About this calculator
This calculator helps estimate how many bags of plaster a wall needs — enter the wall dimensions, the plaster layer thickness, and its coverage rate, and add any openings (doors, windows) separately so they are deducted from the plastered area. Useful for estimating both gypsum and cement-lime plaster quantities.
Formula
The calculator converts the wall dimensions to whole millimetres, then:
gross area = length × height
openings area = Σ (opening width × opening height)
net area = gross area − openings area
volume = net area × layer thickness
mass (excl. waste) = net area × coverage (kg/m²/mm) × layer thickness (mm)
waste mass = round(mass × waste% / 100)
total mass = mass + waste mass
bags required = ceil(total mass / bag size)
Openings are deducted as net area before computing the volume. Volume (net area × layer thickness) is exact geometry, independent of the coverage constant; only the mass calculation depends on the user-entered coverage rate (kg/m² per millimetre of layer thickness). Bag count always rounds up — 11.04 bags means 12 bags.
Worked example
Defaults: wall length 4 m, height 2.5 m, no openings, layer thickness 15 mm, coverage 1.6 kg/m²/mm, waste 15%, bag size 25 kg.
gross area = 4 m × 2.5 m = 10 m² (no openings, so net area is the same)
volume = 10 m² × 0.015 m = 0.15 m³
mass (excl. waste) = 10 m² × 1.6 kg/m²/mm × 15 mm = 240 kg
waste mass = 240 kg × 15 / 100 = 36 kg
total mass = 240 kg + 36 kg = 276 kg
bags required = ceil(276 kg / 25 kg) = ceil(11.04) = 12
For these inputs the calculator returns exactly this: volume 0.15 m³, total mass 276 kg, and 12 bags.
FAQ
Are openings (doors, windows) deducted from the plastered area? Yes. Every added opening is deducted from the wall area before computing the plaster quantity — this differs from the wallpaper calculator, where strip count doesn't deduct openings.
How much waste should be allowed for plastering? The default is 15%, adjustable from 0 to 50. The real requirement depends on subgrade evenness, the plastering method (hand or machine-applied), and losses during mixing and throwing.
What determines the coverage rate (kg/m²/mm)? Coverage is a value you enter, with a default of 1.6 kg/m²/mm — the real figure depends on the specific plaster product's dry density (gypsum plaster is generally lighter, cement-lime heavier), so the calculator doesn't derive it from any standard.
Why doesn't the calculator mention any standard, when EN 13914-1 exists for plastering? EN 13914-1 describes the design and execution of plastering work, but it doesn't fix a specific coverage rate — that depends on the specific product, so it isn't cited here as a calculation source.
Can I plaster in multiple layers of different thickness? The calculator computes one layer by its own thickness. If plastering in several layers of different thickness, each layer needs to be computed separately and the results added.
Assumptions and limits
EN 13914-1 covers the design and execution of plastering work, but doesn't fix a coverage
rate — that depends on the specific product (gypsum or cement-lime, hand or
machine-applied), so meta.standards is deliberately left empty.
Coverage (kg/m²/mm) and bag size (kg) are user inputs with sensible defaults, not constants derived from a standard or manufacturer data.
The volume calculation (net area × layer thickness) is exact geometry; only the mass calculation uses the user-entered coverage rate. The drawing with openings is schematic — once there are more than six openings, the rest aren't drawn (but their area is still included), and the openings themselves are drawn spread evenly across the wall length, since the calculator doesn't collect their exact position in the wall.