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Skim Coat Plaster Calculator

Bags of finish plaster to skim a plasterboard wall or ceiling, from the area and the finished thickness the background actually needs.

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
Who is doing the work?

Waste is set to 15% by hand. Pick a tier above to replace it, or keep your own figure.

The surface being skimmed — walls, ceiling, or both.

Measure the surface rather than the room: a 4 × 3 m room with 2.4 m walls is about 34 m² of wall before openings and 12 m² of ceiling. Openings are worth taking out only when they are large; a single door in a room is inside the waste allowance.

The depth of the finished skim — both passes together, not each.

2 mm over flat, true plasterboard is the nominal figure a bag's coverage is quoted at. Raise it where the surface is being brought level: 3 mm over a patched or bonded wall, and more again over anything uneven. A skim is not a levelling coat — if it needs more than about 5 mm, the wall wants an undercoat first, which is a different order.

One skim is two passes of plaster at the thickness above, not two skims.

The standard skim is a single coat applied in two passes and then trowelled — that is one here. Put two only where a second full skim is genuinely going on over the first, which happens over difficult backgrounds and doubles the material.

How much a given volume of plaster weighs once mixed and on the wall.

1,250 kg/m³ is what the common published coverage implies — a 25 kg bag over about 10 m² at 2 mm. Where the bag gives its own coverage, divide the bag weight by (area × thickness) to get that product's figure and use it here.

Plaster that sets in the bucket, falls off the hawk and is scraped off the board.

Finish plaster has a working time of roughly 45 minutes to an hour once mixed, so whatever is left when it goes off is lost. Fifteen per cent suits a room mixed in sensible batches; raise it for a small job, where one bucket too many is a large share of the order.

Finish plaster needed

188.4 lb

High confidence

About 4 × 50 lb bag(s). The volume is geometry; the weight follows the density, and the thickness is the figure worth checking against the wall.

Plaster volume
2.1 ft³
Before the allowance
163.85 lb
Allowance at 15%
24.58 lb
50 lb bags
4 bags
Coverage at this thickness
80 ft²/bag
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • 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

Inputs used

Area to skim
320 sq ft
Finished thickness
0.08 in
Skim coats
1
Applied density
78.03 pcf
Waste allowance
15

Intermediate steps

Plaster volume
2.1 ft³
Before the allowance
163.85 lb
Allowance at 15%
24.58 lb
50 lb bags
4 bags
Coverage at this thickness
80 ft²/bag
Final result188.43 lb

Confidence note: About 4 × 50 lb bag(s). The volume is geometry; the weight follows the density, and the thickness is the figure worth checking against the wall.

What this calculation does not cover

  • A skim is a finish, not a levelling coat. Where a background is out by more than about 5 mm — call it 3/16 in — the wall takes an undercoat first — bonding on a dense surface, browning on an absorbent one — and that is a separate and much larger order than this.
  • Plasterboard joints, internal angles and any patches are taped and filled before skimming. The scrim and the filler for that are not counted here.
  • The figure assumes the background is prepared as the plaster expects: boards dry and fixed, old paint sealed or bonded, high-suction surfaces controlled. Skim on an unprepared background does not fail by quantity, it fails by adhesion.
  • Coverage figures are for the plaster on the wall. Anything left over cannot be kept: once mixed it goes off, and a part-used bag of finish plaster has a short shelf life after opening.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

skim coat 0.0787 inskim coat 2 mmplasterboard0.0787 in2 mm1 coat

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-23 · v1.0.0

Regulatory standards & verification citations2
  1. 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
  2. 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
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To skim a plaster finish. Generic types, no brands, no prices.

Protection this work requires

  • Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
  • Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Show the 4 tools this job needs

Essential

  • Mixing bucket or tub

  • Paddle mixer

  • Plastering trowel and hawk

Recommended

  • Step ladder

what each plastering tool is for, and the spec that decides which to buy where one does.

Already have some? How much wall will my bag of finish plaster skim?

Worked examples: A room replastered — step 5 of 6 · Stud partition with a door — step 8 of 10

The data behind it: Production rates by operation

How to calculate skim coat plaster in 6 steps

  1. Area to skimThe surface being skimmed — walls, ceiling, or both.
  2. Finished thicknessThe depth of the finished skim — both passes together, not each.
  3. Skim coatsOne skim is two passes of plaster at the thickness above, not two skims.
  4. Applied densityHow much a given volume of plaster weighs once mixed and on the wall.
  5. Waste allowancePlaster that sets in the bucket, falls off the hawk and is scraped off the board.
  6. Finish plaster neededThe tool computes the finish plaster needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Finish plaster needed by area to skim

Page defaults, not your figures above.

Area to skimFinish plaster needed (lb)
200 sq ft118
300 sq ft177
400 sq ft236
500 sq ft294
600 sq ft353

Frequently asked questions

How many bags of plaster to skim a room?
For a 4 × 3 m room with 2.4 m walls, the walls are about 34 m² and the ceiling 12 m² — call it 46 m² in total. At the nominal 2 mm, that is about 0.092 m³ of plaster, roughly 115 kg, and with a sensible allowance around six 25 kg bags. The figure moves with the thickness more than with anything else: the same room at 3 mm is eight.
Is a skim one coat or two?
One coat, applied in two passes. The first pass lays the plaster on at close to the finished thickness, the second flattens and closes it, and both are the same mix from the same bucket — so they are one coat for the purpose of ordering. Enter two coats here only where a second full skim is going on over a set first one, which is a decision about a difficult background rather than normal practice.
Why does my bag say 10 m² and yours says less?
A bag's published coverage is at a stated thickness on a flat background with nothing wasted. This page starts from the same arithmetic and then adds what actually happens: the thickness you are really laying, and the plaster that sets in the bucket. Set the thickness to 2 mm and the allowance to zero and a 25 kg bag comes back at 10 m², which is the bag's own figure.
Can I skim straight over old paint or artex?
Not as a quantity question — as an adhesion one. Sound, matt, well-keyed paint usually takes a bonding agent first; glossy, flaky or powdery surfaces need more than that; and textured coatings applied before the 1980s may contain asbestos, which stops the job until it has been tested. None of that changes the plaster figure on this page, and all of it changes whether the plaster stays on the wall.
What is the difference between this and the finish coat calculator?
The finish coat page works in a three-coat system over lath — the American scratch, brown and finish build-up — and asks for that coat's own thickness. This one is the British and Australian skim: finish plaster straight onto plasterboard, ordered in bags, at the thickness the board actually needs. The arithmetic underneath is the same volume; the defaults, the vocabulary and the background are not.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.