Plaster Calculator
Enter the wall area or room size, the plaster thickness and the cement-sand ratio. You get the number of cement bags and the tons of plaster sand to order, with an allowance for uneven walls and waste.
Everything is calculated in your browser. Nothing you type is sent or stored.
Dry mortar = area × thickness × 1.27 + 10%. Split it by the ratio: cement = dry ÷ (1 + sand parts) × 1,440 kg/m³. 500 sq ft (46.5 m²) of 12 mm plaster at 1:4 needs 0.78 m³ of dry mortar, which is 224 kg of cement and 0.62 m³ of sand, so buy 5 bags of 50 kg and about 1 ton of sand.
How to use this plaster calculator
- Enter the area to plaster, with doors and windows taken out. Plastering both faces of a wall counts twice.
- Or switch to Room and enter the room size, the total area of openings and whether the ceiling is included.
- Choose the thickness. 12 mm for most internal walls, 20 mm for external walls in two coats.
- Choose the mix ratio and bag size. 1:4 or 1:5 is typical for walls.
- Read the order: cement bags, cost and sand in volume and tons.
How to calculate cement and sand for plaster
cement (kg) = dry volume ÷ (1 + sand parts) × 1,440
sand (m³) = dry volume × sand parts ÷ (1 + sand parts)
The 1.27 factor turns finished plaster into loose dry material. The extra 10% covers uneven walls, filling of joints and droppings. Raise it on rough or out-of-plumb walls.
| Ratio | 12 mm: cement per 10 m² | 12 mm: sand per 10 m² | 20 mm: cement per 10 m² | 20 mm: sand per 10 m² |
|---|---|---|---|---|
| 1 : 3 | 60 kg (1.2 bags) | 0.13 m³ | 101 kg (2.0 bags) | 0.21 m³ |
| 1 : 4 | 48 kg (1.0 bag) | 0.13 m³ | 81 kg (1.6 bags) | 0.22 m³ |
| 1 : 5 | 40 kg (0.8 bag) | 0.14 m³ | 67 kg (1.3 bags) | 0.23 m³ |
| 1 : 6 | 34 kg (0.7 bag) | 0.14 m³ | 58 kg (1.2 bags) | 0.24 m³ |
Bags are 50 kg. Figures include the 1.27 dry factor and 10% waste. For 100 sq ft of 12 mm plaster at 1:4, allow about 99 lb of cement and 4.4 ft³ of sand.
Typical plaster thickness and ratio
| Surface | Typical thickness | Typical ratio |
|---|---|---|
| Internal walls, smooth brick or block | 12 mm (½ in) | 1:5 or 1:6 |
| Internal walls, rough brickwork | 15 mm (⅝ in) | 1:5 or 1:6 |
| External walls (two coats) | 20 mm (¾ in) | 1:4 |
| Ceilings | 6 mm (¼ in) | 1:3 or 1:4 |
These are common practice, not rules. Follow your specification. For exterior portland cement stucco over lath, use the stucco calculator.
Worked example
- 500 sq ft of wall, 12 mm thick
- 46.45 m² × 0.012 = 0.557 m³ wet
- Dry volume (× 1.27, + 10%)
- 0.779 m³
- Cement at 1:4 (1 part of 5)
- 0.156 m³ × 1,440 = 224 kg
- Cement bags
- 5 × 50 kg (≈ $60 at $12) or 6 × 94 lb
- Sand (4 parts of 5)
- 0.62 m³ (0.81 yd³) ≈ 1 ton
Frequently asked questions
How do I calculate cement and sand for plastering?
Area (m²) × thickness (m) for wet volume, × 1.27 for dry, + about 10%. Divide by the sum of the ratio. The cement part × 1,440 kg/m³ is the cement weight and the sand part is the sand volume.
How many bags of cement for 100 sq ft of plaster?
For 12 mm at 1:4 with 10% waste, about 99 lb of cement (roughly one 94 lb bag) and 4.4 ft³ of sand. At 1:6, about 71 lb.
How much area does one bag of cement plaster?
A 50 kg bag covers about 10 m² (111 sq ft) of 12 mm plaster at 1:4, or about 14.5 m² at 1:6, including 10% waste.
What thickness should wall plaster be?
Typically 12 mm inside, 15 mm on rough brickwork, 20 mm outside in two coats and 6 mm on ceilings.
Which cement sand ratio is best for plaster?
1:4 for external walls, 1:5 or 1:6 for internal walls and 1:3 for ceilings. Richer mixes are stronger but crack more easily.
Why is the dry volume factor 1.27 for plaster?
Loose cement and sand lose volume when mixed with water and pressed on. Some references use 1.33, which you can enter in the calculator.
What this calculator assumes
- Cement-sand plaster by volume, a dry volume factor of 1.27 (editable) and cement at 1,440 kg/m³.
- Sand weight uses a typical loose dry density of 1,600 kg/m³. Damp sand bulks up, so confirm with your supplier.
- The waste allowance covers uneven surfaces, joint filling and droppings. Use more on rough walls.
- Prices are editable estimates before tax and delivery. Lime, gypsum or ready-mixed plasters have their own coverage, so follow the bag label.