Plaster calculator
Enter the wall or ceiling area, the coat thickness and the coverage printed on the bag. The coverage is adjusted to your thickness, a waste allowance is added and the result is rounded up to whole bags, with the cost.
Area and thickness
The bag
About the plaster calculator
Enter the wall or ceiling area, the coat thickness and the coverage printed on the bag. The coverage is adjusted to your thickness, a waste allowance is added and the result is rounded up to whole bags, with the cost.
Part of SmartCalcy’s home & construction tools. It runs in your browser with no account, uses your own currency and units (change them at the top of the page), and gives estimates for information only, so check official guidance before you rely on a result.
How to use the plaster calculator
What it calculates and when to use it
Works out the bags of plaster for walls or ceilings at your coat thickness, from the coverage printed on the bag, with waste and cost.
How to use it, step by step
- Enter the area to plaster.
- Enter your coat thickness.
- Copy the bag’s coverage and the thickness it is based on.
- Enter the bag weight, waste and price.
What each input means
- Area to plaster – Starts at 30 m².
- Coat thickness – Starts at 2 mm, a typical finishing skim.
- Coverage per bag – Starts at 10 m² at 2 mm, as printed on a 25 kg bag of a common finishing plaster.
- Waste – Starts at 10%.
How to read the results
- Bags to buy – Whole bags with the total weight and cost.
- Coverage at your thickness – The bag’s coverage scaled to your coat.
- Plaster per unit area – Useful for comparing products.
The formula
Coverage at your thickness = bag coverage × stated thickness ÷ your thickness. Bags = area ÷ that coverage × (1 + waste), rounded up.
A worked example
With the starting entries: 30 m² ÷ 10 m² per bag = 3 bags; with 10% waste 3.3, so buy 4 bags of 25 kg.
Assumptions and limits
- Coverage scales with thickness for an even coat.
- Uneven walls use more plaster than their area suggests.
Common mistakes
- Ignoring the thickness that the bag’s coverage is based on.
- Applying finish plaster too thick in one coat.
- Buying plaster long before the job; it has a shelf life.
Frequently asked questions
How thick is a skim coat?
About 2 mm in total, often in two thin coats.
How much does a 25 kg bag cover?
It depends on the product; check the bag. Finishing plasters are often sold as covering about 10 m² at 2 mm.
Do I need a bonding agent?
On very smooth or very absorbent surfaces, often yes. Follow the plaster maker’s advice.
Related tools
How it adds up
- Coverage at your thickness
- 10 m² per bag10 × 2 ÷ 2
- Bags before waste
- 3
- With waste
- 3.3
- Plaster per m²
- 2.5 kg