Plaster Calculator — Bags & Coverage

Plaster coverage depends on the product type and application thickness. Multi-finish plaster (the most common top coat) covers approximately 10–11 m² per 25 kg bag at 2 mm thickness. Bonding coat (base coat for solid walls) covers about 3.5 m² per 25 kg bag at 8 mm thickness, and browning (base coat for brick) covers a similar area. The maths is simple: measure the wall area as length × height, take off doors and windows, and divide by the coverage per bag for each coat. A room with 36 m² of net wall area needing only a skim takes 36 ÷ 10.5 = 3.4 bags of multi-finish, which rounds up to 4 bags.

This calculator takes wall and ceiling areas, subtracts openings, and calculates bags needed based on the plaster type and coat thickness. For a full re-plaster on brick, you typically need a base coat (8–11 mm) plus a finish coat (2 mm). For skim-coating existing plasterboard, only the finish coat is needed (2–3 mm). Enter the area, plaster type and number of coats, and it shows bags of plaster, litres of PVA (for sealing surfaces before plastering, about 5 litres for the 36 m² room above) and approximate cost. One 25 kg bag of multi-finish costs around £8–12. For large areas, always mix from the same batch to avoid colour variations.

Each product has a job. Bonding plaster (for example Thistle Bonding 60) is the base coat for low-suction surfaces such as engineering brick, concrete and painted walls. Hardwall is the standard backing plaster for brick and block walls, applied at 11mm. Browning is the traditional base coat and is in declining use. Multi-Finish is the 2mm finishing skim that goes over any of them. One Coat (Easi-Finish) combines base and top coat in a single application and is the DIY-friendly option. Plasterboard adhesive is used for dot-and-dab fixing of boards to walls, and lime plaster is the right choice for pre-1920s solid-wall properties that need to breathe.

Skimming an existing wall starts with preparation. Old wallpaper has to come off completely, and a steam stripper hires for about £15 a day. Glossy or painted walls need sealing with PVA diluted 1:4 with water, applied in 1 coat and left to dry, and any cracks or holes are filled with bonding or filler and sanded smooth. Allow 1-2 days between the PVA and the plaster, and 7 days or more before painting, since plaster that still looks green is still wet. A DIY skim is possible but technique-dependent: a smooth, flat finish takes practice, and a beginner's finish is usually visible once painted. A professional skim over existing walls costs £40-£80/m².

Plasterboard has its own sums. A standard sheet of 1200×2400mm at 12.5mm thick costs £8-£15 and covers 2.88 m², while fire-rated (red core) and moisture-resistant (green core, for bathrooms) boards cost £15-£25 a sheet. A bathroom with 30 m² of wall needs 11 sheets, or 13 with 10% waste. Dot-and-dab adhesive costs £8-£15 a bag and covers 5 m², and edging beads, scrim tape and screws add £20-£40 for a typical room. Boards are cut by scoring with a Stanley knife, snapping, then cutting the paper backing, and the joints and tapered edges are skimmed over afterwards.

Whether to do it yourself depends on the size of the job. Small patches and repairs are achievable with practice for £50-£100 in materials. A full room is harder: skimming a 4m × 4m room yourself costs £80-£120 in materials plus 1-2 days of your own labour, against £400-£800 for a professional all in, and the difference buys a flat finish without visible defects. Plasterers charge £180-£280 a day and cover roughly 20-40 m² of existing wall in that time, or 30-50 m² when boarding and skimming new plasterboard. Ceilings, large areas, ornate cornicing and anything that must look perfect are worth paying for, if only to avoid the cost of doing it twice.