Skim Coat Calculator — How Much Multi-Finish Do I Need?

Our skim coat calculator divides your total wall or ceiling area by the coverage rate of multi-finish plaster based on your surface type. Coverage rates are based on British Gypsum Thistle Multi-Finish guidelines at the standard 2mm application thickness.

The formula

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

The wastage margin is applied before rounding, then the result is rounded up to the next whole 25kg bag. For example, skimming 36 m² of plasterboard at 10 m²/bag with a 10% margin: 36 ÷ 10 = 3.6 bags, × 1.10 = 3.96 bags → round up to 4 bags of 25kg multi-finish. (Rounding up only once avoids over-ordering a spare bag you do not need.)

Coverage rates by surface

• Plasterboard: 10 m² per 25kg bag at 2mm, the British Gypsum PDS (DS-110-01) figure for a smooth, consistent board face
• Backing coat: 10 m² per 25kg bag at 2mm, the same PDS rate over a flat bonding or hardwall undercoat
• Old plaster / patched walls: allow ~8 m² per 25kg bag, a practical site allowance (not a manufacturer figure) for uneven, absorbent surfaces that drink more plaster

The 10 m²/bag figures are taken at the 2mm skim thickness given in the British Gypsum Thistle Multi-Finish Product Data Sheet. The wastage allowance is a trade rule of thumb, not a manufacturer specification. Around 10% covers mixing losses, dropped plaster and filling minor imperfections; if you are new to skimming or working patchy, cut-up walls it is worth allowing 15-20%.