For 1 m² of 12 mm thick cement plaster, a 1:4 mix needs about 4.39 kg of cement and 0.0122 m³ (0.43 cft) of sand. A 1:6 mix needs about 3.14 kg of cement and 0.0131 m³ (0.46 cft) of sand.
Per 100 sq ft of wall, that is about 0.82 bag of cement for 1:4 and 0.58 bag for 1:6. The working below uses a dry volume factor of 1.27 and cement at 1440 kg/m³.
Cement and sand for 12 mm plaster: step by step
- Wet volume = area × thickness = 1 × 0.012 = 0.012 m³.
- Dry volume = 0.012 × 1.27 = 0.01524 m³. The factor allows for the loss in volume when water is added to dry cement and sand, since fine particles settle into the gaps.
- 1:4 mix (sum 5): cement = 0.01524 ÷ 5 = 0.003048 m³ × 1440 = 4.39 kg. Sand = 0.01524 × 4 ÷ 5 = 0.0122 m³.
- 1:6 mix (sum 7): cement = 0.01524 ÷ 7 = 0.002177 m³ × 1440 = 3.14 kg. Sand = 0.01524 × 6 ÷ 7 = 0.0131 m³.
Quantities per m², per 100 sq ft and per 100 m²
| Area (12 mm plaster) | 1:4 cement | 1:4 sand | 1:6 cement | 1:6 sand |
|---|---|---|---|---|
| 1 m² | 4.39 kg (0.088 bag) | 0.0122 m³ / 0.43 cft | 3.14 kg (0.063 bag) | 0.0131 m³ / 0.46 cft |
| 100 sq ft (9.29 m²) | 40.8 kg (0.82 bag) | 0.113 m³ / 4.00 cft | 29.1 kg (0.58 bag) | 0.121 m³ / 4.29 cft |
| 100 m² | 438.9 kg (8.78 bags) | 1.22 m³ / 43.1 cft | 313.5 kg (6.27 bags) | 1.31 m³ / 46.1 cft |
1:4 is common for ceilings, outer walls and wet areas. 1:6 is common for inside brick walls. Your drawings or specification decide the mix.
The 1.3 plus wastage approach
Many site estimators use a dry factor of about 1.3 and then add around 5% for wastage. That gives 1.3 × 1.05 = 1.365, which is about 7.5% more material than the 1.27 figure. For 100 m² of 12 mm plaster this means about 9.43 bags of cement in 1:4 and 6.74 bags in 1:6.
Some textbooks go further: they first add 20% to the wet volume for filling joints and uneven surfaces, then apply 1.27. That is 1.2 × 1.27 = 1.524 overall, or about 10.53 bags in 1:4 per 100 m². Each method is in use, so pick one and stay consistent across the estimate.
Worked example
Inside walls of a room have 400 sq ft of plaster area after deducting doors and windows, in 1:6. Cement = 4 × 0.58 = about 2.33 bags. Sand = 4 × 4.29 = about 17.1 cft.
The plaster calculator does this for any area, thickness and mix. For painting the same walls, see the paint calculator.
Frequently asked questions
What about 20 mm outside plaster?
Scale by thickness. 20 mm in 1:6 needs 20 ÷ 12 times the 12 mm amount, which is about 0.97 bag of cement and 7.14 cft of sand per 100 sq ft.
Why is sand more in 1:6 than in 1:4?
The dry volume is the same, so a leaner mix has more of that volume as sand and less as cement.
Does this include wastage?
No. The 1.27 factor only converts wet volume to dry volume. Add wastage, often around 5%, or use the 1.3 plus wastage method above. Rough brickwork and uneven walls need more.
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