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. Browning (base coat for brick) covers a similar area.

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. The calculator shows bags of plaster, litres of PVA (for sealing surfaces before plastering) 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.

How much plaster do you need? Wall area = length × height. Coverage rates: skim coat (1-2mm finishing layer) — 1 bag (25kg) Multi-Finish covers 10-12 m². Base coat (Hardwall or Bonding, 8-11mm thick) — 1 bag covers 4-6 m². Sample 4m × 2.4m wall = 9.6 m² requires 1 bag skim + 2 bags Hardwall = £35-£50 materials. Plasterer day rate UK 2026: £180-£280/day; covers ~20-40 m² per day on existing walls. New plasterboard install + skim: skilled plasterer 30-50 m²/day.

UK plaster types and uses. Bonding plaster (e.g. Thistle Bonding 60): for low-suction surfaces (engineering brick, concrete, painted walls). 11mm thick. Hardwall: standard backing plaster for brick and block walls. 11mm thick. Browning: traditional, declining use. Multi-Finish (top coat): 2mm finishing skim. One Coat (Easi-Finish): combined base + top in one application — DIY-friendly. Plasterboard adhesive (dot-and-dab): bonding boards to walls. Lime plaster: for pre-1920s solid-wall properties needing breathability.

Skimming over existing walls. Old wallpaper: strip completely first (steam stripper £15/day hire). Glossy/painted walls: PVA dilute 1:4 with water and apply 1 coat to seal. Sand smooth before plastering. Cracks and holes: fill with bonding/filler first. Time: 1-2 days drying between PVA and plaster; 7+ days drying before painting (plaster green colour means still wet). DIY skim coat possible but technique-dependent: smooth flat finish takes practice — beginner finish often visible after painting. Professional skim: £40-£80/m² existing wall.

Plasterboard and dot-and-dab. Standard plasterboard (1200×2400mm, 12.5mm thick): £8-£15/sheet. Fire-rated (red core): £15-£25/sheet. Moisture-resistant (green core, bathrooms): £15-£25/sheet. Coverage: 2.88 m²/sheet. Sample bathroom 30 m² walls = 11 sheets + 10% waste = 13 sheets. Dot-and-dab adhesive: £8-£15/bag covers 5 m². Edging beads, scrim tape, screws: £20-£40 for typical room. Plasterboard cuts: stanley knife scoring, snap, then cut paper back. Then skim coat over plasterboard joints/tapered edges.

DIY vs professional plastering. DIY rendering small patches/repairs: achievable with practice (£50-£100 materials for typical patch). Full room skim DIY: difficult to achieve smooth flat finish — visible defects when painted. Professional plasterer for: large areas, ceilings, ornate cornicing, perfect finish requirements. Cost compare: DIY 4m × 4m room skim £80-£120 materials + 1-2 days own labour; professional £400-£800 inclusive. Worth paying for: avoiding redo costs, getting it right first time, complex jobs.