Plaster Calculator
The plaster for a wall or ceiling: the volume at your thickness and the bags to buy, from the coverage printed on your bag.
Result…
How it's calculated
Area = walls length × height − openings (or the area you enter) Volume = area × thickness · With waste = volume × (1 + waste %) Volume per bag = coverage × its thickness (or bag weight ÷ density) Bags = volume with waste ÷ volume per bag, rounded up
Plaster is worked out by volume: the area times the thickness. A bag's coverage is quoted at one thickness, so the calculator turns it into a volume per bag and fits that to your thickness: a bag that covers a certain area at 10 mm covers half that at 20 mm. If you have the bag weight and the product's dry density instead, the volume per bag is the weight divided by the density. Lengths in metres or feet; thickness in millimetres or inches (1 in = 25.4 mm). The bag count is rounded up.
Your numbers, step by step
About the Plaster calculator
This plaster calculator estimates how much plaster a wall or ceiling needs and how many bags to buy. Enter the area directly, or the total length and height of the walls with the doors and windows to subtract, then the plaster thickness and a waste allowance. For the bags, use the coverage printed on your product, which is always quoted at one thickness, or the bag weight with the product's dry density from its data sheet. Plaster products differ a lot, from thin finishing coats to thick base coats and renders, so the calculator never assumes a coverage: the starting values are only examples to replace with yours. It works in square metres and millimetres or square feet and inches, and the steps show the volume and the bag count with your numbers. For the wall area of a room, the square footage calculator can help.
How to use it
- Choose “Metric” or “Imperial” and, under “Area from”, pick “Area I know” or “Walls minus openings”.
- Enter the “Area to plaster”, or the “Length of the walls”, “Wall height” and “Doors and windows to subtract”.
- Enter the “Plaster thickness”.
- Under “Bag information”, enter the “Coverage per bag” and the thickness it is quoted “At a thickness of”, or the “Bag weight” and “Dry plaster per volume”.
- Set the “Waste allowance” and read the bags, the volume and the volume with waste.
Worked example
Walls 14 m long in total and 2.4 m high, with 3.6 m² of doors and windows, give 14 × 2.4 − 3.6 = 30 m². At 11 mm thick that is 30 × 11 ÷ 1000 = 0.33 m³, or 0.363 m³ with 10% waste. If the bag says it covers 2.4 m² at 10 mm, one bag makes 2.4 × 10 ÷ 1000 = 0.024 m³, so you need 0.363 ÷ 0.024 = 15.1, rounded up to 16 bags (14 without the waste). In imperial, 300 ft² at 3/8 in is 9.375 ft³, or 10.3125 ft³ with 10% waste; with 50 lb bags and 60 lb of dry plaster per ft³, each bag makes 0.833 ft³, so you need 13 bags.
Updated
Questions people ask
How do I calculate how much plaster I need?
Multiply the area by the thickness to get the volume, add a waste allowance, and divide by the volume one bag makes. One bag's volume is its coverage times the thickness that coverage is quoted for.
How many square metres does a bag of plaster cover?
It depends on the product and the thickness, so read it from the bag or the maker's data sheet. Because coverage halves when the thickness doubles, the calculator converts the printed figure to your own thickness.
What if the data sheet gives kg per m² per mm?
Multiply that figure by 1000 to get kilograms per cubic metre, and enter it as “Dry plaster per volume” with “Bag weight and density” chosen.
How much waste should I allow?
It depends on the job and on how even the walls are. Uneven or very absorbent walls take more than a flat, sealed surface, so a waste allowance covers mixing, spills and hollows.
What does the estimate leave out?
It counts one coat at one average thickness. Base coats and finishing coats are separate products with different coverage, so work out each coat on its own. The volume converter turns cubic metres into litres or cubic feet.