How to use the plaster calculator
Plaster and stucco are sold by the bag, but the amount you need depends on area and thickness, and every coat in a system has a different thickness and yield. This calculator works coat by coat, so a skim over plasterboard, a two-coat gypsum system on blockwork and a three-coat exterior stucco are all estimated the same way.
- Choose units and a coat system. Each preset fills typical thicknesses and yields for up to three coats. Pick Custom to type your own.
- Enter the area of walls and ceilings to be plastered, and deduct windows and doors.
- Check the thickness and yield of each coat. Yield is the dry bag weight used per m² per mm of thickness (or per ft² per inch). It comes straight from the coverage line on the data sheet: bag weight ÷ coverage ÷ thickness.
- Set the bag size and waste. For a site-mixed render, choose Site-mixed and enter the sand ratio to see cement bags and sand tonnage instead.
Plaster quantity formula
Plaster per coat (kg) = net area × thickness × yield × (1 + waste)
Yield (kg/m²/mm) = bag weight ÷ (coverage per bag × thickness)
Bags = ⌈plaster ÷ bag size⌉ · wet volume = area × total thickness
Site mix: dry volume ≈ wet volume × 1.33 · cement = dry ÷ (1 + r) × 1,440 kg/m³ · sand = dry × r ÷ (1 + r) × 1,600 kg/m³
The preset yields below are worked back from published coverage figures or are labelled estimates. Products differ, so overwrite them with the figure from your own data sheet.
| Coat | Typical thickness | Yield used (estimate) | Basis |
|---|---|---|---|
| Gypsum finish skim | 2 mm | 1.25 kg/m²/mm | 10 m² per 25 kg at 2 mm (Thistle MultiFinish data sheet) |
| Lightweight gypsum undercoat | 11 mm | 0.76 kg/m²/mm | about 3 m² per 25 kg at 11 mm |
| Bonding undercoat | 8–11 mm | 0.83 kg/m²/mm | about 2.75 m² per 25 kg at 11 mm |
| One-coat gypsum | 13 mm | 0.85 kg/m²/mm | about 2.25 m² per 25 kg at 13 mm |
| Cement stucco base coat | ⅜ in each | 1.87 kg/m²/mm (9.7 lb/ft²/in) | 20–24 ft² per 80 lb at ⅜ in (Quikrete data sheet) |
| Stucco finish coat, render | ⅛ in / 7–10 mm | 1.8 kg/m²/mm | estimate for cement-based mixes |
Worked examples
Skimming a plasterboard room
A bedroom has 40 m² of plasterboard walls and ceiling. A 2 mm skim at 1.25 kg/m²/mm uses 2.5 kg per m², so 100 kg in total, or 110 kg with 10% waste. That is ⌈110 ÷ 25⌉ = 5 bags of finish plaster.
Two-coat plaster on blockwork
The same 40 m² on new blockwork takes an 11 mm undercoat at 0.76 kg/m²/mm, which is 8.36 kg per m², 334 kg net and 368 kg with waste: 15 bags of undercoat. The 2 mm skim on top adds the same 5 bags of finish as the first example.
Three-coat stucco on a garage wall
A 400 ft² wall gets a ⅜ in scratch coat, a ⅜ in brown coat and a ⅛ in finish. Each base coat uses 0.375 × 9.7 = 3.64 lb per ft², so 1,456 lb net, or 1,602 lb with 10% waste: 21 bags of 80 lb for each coat, 42 for the two base coats. The finish coat at about 9.4 lb/ft²/in needs 0.125 × 9.4 × 400 × 1.1 = 517 lb, or 7 bags.
Who this calculator helps
- Architects and specifiers checking plaster and render build-ups and quantities in a specification.
- Plasterers and renderers pricing a job and ordering bags and bulk sand.
- Builders and contractors comparing plaster with drylining or exterior stucco with cladding.
- Self-builders and renovators working out how much material a room or extension needs.
Practical tips
- Get the plasterboard and joint quantities from the drywall calculator first, then skim the same area here.
- Use the square footage calculator to add up wall and ceiling areas, and the paint calculator for the mist coat and finish.
- Comparing stucco with other facades? The siding calculator gives cladding quantities for the same elevation.
- Moist-cure cement base coats as the product requires, and do not apply gypsum plaster to exterior or persistently damp walls.
- Old, uneven walls can need far more undercoat than the nominal thickness; measure the worst spots with a straightedge.
Not sure what finish or colour the facade should have? Try stucco, render and paint options on a photo with AI Exterior Redesign.
