The amount of plaster depends on three numbers: the area, how thick you lay it and how heavy the product is per millimeter. 40 m² of cement-lime render, 15 mm thick, at 1.4 kg per m² per mm, with 10% extra: 40 × 15 × 1.4 × 1.1 = 924 kg, or 37 bags of 25 kg. Choose a material to start from, type the consumption printed on your bag if it differs, and the calculator gives bags, total weight, what one bag covers and the cost.
Typical consumption, per 10 m²
| Material | Thickness | Per m² | 10 m², +10% | 25 kg bags |
|---|---|---|---|---|
| Cement-lime plaster or render (base) | 15 mm | 21 kg | 231 kg | 10 |
| Gypsum plaster, one-coat or base | 10 mm | 10 kg | 110 kg | 5 |
| Gypsum finishing plaster (skim) | 2 mm | 2 kg | 22 kg | 1 |
| Cement-based skim coat | 2 mm | 2.6 kg | 29 kg | 2 |
These consumptions are typical datasheet values: cement-lime about 1.4 kg per m² per mm, gypsum products about 1.0, cement skim coats about 1.3. Your product prints its own figure, and the calculator uses whatever you type.
Metric and US bags
A consumption of 1 kg per m² per mm is the same as 1 kg per liter of finished plaster, or about 5.2 lb per ft² per inch of thickness. In the US many products come in 50 lb bags and give their coverage in ft² at a thickness such as 1/8 in; the calculator takes thicknesses in inches and bag weights in pounds and converts for you.
Getting the thickness right
The thickness that matters is the average, not the nominal. Hold a straightedge across the wall: if the gaps reach 10 mm in places on a coat planned at 5 mm, the real average may be 7–8 mm, and the bags run out early. Fill deep holes first with a base coat, then finish at an even thickness.
The calculator works offline, so the numbers are there in a van or a store with no signal.
Frequently asked questions
How much plaster do I need per square meter?
Area × thickness in mm × the consumption on the bag. A cement-lime base coat at about 1.4 kg per m² per mm, 15 mm thick, uses 21 kg per m²; a 2 mm gypsum finish at about 1.0 uses 2 kg per m².
How do I read the coverage on a bag?
If the bag says '14 kg/m² at 10 mm', the consumption is 1.4 kg per m² per mm. If it gives a coverage per bag, such as '25 kg covers 10 m² at 2 mm', divide: 25 ÷ 10 ÷ 2 = 1.25.
How thick should plaster be?
Base coats are typically 10–20 mm (3/8–3/4 in); finishing or skim coats 1–3 mm (about 1/16–1/8 in). The datasheet gives the range the product is made for.
How much extra should I buy?
About 10% for mixing losses and uneven walls; more on old, out-of-true walls, where the coat is thicker in places than the average.
How many square feet does a 50 lb bag cover?
It depends on the product and the thickness. At 1.0 kg per m² per mm and 1/8 in (3.2 mm), a 50 lb (22.7 kg) bag covers about 7.1 m², or 77 ft².
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