Show the formula and working
This plaster calculator works out how many bags of finishing plaster or undercoat plaster you need to skim or plaster the walls and ceiling of a room. It is built for UK and European rooms measured in metres, with a feet and inches switch, and it uses the coverage figures that British Gypsum publishes for its Thistle plasters, or any coverage you take from your own bag.
Quick answer: A 25 kg bag of Thistle MultiFinish covers about 10 m² at 2 mm thick. Work out the wall area, take off doors and windows, add 10% and divide by 10. A 4 m by 3.5 m room with 2.4 m walls has 32.4 m² to skim, so the plaster calculator gives 4 bags.
What Is a Plaster Calculator?
A plaster calculator is a tool that turns the area of a wall or ceiling and the coat thickness into the weight and number of bags of plaster to buy. Wall area is the room perimeter, 2 x (length + width), multiplied by the height; the ceiling is length x width. Subtract doors and windows and add a waste allowance.
Plaster needed per square metre comes from the maker’s coverage figure. The Thistle MultiFinish product data sheet gives 10 m² per 25 kg bag at 2 mm, which is 2.5 kg per m². The calculator scales this in proportion to the thickness you enter, multiplies by the area and divides by the bag weight, rounding up to whole bags.
How Do You Use the Plaster Calculator?
- Choose What are you plastering?: all walls of a room, or one wall or one ceiling.
- Enter Length, Width and Height. For one ceiling, enter its width in Height. Tick Include the ceiling to add it to a room.
- Enter Doors and windows to leave out in square metres.
- Pick the Plaster and set the Thickness. For another brand, choose Other plaster and fill in Other plaster: coverage per bag and Other plaster: at thickness from the bag.
- Check Bag weight and Extra for waste.
- Read the bags and weight, then open “Show the formula and working” to see each step with your numbers.
How Many m² Does a Bag of Plaster Cover?
A 25 kg bag of skim plaster such as Thistle MultiFinish or BoardFinish covers about 10 m² at 2 mm, while undercoat plasters cover far less because they go on much thicker. British Gypsum gives 3 m² per bag for Thistle HardWall at 11 mm and 3.5 m² per bag for Thistle BondingCoat at 8 mm. Real coverage drops on uneven walls, so keep a waste allowance.
| Plaster (25 kg bag) | Use | Coverage per bag | Plaster per m² | Bags per 10 m² |
|---|---|---|---|---|
| Thistle MultiFinish | Skim on plaster or plasterboard | 10 m² at 2 mm | 2.5 kg | 1.0 |
| Thistle BoardFinish | Skim on plasterboard | 10 m² at 2 mm | 2.5 kg | 1.0 |
| Thistle HardWall | Undercoat on medium to high suction masonry | 3 m² at 11 mm | 8.3 kg | 3.3 |
| Thistle BondingCoat | Undercoat on low suction backgrounds | 3.5 m² at 8 mm | 7.1 kg | 2.9 |
| Thistle BondingCoat | Undercoat on low suction backgrounds | 2.75 m² at 11 mm | 9.1 kg | 3.6 |
Bags per 10 m² are before any waste allowance. Figures are from British Gypsum product data sheets, including the Thistle HardWall data sheet.
How Thick Should Plaster Be?
A finishing skim is 2 mm thick, and British Gypsum lists 2 mm as the application thickness for both MultiFinish and BoardFinish on walls and ceilings. Undercoats are much thicker: HardWall is applied at 11 mm on walls and 8 mm on ceilings, and BondingCoat at 8 to 11 mm. The calculator warns if your thickness is far from the maker’s figure.
How Much Plaster Do I Need to Skim a Room?
An average bedroom of about 4 m by 3.5 m with 2.4 m walls takes 4 bags of 25 kg skim plaster for the walls, and about 6 bags if you add the ceiling. Bigger rooms scale in proportion: allow roughly one 25 kg bag of skim for every 9 m² of wall once waste is included.
After plastering, the paint calculator works out how much emulsion the new walls will need, and the paint and decorating calculators hub covers ceilings, wallpaper and painting time.
Worked Example: Skimming a Bedroom
Example: you are skimming the walls of a bedroom 4 m long and 3.5 m wide with 2.4 m high walls, using Thistle MultiFinish at 2 mm. The wall area is 2 x (4 + 3.5) x 2.4 = 36 m². Leaving out a door and window of 3.6 m² gives 32.4 m², and 10% extra makes 35.64 m². At 2.5 kg per m² you need 89.1 kg, which is 3.56 bags, rounded up to 4 bags of 25 kg. These are the default inputs, so the calculator shows the same result.
Plaster Calculator FAQs
How many bags of multi finish do I need?
Divide the area to skim in square metres by 10, because a 25 kg bag of Thistle MultiFinish covers about 10 m² at 2 mm, then add around 10% and round up. A 30 m² area needs 3.3 bags, so buy 4 bags to allow for waste and uneven walls.
What is the difference between MultiFinish and BoardFinish?
BoardFinish is a skim plaster made for plasterboard and other low suction backgrounds, while MultiFinish is designed to go over most common backgrounds, including undercoat plasters and plasterboard. Both cover about 10 m² per 25 kg bag at 2 mm, so the bag count is the same.
How much does a 25 kg bag of plaster cover?
It depends on the plaster and the thickness. A skim plaster covers about 10 m² at 2 mm, HardWall about 3 m² at 11 mm and BondingCoat about 3.5 m² at 8 mm, according to British Gypsum. Thicker coats cover proportionally less, so a 3 mm skim covers about 6.7 m².
Do I include doors and windows when measuring for plaster?
Measure the full wall area, then subtract doors and windows. A typical internal door is about 1.6 m². Window reveals still need plastering, so add the reveal area back in if the windows are deep or numerous.
How long does skim plaster take to set?
British Gypsum gives a setting time of about 1.5 hours for Thistle MultiFinish. Setting is not the same as drying: the plaster must dry out fully before decorating, which takes much longer and depends on thickness, temperature and ventilation. Mix only what you can apply within the working time.
How much plaster do I need for a ceiling?
Multiply the ceiling length by its width to get the area, add 10%, then divide by the coverage per bag. A 4 m by 3.5 m ceiling is 14 m², or 15.4 m² with waste, so it needs 2 bags of skim plaster at 2 mm. Choose One wall or one ceiling in the tool.
Checked October 2026. Sources: British Gypsum: Thistle MultiFinish product data sheet, British Gypsum: Thistle BoardFinish product data sheet, British Gypsum: Thistle HardWall product data sheet, British Gypsum: Thistle BondingCoat product data sheet.