Plaster Calculator
Estimate cement bags and sand for wall or ceiling plaster.
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About the Plaster Calculator
Plastering covers brick walls with a smooth layer of cement and sand. Enter the wall size, the thickness and the mix to estimate how many bags of cement and how much sand to buy.
Common practice: 12 mm in a 1 : 6 mix for inside walls, 15 mm for ceilings, and 20 mm in a 1 : 4 mix for outside walls.
How to use the Plaster Calculator
- Choose feet or metres and enter the wall length and height.
- Enter the doors and windows area to deduct, and whether both sides are plastered.
- Choose the thickness and mix, and the extra allowance.
- Click Calculate plaster.
Formula
Wet volume = plaster area × thickness.
Dry volume = wet volume × (1 + extra for uneven surface) × 1.27 (dry materials shrink when mixed with water).
Cement = dry volume ÷ (1 + sand parts); one 50 kg bag ≈ 0.0347 m³. Sand = dry volume × sand parts ÷ (1 + sand parts).
Examples
- A wall 20 × 10 ft (200 sq ft), 12 mm, 1 : 4, 20% extra → about 2 bags of cement and 9.6 cft of sand.
Frequently asked questions
How accurate is this estimate?
It is a planning estimate based on standard formulas. Actual quantities depend on workmanship, materials and site conditions, so confirm with your contractor before buying.
Why add 20% extra?
Brick walls are not perfectly flat, joints need filling and some mortar falls during work. 15–25% extra is common.