1529
Plaster chart calculator
Put your area, coverage per bag and waste in: 400 square feet at 10% waste with 80 square feet per bag is 5.50 bags exact, order 6.
★ CONSTRUCTION · PLASTERING · PLASTER CHART
Plaster Coverage Chart: 6 Bags for 400 Sq Ft at 10% Waste
On the defaults — 400 square feet at 10% waste with 80 square feet per bag coverage — you need 440.00 square feet of plaster coverage, 5.50 bags exact, so order 6 bags. This is the PLASTER version — area times waste divided by coverage, with a lookup table for every common area and waste level — the tile-adhesive-chart page is the TILE-ADHESIVE version — the same divide-and-round-up model counted in adhesive bags; the thinset-chart page is the THINSET version — the same model counted in thinset bags; the grout-chart page is the GROUT version — the same model counted in grout bags; and the mortar-chart page is the MORTAR version — the same model counted in mortar bags.
What does 5.50 bags, order 6, mean?
It means that 400 square feet of wall, once 10% waste is added, needs 440.00 square feet of plaster coverage, which is 5.50 bags at 80 square feet a bag — and since plaster is sold by the whole bag, you order 6. The exact figure and the order figure answer different questions: 5.50 is the arithmetic, 6 is the purchase, and the half bag of headroom is the waste allowance doing its job rather than an over-order. The part people get wrong is ordering the exact figure rounded down: 5.50 rounded down is 5 bags, 400.00 square feet of coverage against a 440.00-square-foot need, and the last wall runs short. The other part people get wrong is applying the waste percentage to the bags instead of the area: waste is 10% more surface to cover, not 10% more bags on top of a rounded order, and the two give different answers once rounding enters.
Three things to check before you trust the number
Is 80 square feet per bag really your coverage? Eighty square feet is the coverage for the thickness on the bag, and it is the figure the exact-bags line turns on: the 100-square-foot-coverage branch totals 4.40 bags exact against the 5.50 headline, so a 20-square-foot coverage difference is a whole bag on 400 square feet. If you are applying a thicker coat than the bag assumes, your real coverage is lower and your real bag count higher — check the thickness on the product sheet against the thickness you are actually trowelling.
Is 10% waste enough for your walls? Ten percent is the waste allowance, and it is the figure the needed-area line turns on: the 20%-waste branch totals 6.00 bags exact against the 5.50 headline, so doubling waste adds half a bag here and a full bag on larger jobs. Smooth new plasterboard at 10% is reasonable; rough masonry, patchy repairs, or walls full of corners and reveals want 15% or 20%, because every cut edge and every uneven patch eats plaster that a flat-wall figure never sees.
Did you measure every wall? Four hundred square feet is the area, and it is the figure everything else multiplies: the 800-square-foot branch totals 11.00 bags exact against the 5.50 headline, so a missed wall doubles the order. Measure wall by wall, subtract large openings only if you genuinely will not plaster them, and add the reveals and returns — small strips that are easy to forget and add up to a bag on a whole house.
Worked example
On the defaults: 400 square feet, 80 square feet per bag, 10% waste.
- Needed area: 400 × 1.10 is 440.00 sq ft.
- Exact bags: 440.00 ÷ 80 is 5.50 bags.
- Order: 5.50 rounded up is 6 bags — area times the waste factor, divided by coverage, rounded up to order.
And the other way: the area behind 5.50 bags is 400 square feet, the coverage behind 5.50 bags is 80 square feet per bag, and the waste behind 5.50 bags is 10%. With 0% waste the model honestly shows 5.00 bags exact and an order of 5 — an exact fit where no rounding up is needed, because the need divides evenly by the coverage.
Every branch, checked
- 200 square feet, 80 square feet per bag, 10% waste: 200 square feet, 80 square feet per bag, 10% waste: 2.75 bags exact — order 3 bags.
- 800 square feet, 80 square feet per bag, 10% waste: 800 square feet, 80 square feet per bag, 10% waste: 11.00 bags exact — order 11 bags.
- 400 square feet, 100 square feet per bag, 10% waste: 400 square feet, 100 square feet per bag, 10% waste: 4.40 bags exact — order 5 bags.
