How to use this calculator
- 1Measure the length and height of a wall (or the length and width of a ceiling), enter the total area, or enter a whole room with its doors and windows.
- 2Choose finishing plaster, undercoat, or cement–sand plaster mixed on site. The usual thickness and consumption fill in.
- 3Change the thickness if you are applying more or less, and replace the consumption with the figure on your bag if it differs.
- 4Enter the bag weight to see how many bags to buy. For cement–sand plaster, choose the mix and your cement bag instead.
The formula
- 01Plaster (kg) = area (m²) × thickness (mm) × consumption (kg per m² per mm) × (1 + extra %)
- 02Bags = plaster ÷ bag weight, rounded up
- 03Area per bag = bag weight ÷ (consumption × thickness)
- 04Room: area = 2 × (length + width) × height − doors − windows (+ ceiling)
- 05Cement–sand: dry volume = area × thickness × (1 + extra %) × 1.33
- 06Cement = dry volume ÷ (1 + sand parts) × 1,440 kg/m³; bags = cement ÷ bag weight, rounded up; sand = dry volume × sand parts ÷ all parts
Plaster manufacturers state coverage as the area one bag covers at a given thickness. Turning that into a consumption per millimetre lets you work out any thickness. For example, a 25 kg bag of finishing plaster that covers 10 m² at 2 mm uses 1.25 kg per m² for each millimetre.
Undercoat plasters are lighter per millimetre but go on much thicker. British Gypsum's HardWall data (about 3 m² per 25 kg bag at 11 mm) and Knauf's Rotband (about 0.8 kg per m² per mm) are close to 0.8; one large retailer quotes 2.25 m² per bag at 13 mm, which is 0.85. We use 0.85 so you are less likely to run short.
In the US, veneer plaster finish is applied 1/16–3/32 in thick; at 1/16 in our finishing figure gives about 123 ft² per 50 lb bag, a little below the 130–160 ft² manufacturers quote, which leaves some margin.
Cement–sand plaster (render) is mixed on site by volume, as in Pakistan, India and the Gulf. The simple method — wet volume × 1.33, split by the mix — gives 4.6 kg of cement per m² for 12 mm of 1 : 4. India's CPWD, which publishes the consumption used for estimates, allows 5.47 kg (and 3.60 kg for 1 : 6, 8.51 kg for 20 mm of 1 : 4), because real walls are uneven and the mortar fills the joints. Adding about 20 % brings the simple method up to those figures, so you do not run short.
Worked example
Skimming a 4 m × 2.5 m wall at 2 mm with finishing plaster at 1.25 kg/m² per mm takes 25 kg.
With 10% extra that is 27.5 kg: buy 2 bags of 25 kg. Each bag covers about 10 m² at this thickness.
Tips before you buy
- Mix only as much as you can apply and trowel before it starts to set — the setting time is on the bag. Never add more water to plaster that is already setting.
- Seal very absorbent walls or old paint with the primer or bonding agent the plaster maker recommends, or the plaster dries too fast and cracks.
- Check the date on the bag: gypsum plaster that is past its date or has got damp sets unpredictably.
- Buy an extra bag for a large job. Running out halfway across a wall leaves a visible joint.
Frequently asked questions
How many m² does a bag of plaster cover?
A 25 kg bag of finishing (skim) plaster covers about 10 m² at 2 mm thick. A 25 kg bag of undercoat covers only about 2.5–3 m² at 11 mm, because it goes on so much thicker. Check your own bag's figure.
How much plaster do I need to skim a room?
Add up the wall and ceiling areas. At 2 mm, allow about 2.5 kg per m² plus 10 % extra — roughly one 25 kg bag for every 9 m². A 3.5 × 3 m bedroom with 2.4 m high walls and its ceiling is about 40 m² before openings, so around five bags.
How thick should a skim coat be?
About 2 mm in total, applied as two thin coats. Undercoats are usually around 10–11 mm on walls, finished with a 2 mm skim, but always follow the thicknesses on the bag.
How many square feet does a 50 lb bag of veneer plaster cover?
USG quotes roughly 130–160 ft² per 50 lb bag for its veneer finish plasters, applied 1/16–3/32 in thick. This calculator uses a slightly more cautious figure; change it to your bag's coverage if you prefer.
How much cement and sand do I need for 100 ft² of plaster?
For 12 mm (½ in) of 1 : 6 plaster with 20 % extra, about 0.73 bag of 50 kg cement and 5.4 ft³ of sand. A 1 : 4 mix needs about 1 bag and 5.0 ft³ of sand. Choose "Cement–sand plaster" to work out your own walls.
How thick should cement plaster be?
Usually about 12 mm (½ in) on inside walls, up to 15 mm on uneven brickwork and about 20 mm (¾ in) on outside walls, where it is applied in two coats. Ceilings get a thinner coat. Follow your drawings or the local practice.
Can I use this for drywall joint compound?
Not directly. Joint compound is sold ready-mixed in tubs as well as in bags, and its coverage is usually given per area of drywall (plasterboard) finished, including taping, rather than per millimetre of thickness. Use the maker's coverage figure for joint compound.
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Sources
- British Gypsum Thistle Multi-Finish data sheet: 10 m² per 25 kg at 2 mm
- British Gypsum Thistle HardWall data sheet: 3.0 m² per 25 kg at 11 mm
- Knauf Rotband technical sheet: about 0.8 kg/m² per mm
- USG Imperial veneer finish: coverage per 50 lb bag
- CPWD cement consumption: 12 mm plaster 0.109 bags/m² (1:4) and 0.072 (1:6); 20 mm 0.170 (1:4) — The Constructor
- Cement for 12 mm plaster by the dry-volume method (factor 1.33, 1,440 kg/m³) — Civil Sir
Formulas last reviewed: September 30, 2026 ·How we check our formulas