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Plaster calculator for walls and render

This plaster calculator works out how much plaster or render you need: area × thickness × consumption per millimetre from the manufacturer’s data sheet, plus a waste allowance. Gypsum plaster, lime-cement plaster, lime plaster and external render are built in; for any other product enter the coverage and bag size printed on the bag.

Example: Wall area 40 m² · Subtract windows and doors 0 m² · Plaster thickness 10 mm · Type of plaster: Gypsum plaster (Knauf Rotband, 0.8 kg/m²/mm, 30 kg) · Waste allowance 10 % → Plaster needed: 12 bags of 30 kg (352 kg). Source: Knauf Rotband Haftputzgips, Technisches Blatt P121.de 06/2014 – 8,0 kg/m² bei 10 mm, 30-kg-Sack (gelesen 09.10.2026). Updated: .

m²
m²
mm
indoors usually 10–15 mm, render outside at least 20 mm
%
Baumit allows about 10% extra
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Result

Plaster needed
12 bags of 30 kg (352 kg)
Calculation
ItemAmountBasis
Area to plaster40 m²wall area minus windows and doors
Consumption per m²8 kg/m²0.8 kg/m² per mm × 10 mm
Plaster without allowance320 kgarea × consumption per m²
Plaster with allowance352 kg+ 10% for background and losses
Bags12 × 30 kgone bag covers about 3.75 m²
Gypsum plaster (Knauf Rotband): 0.8 kg/m² per mm, 5–50 mm in one coat, indoors only. Estimate based on the data sheet: actual use depends on the background, flatness and application – check with a test area.

How it is calculated

40 m² of wall with 10 mm of gypsum plaster needs about 320 kg of plaster, 352 kg with a 10% allowance – that is 12 bags of 30 kg. The formula is area × thickness in mm × consumption in kg/m² per mm.

Coverage by type of plaster

PlasterPer mmAt 10 mmBag
Gypsum plaster (Knauf Rotband)0.8 kg/m²8 kg/m²30 kg ≈ 3.75 m²
Lime-cement, indoor (Baumit MPI 25)1.4 kg/m²14 kg/m²35 kg = 2.5 m²
Lime plaster (Baumit KP 35)1.4 kg/m²14 kg/m²35 kg = 2.5 m²
External render (Baumit MPA 35)1.4 kg/m²14 kg/m²40 kg

Gypsum plaster is lighter, so the same wall needs only a little over half as many kilograms as with lime-cement plaster. For a thin skim coat use the coverage on the bag of your finishing plaster with the “my product” option.

Examples

For the paint afterwards use the paint calculator; for stud walls the drywall calculator.

Limits

The coverage figures come from the data sheets and assume a flat background. Uneven masonry, chases and repairs raise consumption, which is why Baumit adds about 10%. Primer, beads, mesh and the finishing coat are not included. Other products have their own figures – choose “my product” and copy the value from the bag.

Frequently asked questions

How much plaster do I need per m²?

Gypsum plaster uses 0.8 kg per mm of thickness, so 8 kg/m² at 10 mm. Lime-cement plaster uses about 1.4 kg per mm, 14 kg/m² at 10 mm.

How many bags of plaster for 10 m²?

At 10 mm of gypsum plaster (30 kg bags) you need 80 kg plus 10% = 88 kg, so 3 bags. With lime-cement plaster (35 kg bags) it is 154 kg, so 5 bags.

How thick should plaster be?

Indoor backing coats are usually 10 to 15 mm. Knauf Rotband can be applied 5 to 50 mm thick in one coat; Baumit states at least 10 mm for MPI 25.

How thick should external render be?

For Baumit MPA 35 the manufacturer states at least 20 mm on external walls, followed by a finishing render.

How do I calculate plaster quantity by hand?

Area in m² × thickness in mm × consumption in kg/m² per mm (printed on the bag). Divide by the bag size and round up.

Gypsum or lime-cement plaster?

Gypsum plaster is light, goes further and is easy to smooth, but only suits dry indoor rooms. Lime-cement plaster tolerates moisture and is used in bathrooms, cellars and outside.

Sources and legal basis

As of: · Editorial policy

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