Plaster & Putty Calculator
Dry mix and bags for levelling walls at a given layer thickness.
How it works
Dry mix weight is the surface area multiplied by the average layer thickness and the consumption per square metre for each millimetre. The preset gives a sensible starting point, but the rate printed on the exact bag is the number to enter.
Use the average thickness across the wall, not the minimum measured at the highest spot. A few deep hollows can move the total considerably. The allowance covers what remains in the tub, falls from the hawk and is lost while ruling off and sanding.
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How the Plaster & Putty Calculator works
A plaster and putty calculator turns surface area and average layer thickness into dry-mix weight and whole bags. The presets provide representative coverage rates, but the final calculation should always use the figure printed on the product you are buying.
Coverage is measured for every millimetre
Plaster rates are often quoted for a 10 mm coat: around 8.5 kg/m² for a lightweight gypsum plaster or 14 kg/m² for a cement product. The calculator normalises those to 0.85 and 1.4 kg/m² per millimetre, then multiplies by the actual layer thickness.
Putty is spread much thinner but its per-millimetre rate is similar: roughly 0.9 kg/m² for a base gypsum coat and 1.2 kg/m² for a polymer finish. These are starting values, not universal constants — the formulation, grain and density of the exact product move the result.
Average thickness matters more than area
Wall area is easy to measure; the common mistake is entering the thinnest coat instead of the average. If the screeds stand 10 mm proud at a high spot and 22 mm at a hollow, the whole wall cannot be priced at 10 mm. Take readings on a grid and average them: on an uneven substrate thickness drives most of the cost.
A ten-percent allowance covers mix left in the tub and on tools, droppage, ruling-off and sanding loss. Subtract large windows and doors from the area. Once the wall is level, calculate the paint or wallpaper separately.
FAQ
How many bags of gypsum plaster cover 40 m²?expand_more
At a 10 mm average coat and 0.85 kg/m²·mm, the net weight is 340 kg. A ten-percent allowance brings that to 374 kg, or 13 whole 30 kg bags after rounding up.
How is putty calculated differently from plaster?expand_more
The formula is the same, but the thickness is not. Plaster corrects wall geometry in layers measured in millimetres or centimetres; putty closes small defects, commonly in a 0.2–3 mm coat. Never reuse one product's coverage rate for the other.
Can the whole thickness be applied in one coat?expand_more
Not always. Every product has a minimum and maximum thickness per application. The calculator warns when the value falls outside the preset's representative range, but the technical sheet is decisive: an over-thick coat can slump or crack.