Plaster Calculator
Cement bags and sand needed for wall or ceiling plaster at 12, 15 or 20 mm thickness.
About Plaster Calculator
This plaster calculator estimates the cement bags and sand needed to plaster a wall or ceiling. Enter the plaster area in square feet or square metres, pick the thickness (12 mm for inside walls, 15 or 20 mm for outside or rough surfaces) and choose the cement to sand ratio.
It applies the dry volume factor and wastage for you, so you get a practical figure to order. It is useful for homeowners, masons and contractors planning cement plaster work, and for students learning how to calculate plaster quantity. The calculator is free, needs no sign-up and runs in your browser.
How to use Plaster Calculator
- Enter the plaster area in sq ft or m².
- Pick the thickness — 12 mm inside, 15/20 mm outside or on rough walls.
- Choose the cement:sand ratio.
- Read cement bags and sand quantity.
Frequently asked questions
What thickness should I use?
12 mm for internal walls on smooth brickwork, 15 mm for rough internal walls, and 20 mm (often in two coats) for external walls. Ceilings are usually 6–10 mm.
What ratio is used for plaster?
1:4 for external and ceiling plaster, 1:5 or 1:6 for internal walls are common practice.
Why add wastage on top of the 1.27 factor?
1.27 converts wet mortar to dry volume. Extra 10–20% covers uneven brick surfaces, filling joints and site spillage.
How do I calculate plaster quantity?
Multiply area by thickness for wet volume, multiply by 1.27 for dry mortar, add wastage and split by the mix ratio. The calculator shows cement bags and sand directly.
How much cement is needed for 100 sq ft of plaster?
At 12 mm thickness in 1:6 ratio, about 1 bag of cement; thicker plaster or richer mixes need more.
Which is better for plaster, 1:4 or 1:6?
1:4 is used for outside walls and ceilings where strength matters; 1:6 is common for inside walls.
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