Plaster Coverage Calculator — How Many Bags Do I Need?

Our plaster coverage calculator works by dividing your total wall area by the coverage rate of your chosen plaster type. Coverage rates are based on manufacturer guidelines from British Gypsum (Thistle range) and Knauf, using standard UK 25kg bags.

The formula

Bags needed = (Total area ÷ Coverage per bag) × Wastage factor

For example, if you have 36 m² of wall area and you're using multi-finish plaster (10 m² per bag at 2mm skim), you need: 36 ÷ 10 = 3.6 bags, plus 10% wastage = 4 bags.

Coverage rates we use (British Gypsum Thistle datasheets)

• Multi-Finish Skim: 10 m² per 25kg bag at 2mm thickness
• Bonding Coat: 3.5 m² per 25kg bag at 8mm thickness (≈2.75 m² at 11mm)
• Hardwall: 3 m² per 25kg bag at 11mm thickness
• Universal OneCoat: 2.25 m² per 25kg bag at 13mm thickness
• Browning: 3.5 m² per 25kg bag at 11mm thickness

For a fixed bag volume, coverage scales roughly inversely with thickness — applying it thicker covers proportionally less area. Set the Application Thickness in the calculator to match your job and it adjusts the reference coverage automatically. Bonding Coat is the one product whose datasheet publishes two coverage points (3.5 m² at 8mm and 2.75 m² at 11mm); for it the calculator uses the manufacturer's own two-point figures rather than scaling a single value, so the 11mm result matches the datasheet exactly.

These rates are sourced from British Gypsum's published Thistle range datasheets. Actual coverage may vary depending on the surface condition — rough blockwork will use more plaster than smooth plasterboard. The 10% wastage factor accounts for normal mixing and application losses.