How to use this calculator
- Enter or choose units.
- Enter or choose wall length.
- Enter or choose wall height.
- Enter or choose doors and windows (total area).
- Enter or choose plaster thickness.
- The cement bags (rounded up) updates as you type. Use Copy result or Share to keep it.
Method
Area = L × H − Openings; Wet = Area × t; Dry = Wet × (1 + Waste) × Dry factor; Cement = Dry × c ÷ (c + s) × density; Sand = Dry × s ÷ (c + s)
- t
- plaster thickness
- c : s
- cement-to-sand ratio
Worked example
- Area = 5 × 3 − 3 = 12 m²; wet volume = 12 × 0.012 = 0.144 m³.
- Dry volume = 0.144 × 1.10 × 1.27 = 0.2012 m³.
- Cement = 0.2012 × 1/5 × 1440 = 57.9 kg → 2 bags of 50 kg; sand = 0.2012 × 4/5 = 0.161 m³.
Plastering 12 m² at 12 mm in 1:4 takes about 58 kg of cement (2 bags) and 0.16 m³ of sand.
What the result means
Plaster both faces of a wall by doubling the area. Rough brickwork or block takes more than smooth concrete, so raise the waste allowance for poor backgrounds.
Assumptions and limits
- Uniform thickness over the whole area.
- Cement-sand plaster; gypsum and lime plasters are batched differently.
- Reveals, beads and ceiling work are not itemised.
Questions
Why add a waste allowance as well as the dry factor?
The dry factor converts wet to dry volume; the allowance covers material that fills hollows in the wall and drops to the floor.
How thick should plaster be?
Commonly 12 mm on internal brick walls and 15–20 mm in two coats outside, but follow your specification.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.