The job
Modelled, not recorded. Every figure here is the answer the Skim Coat Plaster Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The room is a living room whose walls are being given a fresh skim: 42 m² (452.1 sq ft) of wall once the windows, the door and the chimney breast's fireplace opening are taken off. Some of the walls are new plasterboard and some are the old plaster, sound but marked by years of filling, and both are being brought to one smooth finish.
The finished thickness is left at 2 mm, the thickness the calculator's source gives for a gypsum finish plaster's published coverage, applied in the usual two passes that together make one skim. The calculator counts the two passes as a single coat, which is how the trade describes a skim.
The density stays at 1,250 kg/m³, the applied density the calculator's second source derives from a standard bag's printed coverage at that thickness, and the allowance at 15 per cent, because a skim is mixed in small batches against a setting time, and the end of a batch that has started to go off is scraped off the board rather than forced onto the wall.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Area to skim | 42 m² (452.1 sq ft) | The wall area only, measured with the openings taken off; the ceiling is a separate skim with its own count. |
| Finished thickness | 2 mm (0.08 in) | Left at the calculator's own default. |
| Skim coats | 1 | Left at the calculator's own default. |
| Applied density | 1,250 kg/m³ (78.03 pcf) | Left at the calculator's own default. |
| Waste allowance | 15 | Left at the calculator's own default. |
The working, step by step
1. Plaster volume
84 L (22.19 gal)
Area times finished thickness gives the volume of plaster on the walls: 84 L. It is a thin layer over a large area, which is why it is so sensitive to thickness: every extra millimetre over the whole room is a large share of that volume.
2. Before the allowance
105 kg (231.5 lb)
The volume times the applied density gives the mass of plaster that ends up on the wall: 105 kg. Finish plaster is sold by weight, so this is the first figure that means anything at the merchant's counter.
3. Allowance at 15%
15.75 kg (34.72 lb)
The allowance adds 15.75 kg for the plaster that never reaches the wall: the ends of batches that set before they are used, what is left on the hawk and the bucket, and the scrapings from the floor. A skim has a short working time, which is why this allowance is larger than on most materials.
4. 25 kg bags
5 bags
The mass with its allowance, divided by the bag and rounded up, is 5 bags. Check the date on each bag before it is opened: finish plaster has a shelf life, printed on the bag.
5. Coverage at this thickness
8.4 m²/bag (90.42 ft²/bag)
The calculator turns its answer back into the kind of number printed on a bag: the area divided by the bags bought, 8.4 m²/bag at this thickness. It is lower than the bag's own printed coverage because it carries the allowance and the round-up to whole bags; if the two are far apart, the thickness or the density entered is wrong.
Finish plaster needed
120.7 kg
266.2 lb
Order 5 bags of finish plaster for the walls, which holds 120.7 kg with the allowance. New plasterboard is skimmed directly, usually with scrim tape over the joints first; old plaster that is very absorbent or has been painted is treated with a bonding agent or a primer first, as the plaster's maker specifies, and none of that is in this count. A skim is a skilled job, and the figure is for comparing against a plasterer's quotation as much as for buying.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Finish plaster needed | 120.7 kg (266.2 lb) | Gypsum finish plaster for one skim of the walls, allowance included. |
| 25 kg bags | 5 bags | Whole bags of finish plaster, within the date printed on the bag. |
The allowance, and what it does here
The 15 per cent goes on the mass once and is shown as its own row. It is generous on purpose: finish plaster is mixed by the bucket against a setting time, and the last of a batch is often lost. It does not cover a wall that is far out of true, which needs a backing coat before it can be skimmed at all, or a thicker finish than the one entered, which is a different thickness, not a larger allowance, as the variant below shows.
The common mistake
Taking the bag's coverage as the coverage on your wall
A bag of finish plaster prints its coverage at a stated thickness, and that number assumes a flat, sound background skimmed at exactly that depth. An old wall with dips, filled patches and a few high spots takes more, because the plasterer fills the low points to bring the face flat, and the extra goes in where nobody measures it. Buying by the printed coverage for an uneven wall runs out on the last one. Look at the walls in a raking light before ordering and, if they are poor, enter a thicker finish as the variant below does.
Change the job, and see what moves
The same walls, skimmed over an uneven old finish
Finished thickness: 3 mm (0.12 in) → 181.1 kg (399.3 lb)
Allow a finish of 3 mm instead, for old walls that need the low spots filled as the skim goes on, and the plaster rises to 181.1 kg, or 8 bags. Half as much again of thickness is half as much again of plaster, and the coverage per bag falls to 5.25 m²/bag.
The same walls, by a plasterer who wastes little
Waste allowance: 5 → 110.3 kg (243.1 lb)
An experienced plasterer mixing small batches might lose only 5 per cent, and then the plaster is 110.3 kg: still 5 bags. On a room this size the allowance moves the mass but not the bag count, which is the thickness's job.
What to run next
- Plaster Angle and Stop Bead Calculator
The external corners and the window reveals are finished with bead before the skim, counted by length around the same room.
- Paint Primer Calculator
New plaster is sealed with a mist coat or primer before painting, counted from the same wall area.
- Dot-and-Dab (Direct Bond) Drywall Adhesive Calculator
If any walls are reboarded on masonry first, the boards are bonded with adhesive counted from the board area.
Sources
The method and every constant behind these figures belong to the Skim Coat Plaster Calculator, version 1.0.0, last verified 2026-09-23. It rests on:
- Geometry: the plaster in a skim is the area times the finished thickness of the two passes, and its mass is that volume times the applied density of the mixed plaster
- Gypsum finish plaster is sold by weight and its coverage published by area at a nominal thickness — a 25 kg bag covering about 10 m² at 2 mm, which is an applied density of about 1,250 kg/m³
Frequently asked questions
- How much skim plaster for 42 m² of walls?
- At 2 mm with the calculator's 15 per cent allowance, 120.7 kg, which is 5 bags.
- What area does a bag of finish plaster cover?
- At this thickness and with the allowance included, the calculator gives 8.4 m²/bag. The bag's own printed figure is higher because it carries neither the allowance nor the round-up to whole bags.
- How much more plaster does an uneven wall take?
- Entered as a thicker finish of 3 mm, the same walls take 181.1 kg instead of 120.7 kg.