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Float and Set Plaster Calculator

Undercoat and finish plaster for a masonry wall, as two separate bag counts — because the two coats are different products at very different densities.

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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 masonry surface taking both coats.

Measure the wall, and take out openings only where they are large — a window in a small room is a real deduction on an 11 mm undercoat, which is the coat where a square metre is a third of a bag.

The levelling coat, at the nominal thickness its coverage is quoted for.

11 mm on walls and 8 mm on soffits are the thicknesses gypsum undercoats quote their coverage at. What a wall actually takes depends on how true it is: an old brick wall out by 10 mm in its length is being levelled by this coat, and the average thickness is what to enter, not the thinnest point.

Lightweight, because it has to hold at a levelling thickness without slumping.

850 kg/m³ sits in the middle of what published undercoat coverage implies. Where the bag gives its own coverage, divide the bag weight by (area × thickness) to get that product's figure — a sand-and-cement float is far heavier, nearer 1,900 kg/m³, and is a different order again.

The setting coat over the float, at its own nominal thickness.

The finish is a thin skim laid in two passes over the levelled undercoat, and 2 mm is what its coverage is published at. It is not a levelling coat: anything the float left out of true stays out of true.

Denser than the undercoat, and the reason the two are ordered separately.

1,250 kg/m³ is what the common published finish coverage implies — about 10 m² per 25 kg bag at 2 mm. It is roughly half as heavy again as the undercoat per unit of volume, while the coat is five times thinner.

What sets in the bucket and falls to the floor, on both coats.

Both coats have a working time and both are mixed in batches, so both lose material the same way. Fifteen per cent is normal for a room; raise it where the wall is rough enough that the undercoat is being pushed in rather than laid on.

Plaster needed, both coats

888.2 lb

High confidence

About 14 bag(s) of undercoat and 4 of finish. Order them separately — they are different products.

Undercoat
699.73 lb
Undercoat, 50 lb bags
14 bags
Finish
188.43 lb
Finish, 50 lb bags
4 bags
Allowance at 15%
115.85 lb
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Geometry, coat by coat: each coat's plaster is the area times that coat's thickness, and its mass is that volume times that product's applied density
  • Gypsum undercoat plasters publish coverage at a nominal 11 mm — commonly 2.5 to 3 m² per 25 kg bag, an applied density of about 760 to 900 kg/m³ — while finish plaster covers about 10 m² per 25 kg bag at 2 mm, about 1,250 kg/m³

Inputs used

Area to plaster
320 sq ft
Undercoat thickness
0.43 in
Undercoat applied density
53.06 pcf
Finish thickness
0.08 in
Finish applied density
78.03 pcf
Waste allowance
15

Intermediate steps

Undercoat
699.73 lb
Undercoat, 50 lb bags
14 bags
Finish
188.43 lb
Finish, 50 lb bags
4 bags
Allowance at 15%
115.85 lb
Final result888.17 lb

Confidence note: About 14 bag(s) of undercoat and 4 of finish. Order them separately — they are different products.

What this calculation does not cover

  • The two coats are different products and are bought separately. The headline here is their combined weight; the bag counts in the breakdown are what to order, and they cannot be added together.
  • The undercoat thickness is an average over the wall. A wall that is badly out of true takes far more than its nominal 11 mm, or 7/16 in, and the only honest way to know is to string a line across it before ordering.
  • A sand-and-cement float is not this. It is mixed on site from separate materials at a density more than twice the gypsum figure here, and the render and plaster mix calculator is the page for it.
  • Beads, scrim, bonding agents and the preparation the background needs are not counted. On a masonry wall the beads alone are a real line: every external angle and every stop.

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.

finish 0.0787 infinish 2 mmundercoat 0.43 inundercoat 10.92 mmmasonry0.509 in12.92 mm

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, coat by coat: each coat's plaster is the area times that coat's thickness, and its mass is that volume times that product's applied density
  2. Gypsum undercoat plasters publish coverage at a nominal 11 mm — commonly 2.5 to 3 m² per 25 kg bag, an applied density of about 760 to 900 kg/m³ — while finish plaster covers about 10 m² per 25 kg bag at 2 mm, 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.

Worked example: A room replastered — step 4 of 6

The data behind it: Production rates by operation

How to calculate float and set plaster in 7 steps

  1. Area to plasterThe masonry surface taking both coats.
  2. Undercoat thicknessThe levelling coat, at the nominal thickness its coverage is quoted for.
  3. Undercoat applied densityLightweight, because it has to hold at a levelling thickness without slumping.
  4. Finish thicknessThe setting coat over the float, at its own nominal thickness.
  5. Finish applied densityDenser than the undercoat, and the reason the two are ordered separately.
  6. Waste allowanceWhat sets in the bucket and falls to the floor, on both coats.
  7. Plaster needed, both coatsThe tool computes the plaster needed, both coats from those figures and shows the formula, its sources, and a confidence rating alongside it.

Plaster needed, both coats by area to plaster

Page defaults, not your figures above.

Area to plasterPlaster needed, both coats (lb)
200 sq ft558
300 sq ft837
400 sq ft1,116
500 sq ft1,396
600 sq ft1,675

Frequently asked questions

Why are there two bag counts instead of one?
Because they are two different products. The undercoat is a lightweight levelling plaster laid around 11 mm; the finish is a dense setting plaster laid around 2 mm. On a 30 m² wall that is roughly 280 kg of undercoat and 75 kg of finish before any allowance — about twelve bags and three — and a merchant cannot supply fifteen bags of "plaster".
How much does an out-of-true wall add?
Undercoat in direct proportion to its average thickness: a wall averaging 18 mm (0.71 in) instead of the nominal 11 mm (0.43 in) takes about 64% more, the largest single error in a float and set order. The undercoat is the coat doing the levelling, so its average thickness is what matters. String a line along the wall and measure the worst and the best point before ordering; the average of the two is a better figure than the nominal.
Can I use one coat instead of two?
Only where the background is already flat and suitable, which on masonry it rarely is. A finish plaster laid straight onto brick or block will follow every hollow in it, and at 2 mm it has nothing to level with. The undercoat exists to make a flat, even-suction surface for a coat that is too thin to make one itself.
Is this the same as render?
No. Render is a sand-and-cement or lime mix, usually external, mixed on site from separate materials at a density more than twice the gypsum plasters here — and it is ordered as cement, lime and sand rather than as bags of plaster. The render and plaster mix page covers that; this one is gypsum plaster indoors.
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.