- 400 square feet, 80 square feet per bag, 20% waste: 400 square feet, 80 square feet per bag, 20% waste: 6.00 bags exact — order 6 bags.
The reverses, worked backwards
- Area behind 5.50 bags: 400 sq ft — at 400 sq ft the figures are 5.50 bags exact — order 6 bags.; at the lower neighbor the figures are 4.81 bags exact — order 5 bags.; at the upper neighbor the figures are 6.19 bags exact — order 7 bags..
- Coverage behind 5.50 bags: 80 sq ft per bag — at 80 sq ft per bag the figures are 5.50 bags exact — order 6 bags.; at the lower neighbor the figures are 5.87 bags exact — order 6 bags.; at the upper neighbor the figures are 5.18 bags exact — order 6 bags..
- Waste behind 5.50 bags: 10% waste — at 10% waste the figures are 5.50 bags exact — order 6 bags.; at the lower neighbor the figures are 5.25 bags exact — order 6 bags.; at the upper neighbor the figures are 5.75 bags exact — order 6 bags..
What the defaults assume
The defaults are 400 square feet, 80 square feet per bag and 10% waste, so the headline is one medium room of plaster at a standard coat thickness with a standard waste allowance. The model is a multiplication, a division and a round-up: the needed area is area times one plus waste over one hundred, the exact bags are needed area divided by coverage, and the order is exact bags rounded up to the next whole bag. The area reverse is checked at 50 square feet either side, because 50 square feet at 10% waste is 55 square feet, or 0.69 bags; the coverage reverse is checked at 5 square feet either side; the waste reverse is checked at 5 points either side, because 5 points on 400 square feet is 20 square feet, or 0.25 bags. The model does not include coat thickness beyond what the coverage figure already assumes, primer, mesh, or different products for different coats — price those separately.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Needed area | area × (1 + waste ÷ 100) | 440.00 sq ft — 400 × 1.10. |
| Exact bags | needed area ÷ coverage per bag | 5.50 bags — 440.00 ÷ 80. |
| Order | exact bags rounded up | 6 bags — 5.50 rounds up to the next whole bag. |
| Area behind a target exact figure | exact bags × coverage ÷ (1 + waste ÷ 100) | 400 sq ft — 5.50 × 80 ÷ 1.10. |
| Coverage behind a target exact figure | needed area ÷ exact bags | 80 sq ft per bag — 440.00 ÷ 5.50. |
| Waste behind a target exact figure | (exact bags × coverage ÷ area − 1) × 100 | 10% — (5.50 × 80 ÷ 400 − 1) × 100. |
| One extra bag of headroom | order − exact bags | 0.50 bags — 6 − 5.50, the waste allowance doing its job. |
| 200 square feet, 80 square feet per bag, 10% waste | same model | 2.75 bags exact — order 3 bags. |
| 800 square feet, 80 square feet per bag, 10% waste | same model | 11.00 bags exact — order 11 bags. |
| 400 square feet, 100 square feet per bag, 10% waste | same model | 4.40 bags exact — order 5 bags. |
| 400 square feet, 80 square feet per bag, 20% waste | same model | 6.00 bags exact — order 6 bags. |
| Area behind 5.50 bags | reverse | 400 sq ft: 5.50 bags exact — order 6 bags. |
| Coverage behind 5.50 bags | reverse | 80 sq ft per bag: 5.50 bags exact — order 6 bags. |
| Waste behind 5.50 bags | reverse | 10% waste: 5.50 bags exact — order 6 bags. |
| 0 square feet | same model | 0.00 bags exact — order 0 bags. |
| Negative area | same model | Area — 0 or more. |
| Zero coverage | same model | Coverage — more than 0. |
| 400 square feet, 0% waste | same model | 5.00 bags exact — order 5 bags. |
Lookup table: bags by area and waste
All rows at 80 square feet per bag. Find your area and waste level, read across to the exact figure and the order.
