How Many Bags of Plaster for a Room?

How We Calculated This

For a standard UK room measuring 3.5m x 4m with 2.4m high ceilings, the total wall area is calculated as follows:

• Two walls at 3.5m wide: 2 x 3.5m x 2.4m = 16.8m²
• Two walls at 4m wide: 2 x 4m x 2.4m = 19.2m²
• Total wall area: 36m²

Coverage Rate

A standard 25kg bag of Thistle Multi-Finish plaster covers approximately 1.8m² at a 2mm skim coat thickness. This is the most common finishing plaster used in UK domestic work.

• Bags needed (no wastage): 36 ÷ 1.8 = 20 bags
• With 10% wastage: 20 x 1.10 = 22 bags

What About Doors and Windows?

This calculation uses the gross wall area without deducting for doors or windows. Most plasterers work on the gross area because the extra material accounts for:

• Thicker application in hollows and uneven patches
• Plaster around reveals (window and door edges)
• Material lost during mixing and application
• Finishing waste in the bucket

Different Room Sizes

As a rough guide for multi-finish plaster at 2mm thickness:

• Small bedroom (3m x 3m): ~24m² walls = ~15 bags
• Standard room (3.5m x 4m): ~36m² walls = ~22 bags
• Large room (5m x 4m): ~43.2m² walls = ~27 bags

Tips for Getting It Right

Always buy at least one extra bag beyond your calculation — running out mid-wall means a visible join line. Check the “use by” date on every bag, as out-of-date plaster sets too fast and won’t trowel up properly. Store bags off the ground on a pallet and keep them dry.

If you are skimming over old plaster or plasterboard, apply a coat of PVA (diluted 1:4 with water) first to control suction and give yourself more working time.