How plaster quantities are calculated
Wet volume = area × thickness × (1 + wastage)
Dry volume = wet volume × 1.33
Cement = dry volume ÷ (1 + sand parts) × 1,440 kg/m³
Sand = dry volume × sand parts ÷ (1 + sand parts)
Dry volume = wet volume × 1.33
Cement = dry volume ÷ (1 + sand parts) × 1,440 kg/m³
Sand = dry volume × sand parts ÷ (1 + sand parts)
The factor of 1.33 makes up for the air voids in loose cement and sand, which disappear once water is added and the mortar is compacted onto the wall.
Worked example
140 m² of 15 mm plaster in 1:4 with 5% wastage: 140 × 0.015 × 1.05 = 2.205 m³ wet. Dry volume is 2.93 m³. Cement is 2.93 ÷ 5 × 1,440 = 845 kg, or 17 bags. Sand is 2.35 m³.
Typical thickness and mix
| Work | Thickness | Mix |
|---|---|---|
| Internal walls, smooth finish | 12–15 mm | 1:4 to 1:6 |
| External render | 18–20 mm | 1:4 |
| Two-coat external render | 20–25 mm | 1:3 base, 1:4 finish |
| Ceiling | 6–10 mm | 1:3 or 1:4 |
Frequently asked questions
How many bags of cement per m² of plaster?
For 15 mm in 1:4, about 0.12 bags (6 kg) per m². For 12 mm in 1:6, about 0.07 bags per m².
Why is external plaster thicker?
External render protects the wall from weather and sun, so it is laid thicker and richer, often in two coats.