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Plaster Quantity Calculator

Start with your net plastering area, then estimate whole bags from a matching product yield at your already-specified total skim, backing or coating thickness.

Start with your measurements

Measured surface

Keep upstairs and ground-floor areas separate.

Leave out a measured opening optional

Add it to this surface only. A wall window never reduces your floor.

Use the actual surface measurements, not a guessed size.

All included surfaces use the same product facts you enter below. Keep different products or layers in separate estimates.

Choosing an option supplies its stated facts, not a suitability recommendation. Different variants need their own quantities.

Details from your chosen product or plan
Blank means unknown. This choice does not recommend a system or establish product suitability.
Your existing specification, not a calculator recommendation. Total thickness is not thickness per application.
Use only a yield explicitly stated for your chosen product, system, total thickness and substrate.
Optional reference, for example the product name and its stated bag size. This does not select or validate a product.
Copy the thickness attached to the yield. It must exactly match your specified total thickness; no scaling is applied.
For that exact pack variant at the stated total thickness and matching layer/system. Bag mass alone does not establish coverage.
At the stated total thickness. Not kg/m²/mm, wet mix mass, dry set weight, or a rate inferred from bag mass.
Copy the actual selected bag weight. Leave unknown to see dry product mass without a bag count.
Confirm only from the product documentation or a reliable existing specification. This is a record of your facts, not a suitability approval.
Optional and explicitly chosen by you. Blank adds no arithmetic extra; no waste percentage is recommended.

Your current estimate will appear here. No dimensions or product facts are assumed.

Separate local inputs only. This calculator does not add to My Materials List.

How the estimate works

Calculator.Shop quantity arithmetic: A is the already-net area. Extra area E = A × supplied allowance% / 100; adjusted area T = A + E. At an exactly matching specified total thickness and layer/system, bags = ceil(T / product-stated m² per bag), or dry product M = T × product-stated dry kg per m² and bags = ceil(M / selected bag kg). No yield is inferred from bag weight, dry set weight, density or volume. Thickness is a compatibility check, never a scaling factor. Retain full precision until the final whole-bag ceiling, using the shared helper’s near-integer floating-point normalisation.

Quantity estimate for one already-decided layer/system and one compatible dry plaster product/pack variant. Blank layer/system, unknown yield basis or unmatched thickness cannot produce bag counts. No default 25 kg yield, thickness interpolation, coat multiplication, backing-plus-skim combination, mix/water calculation, substrate approval, installation design or Materials List output. No stock, price, sufficient-materials, engineering or permission guarantees.

Worked example

Verified example only, not a preset: British Gypsum Thistle MultiFinish in the nominal 25 kg bag states 10 m² per bag at 2 mm total skim thickness in its 15 September 2026 product data sheet. For an already-net 23 m², skim selected, both thickness fields 2 mm, bag-coverage basis and the matching-yield confirmation, entering 10 m²/bag and a user-chosen 10% allowance gives 2.3 m² extra, 25.3 m² adjusted and ceil(25.3 / 10) = 3 bags, with 4.7 m² coverage-based surplus. The two applications used to build this total finish are not two independent full-thickness layers. The example does not apply to another thickness, bag variant, backing plaster or coating.

What to measure and check

Measure the surfaces once: the supplied area is already net of any exclusions. Choose the layer/system and its already-specified total thickness, then enter the matching yield from your selected product label or technical sheet. Use one calculation per product and layer; do not double the area for the applications used to build one finish.

Can I calculate bags from area alone?

No. The calculator shows your already-net area first. Bag counts require a selected layer/system, an explicit yield basis and matching product facts at the already-specified total thickness. A blank layer/system is unknown, not an automatic skim selection.

Does every 25 kg bag cover 10 m²?

No. The cited 10 m² figure is specific to the nominal 25 kg Thistle MultiFinish bag at 2 mm skim thickness. Enter your actual product’s coverage or its stated dry kg/m² consumption and actual bag kg. Bag weight is not a universal yield.

Can I use a 2 mm yield for a 3 mm finish?

Not here. Different specified and yield thicknesses return missing compatible-yield guidance, not a scaled estimate. Obtain an explicitly supported yield for the exact already-specified total thickness; this calculator does not choose an allowable thickness.

Should two applications or coats double the bags?

Not automatically. A stated yield can already cover the total thickness built in more than one application. This calculator has no coat multiplier. Estimate separately only for genuinely separate, already-specified product/layer requirements, using each matching yield.

Are doors and windows deducted again?

No. The shared measurement supplies already-net area. Openings are not deducted a second time. Any reveal surfaces that need plaster must already be included in that measured basis.

What if I know kg per m² but not the bag weight?

With the matching layer, total thickness and confirmed dry consumption, the mass-rate basis shows dry product kilograms. Whole bags stay unavailable until you enter the actual selected bag weight. Wet mix weight and the manufacturer’s dry set weight are not dry bag consumption.

Does the allowance guarantee enough material?

No. The optional allowance is your own explicit addition, not a recommended waste factor. Bag rounding and coverage-based surplus are arithmetic estimates; uneven backgrounds, product tolerances and site conditions can change use. This is not installation, substrate, engineering or permission advice, and does not guarantee stock or price.

Where the numbers come from

Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.

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