| Area | Waste | Needed area | Exact bags | Order |
|---|---|---|---|---|
| 100 sq ft | 0% | 100.00 sq ft | 1.25 | 2 bags |
| 100 sq ft | 5% | 105.00 sq ft | 1.31 | 2 bags |
| 100 sq ft | 10% | 110.00 sq ft | 1.38 | 2 bags |
| 100 sq ft | 15% | 115.00 sq ft | 1.44 | 2 bags |
| 200 sq ft | 0% | 200.00 sq ft | 2.50 | 3 bags |
| 200 sq ft | 5% | 210.00 sq ft | 2.63 | 3 bags |
| 200 sq ft | 10% | 220.00 sq ft | 2.75 | 3 bags |
| 200 sq ft | 15% | 230.00 sq ft | 2.88 | 3 bags |
| 400 sq ft | 0% | 400.00 sq ft | 5.00 | 5 bags |
| 400 sq ft | 5% | 420.00 sq ft | 5.25 | 6 bags |
| 400 sq ft | 10% | 440.00 sq ft | 5.50 | 6 bags |
| 400 sq ft | 15% | 460.00 sq ft | 5.75 | 6 bags |
| 800 sq ft | 0% | 800.00 sq ft | 10.00 | 10 bags |
| 800 sq ft | 5% | 840.00 sq ft | 10.50 | 11 bags |
| 800 sq ft | 10% | 880.00 sq ft | 11.00 | 11 bags |
| 800 sq ft | 15% | 920.00 sq ft | 11.50 | 12 bags |
Who uses this
- A homeowner plastering one room — 400 square feet at 10% waste is 5.50 bags exact, order 6 — the lookup table covers the room sizes around it without touching the calculator.
- A plasterer pricing a job — the coverage and waste branches price a thicker coat or rougher walls at once, so a quote can be adjusted before the bags are ordered.
- A builder ordering for a whole house — the 800-square-foot rows scale the same model, and the reverses recover the area, coverage or waste behind any quoted bag count.
- A DIYer checking a store recommendation — the exact-fit edge shows when an order needs no rounding up, so a recommendation of 5 bags for 400 square feet at 0% waste can be checked rather than trusted.
Accuracy & limitations
Figures are the model’s own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp. The main limitation is the inputs, not the arithmetic: whether the area includes every wall and reveal, whether the coverage figure matches the coat thickness actually applied, and whether the waste allowance suits the wall condition are all judgement calls, and this page prices the figures you enter without checking them. This page is a calculation tool, not building advice.
Cite this calculator
400 square feet at 10% waste with 80 square feet per bag needs 440.00 square feet, 5.50 bags exact: order 6 bags. Source: Bright Side Kit, https://brightoolbox.com/plaster-chart/, accessed 29 October 2026.
Frequently asked questions
Why order 6 bags when the exact figure is 5.50?
Because plaster is sold by the whole bag and you cannot buy half a bag. Rounding down to 5 would leave 40.00 square feet uncovered; rounding up to 6 covers the full need with half a bag of headroom, which is exactly what the waste allowance is for.
When does the order need no rounding up?
When the needed area divides evenly by the coverage: 400 square feet at 0% waste is 400.00 square feet, which is exactly 5.00 bags at 80 square feet a bag, so the order is 5. Exact fits are the exception, not the rule, and they vanish the moment waste is added.
Should waste apply to the area or to the bags?
To the area. Waste is extra surface to cover — offcuts, spillage, uneven patches — so it belongs in the needed area before dividing by coverage. Applying waste to the bags after rounding gives a different, usually smaller, answer that understates the need.
What coverage figure should I use?
The one on the bag or product sheet for the thickness you are actually applying. A bag rated at 80 square feet for a thin coat may cover far less at a thick coat, and using the wrong thickness figure is the fastest way to run short mid-wall.
Do I subtract windows and doors?
Only openings you genuinely will not plaster. A door or window opening saves the flat area but adds reveals to plaster, so subtracting the full opening without adding the reveals back understates the job. When in doubt, leave small openings in and treat the saving as extra headroom.
How is a plaster chart different from a floor leveler chart?
Only the material changes: plaster at 80 square feet per bag against the floor leveler page at 25 square feet per bag. The model — area times waste divided by coverage, rounded up — is identical, which is what makes the chart pages comparable.