How many bags of render or plaster you need for walls or ceilings. Single and two-coat systems, standard AU 20 kg bags, with waste margin included.
Render and plaster products in Australia are sold in 20 kg bags. Coverage depends on the product type and how thick you apply each coat — thicker coats use more material per square metre.
The calculator uses approximate coverage rates per 20 kg bag based on typical product specifications from brands like Boral, Rockcote, Dulux AcraTex and BGC. Always check the coverage on your specific product's data sheet, as rates vary slightly between brands.
| Application | Thickness | Coverage per bag |
|---|---|---|
| Fine finish plaster | 3 mm | ~5.5 m² |
| Medium finish coat | 5 mm | ~3.5 m² |
| Standard plaster / render | 8 mm | ~2.5 m² |
| Heavy base coat / render | 13 mm | ~1.4 m² |
A single coat is used where the surface is already even — such as a skim coat over plasterboard, or a textured finish coat over a prepared base.
A two-coat system is standard for masonry walls (brick, block, hebel) where the base coat builds up the surface and fills irregularities, and the finish coat provides the final smooth or textured surface. The base coat is applied thicker; the finish coat is thin.
Plaster is used internally on walls and ceilings. It's typically a gypsum-based product, sets hard, and is designed for painting.
Internal render is a cement-based product used on masonry internal walls before plastering or tiling. It's harder and more moisture-resistant than gypsum plaster.
External render is cement or acrylic-based, designed to handle weather exposure. It requires a primer coat and is usually finished with a texture coat or paint system.
Each internal or external corner adds material where the beads and angles are set. A 15% waste margin covers projects with lots of window reveals, door openings, and corners. For simple flat walls with few openings, 10% is sufficient.
A standard 20 kg bag of cement render covers approximately 1–1.5 m² at 10 mm thickness. At 15 mm thickness, one bag covers about 0.7–1 m². The exact coverage varies by brand and mix — always check the bag for the manufacturer's stated yield and use that figure to confirm the calculator's result.
Render is applied to external masonry surfaces (brick, block, hebel) and is designed to handle weather exposure. Plaster (or internal set) is applied to interior walls and ceilings — typically over plasterboard joints or masonry. Render is harder and coarser; plaster is finer and smoother for interior finish work.
Rough or highly absorbent masonry surfaces usually need a scratch coat (first coat, 10–12 mm) followed by a finish coat (5–8 mm) for a total of two coats. Smooth block or hebel surfaces may only need one coat if the finish render is applied carefully. The calculator lets you select single or two-coat systems.
In Queensland's warm, humid climate, cement render typically takes 24–48 hours to firm up enough for a second coat or painting. Full cure takes 7–28 days depending on thickness and conditions. Avoid rendering in direct sun or on hot days — rapid drying causes shrinkage cracks. Keep freshly rendered surfaces damp for the first 48 hours.
Yes, but allow full cure first — typically 28 days for cement-based render. Apply a sealer or render primer before painting to even out porosity and improve paint adhesion. In Queensland's climate, use an exterior paint with high water resistance and flexibility to handle temperature movement in rendered surfaces.