Short answer: a 25 kg bag of finishing (skim) plaster covers about 10 m² at the usual 2 mm, so a skim coat takes about 2.5 kg per m². Undercoat (backing) plaster is laid about 11 mm thick and covers only about 2.7–3 m² per 25 kg bag. In the US, a 50 lb bag of veneer plaster covers about 123 ft² at 1/16 in.
Plaster is weighed, not measured by area, so the thickness decides everything: double the thickness and you need double the plaster.
The rule: area × thickness × consumption
Every bag states its consumption, usually as kilos per m² for each millimetre of thickness, or as "covers X m² at Y mm". The plaster calculator uses:
- Finishing (skim) plaster: about 1.25 kg per m² per mm — British Gypsum's Multi-Finish figure is 10 m² per 25 kg at 2 mm.
- Undercoat / backing plaster: about 0.85 kg per m² per mm — a 25 kg bag of Thistle HardWall covers about 3 m² at 11 mm.
Then: plaster (kg) = area (m²) × thickness (mm) × kg per m² per mm, plus about 10 % for what is left in the bucket and dropped on the floor.
How far does one 25 kg bag go?
Before the 10 % extra:
| Plaster and thickness | Plaster per m² | One 25 kg bag covers |
|---|---|---|
| Skim, 1 mm (one thin coat) | 1.25 kg | 20 m² |
| Skim, 2 mm (two coats, the usual) | 2.5 kg | 10 m² |
| Skim, 3 mm | 3.75 kg | 6.7 m² |
| Undercoat, 8 mm | 6.8 kg | 3.7 m² |
| Undercoat, 10 mm | 8.5 kg | 2.9 m² |
| Undercoat, 11 mm (the usual) | 9.35 kg | 2.7 m² |
| Undercoat, 15 mm | 12.75 kg | 2 m² |
In the US, veneer plaster comes in 50 lb bags: about 123 ft² per bag at 1/16 in, 82 ft² at 3/32 in and 62 ft² at 1/8 in.
How many bags of plaster per board?
A standard plasterboard of 2.4 × 1.2 m is 2.88 m², so one 25 kg bag of skim at 2 mm covers about 3½ boards. A 4 × 8 ft drywall sheet is 32 ft², so one 50 lb bag of veneer plaster at 1/16 in covers about 3¾ sheets. The drywall calculator works out the boards themselves.
A whole room, worked out
A 3 × 4 m room with walls 2.5 m high has 35 m² of wall; minus a door and a window that leaves about 31.7 m².
- Skim at 2 mm: 31.7 × 2.5 kg = 79 kg, plus 10 % = 87 kg → 4 bags of 25 kg.
- Undercoat at 11 mm: 31.7 × 9.35 kg = 296 kg, plus 10 % = 326 kg → 14 bags of 25 kg.
Add the ceiling (12 m² in this room) if you plaster it too.
Cement plaster and render
In Pakistan, India and the Gulf, walls are often plastered with cement and sand instead of gypsum. For a 12 mm coat, public works figures give about 0.109 bags of 50 kg cement per m² for a 1 : 4 mix and 0.072 bags for 1 : 6. The plaster calculator has a cement–sand mode that adds the sand too.
Frequently asked questions
How many m² does a 25 kg bag of plaster cover?
Finishing (skim) plaster covers about 10 m² at 2 mm. Undercoat plaster covers about 2.7–3 m² at 11 mm. Check your bag: products differ.
How many bags of plaster do I need to skim a room?
For a 3 × 4 m room with 2.5 m walls, about 4 bags of 25 kg for the walls at 2 mm, including 10 % extra. Add one more if you skim the ceiling.
How many bags of plaster per plasterboard?
At 2 mm, one 25 kg bag of skim covers about 3½ boards of 2.4 × 1.2 m. In the US, one 50 lb bag of veneer plaster at 1/16 in covers about 3¾ sheets of 4 × 8 ft.
How thick should skim plaster be?
About 2 mm in total, usually in two thin coats. Undercoats are thicker, about 10–11 mm.
Why does the calculator add 10 %?
For plaster left in the bucket, dropped on the floor and soaked up by absorbent walls. Add more for very uneven or very dry walls.
Calculators in this guide
Enter your own sizes for an exact answer.
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