Plaster Calculator

How plaster coverage is worked out

A 25kg bag of multi-finish plaster covers roughly 10 m² at 2mm thickness (a typical skim coat). Multiply your wall area by the thickness and divide to get the bag count. Always allow for two coats and some waste.

Worked example: skim-coating a 4 m × 3.5 m room

With 2.4 m ceilings the wall area is about 36 m². Take off 2 m² for a door and window and you are skimming 34 m². A 25 kg bag of multi-finish covers roughly 10–11 m² at 2–3 mm, so that is 3–4 bags. Buy four: running out mid-wall means a cold joint that shows through paint.

Backing coats versus finish

Bare brick or block needs a backing coat first — bonding, browning or hardwall, around 11 mm — then a 2–3 mm skim of multi-finish. Plasterboard only needs the skim. If you are skimming over sound old plaster, two thin coats of multi-finish are enough, but the surface must be sealed with PVA or a bonding agent first or the new plaster will craze as it dries.

The extras that decide the finish

Angle beads on every external corner, scrim tape over board joints and any crack, and a damp check before you start — plaster over a damp wall will blow within a year. Mix in small batches: multi-finish sets in 20–30 minutes, so a 25 kg bag is about the limit of what one person can lay in one go.

Drying and painting

Fresh plaster takes 4–6 days per 25 mm of thickness to dry out. Before painting, apply a mist coat — emulsion thinned about 20% with water — and let it dry. A full-strength coat straight onto new plaster will peel. Give it a week of ventilation in winter rather than forcing it with heaters, which causes hairline cracks.

Common mistakes

Skimming over Artex on a pre-2000 ceiling without testing for asbestos. Laying one thick coat instead of two thin ones. And skimming walls that are still damp or that were never sealed, which leads to the finish flaking months later.