about 3.1 bags, 4.43 × 5.2 = about 23 cft sand
- Ceiling (144 sq ft, 6 mm, 1:3): 1.44 × 0.6 = about 0.9 bag, about 3.3 cft sand
Plaster Good Practice
- Dampen the wall before plastering.
- Fix mesh across junctions of walls with columns and beams.
- Apply external plaster in two coats, the first roughened to key the second.
- Cure for at least 7 days.
- Avoid plastering in direct afternoon sun in Hyderabad's summer.
The Short Version
Area × thickness × 1.27 × 1.2, split by the mix ratio. Per 100 sq ft, plan about 0.7 bag for 12 mm internal plaster, 1.6 bags for 20 mm external, and 0.6 bag for ceilings.
