Plaster and render calculator
Area times thickness times yield: kilos and bags.
Your inputs
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Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
Plaster gets painted: the area is already measured and only the coverage changes.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Enter the area you are covering.
- Set the thickness: a plaster skim is usually 10-15 mm.
- Check the yield on the bag and change it if it differs.
- Enter the bag weight to see how many to buy.
How it is calculated
Consumption of a render or plaster is linear with thickness:
Yield is expressed in kilograms per square metre per millimetre of thickness. Gypsum plaster runs about 0.8-1.0 kg/m²·mm; one-coat renders and adhesives are very different. The bag always states it, and that figure wins.
Nominal thickness is misleading: a wall that is out of plumb or uneven consumes considerably more, because the render fills everything the wall is not.
Worked example
A 215 sq ft skim at 10 mm, yield 0.9 kg/m²·mm, 10 % waste:
- Consumption per m²: 10 x 0.9 = 9 kg/m²
- Without waste: 20 m² x 9 = 180 kg
- With waste: 198 kg
- In 25 kg bags: 8 bags
Good to know
- Measure the real thickness. If you are correcting a wall that is out of plumb, the average thickness can be double the nominal in places.
- Gypsum, lime and cement are not interchangeable. Yield, working time and use all differ. Gypsum plaster does not belong outdoors or in wet areas.
- Mix in batches. Gypsum plaster sets in 20-40 minutes. Mixing a whole bag without experience ends with a wasted bucket.
- 10 % waste. Make it 15-20 % if this is your first time: plaster on the floor is not coming back.
Frequently asked questions
How much plaster do I need per square metre?
At a yield of 0.9 kg per m² per mm, a 10 mm skim uses about 9 kg per square metre. For 20 m² that is 180 kg, or 198 kg with waste.
How many bags of plaster per square metre?
At 10 mm thickness with 25 kg bags, one bag covers about 2.8 m². At 15 mm, under 2 m². Thickness matters more than area.
Is the yield on the bag reliable?
It is the best figure available, but it assumes ideal application on a regular surface. On an uneven wall you will use more.
Does this work for one-coat render?
Yes, by changing the yield to the manufacturer's figure. One-coat renders are applied thicker and have quite different yields.