How plaster quantity is calculated
Plaster quantity starts with the area to be covered, multiplied by the plaster thickness to give the wet volume. Because dry cement and sand bulk up before water is added, the calculator multiplies the wet volume by 1.33, adds wastage, and splits the result by the cement:sand ratio. Cement is then converted to 50 kg bags.
You enter the wall length and height, the number of identical walls, the thickness in millimetres (1 to 50 mm, default 12), the mix ratio (1:4, 1:5 or 1:6), and wastage (default 5%).
The formulas
- Plaster area = Wall length × Wall height × Number of walls
- Wet volume = Plaster area × Thickness
- Dry volume = Wet volume × 1.33, then × (1 + wastage %)
- Cement volume = Dry volume × 1 ÷ (1 + sand parts); sand is the rest
- Cement bags = Cement volume × 1,440 kg/m³ ÷ 50 kg
The 1,440 kg/m³ figure is the bulk density of cement the calculator assumes, and bags are 50 kg.
Worked example: a whole room
A room with four walls, each 5 m long and 3 m high, plastered 12 mm thick in 1:4 with 5% wastage:
- Area: 5 × 3 × 4 = 60.00 m².
- Wet volume: 60 × 0.012 = 0.720 m³.
- Dry volume: 0.72 × 1.33 = 0.9576 m³; with wastage, × 1.05 = 1.0055 m³.
- Cement: 1.0055 ÷ 5 = 0.2011 m³, × 1,440 ÷ 50 = 5.79 bags.
- Sand: 1.0055 × 4 ÷ 5 = 0.804 m³.
A single 5 m × 3 m wall in 1:6 at the same thickness needs 1.03 bags of cement and 0.215 m³ of sand.
Cement and sand per 100 m²
The calculator's results for 100 m² with wastage set to 0:
| Thickness | Mix | Cement (50 kg bags) | Sand (m³) |
|---|---|---|---|
| 12 mm | 1:4 | 9.19 | 1.277 |
| 12 mm | 1:5 | 7.66 | 1.330 |
| 12 mm | 1:6 | 6.57 | 1.368 |
| 20 mm | 1:4 | 15.32 | 2.128 |
Doubling thickness from 10 mm to 20 mm doubles the materials, so the thickness you actually achieve on site matters as much as the mix.
Choosing thickness and mix
IS 1661:1972, the Indian code of practice for cement and cement-lime plaster finishes, sets out typical values in its Table 2 and clause 7.3:
| Plaster type | Thickness per IS 1661 |
|---|---|
| Single coat | 10 to 15 mm |
| Two coat (total) | Generally about, and not normally over, 20 mm |
| Two coat on in-situ concrete soffits | Not over 15 mm |
| Three coat (total) | Not normally over 25 mm |
Its single-coat specifications include cement:sand mixes of 1:3, 1:4 and 1:6 by volume (written there as cement:lime:sand, such as 1:0:4). Your project specification decides which applies.
How to use it
- Measure each wall. Group walls of the same size and use the number-of-walls field.
- Count both faces of a wall if both will be plastered.
- For a ceiling, enter its length and width as the wall length and height.
- Subtract large openings by running them as a separate "wall" and taking the result away.
- Set thickness from the specification, plus a little for uneven backgrounds, and set wastage.
The room calculator gives wall and ceiling areas in one step. The brick calculator and block calculator estimate the wall itself, and the paint calculator covers the finish. To turn sand volume into kilograms, use the construction materials converter.
Results are estimates for ordering. Actual consumption depends on the wall surface, sand grading and workmanship, so check against your contractor's figures or a site trial.
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