Ceiling Plaster Calculator — How Many Bags for Your Ceiling?

Our ceiling plaster calculator uses the manufacturer-published coverage rate for your chosen plaster type at the standard application thickness, then scales linearly if you set a different thickness. Overhead application losses are captured in the higher default wastage allowance (15% for ceilings vs 10% for walls) rather than a reduced coverage rate.

The formula

Coverage at your thickness = standard coverage × (standard thickness ÷ your thickness)

Bags needed = ceil(area ÷ coverage × wastage factor)

Single Math.ceil at the end — not two separate rounding steps — so the wastage allowance is applied to the exact fractional bag count, then rounded up once to the nearest whole bag. This is industry convention (Wickes, B&Q, NHBC).

For example, a 14 m² ceiling using Multi-Finish at 2mm: 14 ÷ 10 × 1.15 = 1.61 → round up to 2 bags total (15% wastage already included).

Coverage rates (British Gypsum Thistle datasheets)

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

Coverage scales linearly with thickness because each bag yields a fixed volume of mortar. Setting the Application Thickness in the calculator adjusts the coverage automatically. The 15% wastage default for ceilings accounts for the overhead application losses that British Gypsum's published rates do not include.