Plaster calculator — how many bags of plaster do you need?

Plaster quantity (kg) = area (m²) × layer thickness (m) × density (kg/m³) × (1 + waste/100). Standard plaster density is about 1,600 kg/m³. Example: 20 m², 15 mm thick, 10% waste: 20 × 0.015 × 1,600 × 1.1 = 528 kg = 22 × 25 kg bags.

Standard layer thickness: gypsum plaster (interior) 10–15 mm; cement-lime plaster (interior) 15–20 mm; external render 15–25 mm; levelling plaster on uneven surfaces up to 30 mm. Smooth new concrete block walls need as little as 10 mm; old brick walls often require 20 mm or more.

Gypsum plaster: faster to apply, smooth finish ready to paint, interior use only, not moisture-resistant. Cement-lime plaster: more moisture and impact resistant, used inside and outside, requires skimming before painting. Gypsum is more popular in new builds; cement-lime in renovations.

At 15 mm thickness and 1,600 kg/m³ density: 1 m² × 0.015 m × 1,600 = 24 kg per m². One 25 kg bag covers about 1 m² at 15 mm. Adding 10% waste, 10 m² needs about 11 bags. Check the manufacturer's stated coverage on the packaging, as it may differ.

The waste allowance compensates for: losses during mixing and application (plaster sticks to tools, spill-off during throwing), substrate unevenness (actual thickness greater than nominal), patching and touch-ups, and batch differences. Recommended: 10–15% for smooth substrates, 15–20% for uneven ones.

Priming before plastering is usually necessary. Primer evens out substrate absorbency and prevents the plaster from drying too quickly, which would weaken adhesion and strength. Use deep-penetrating primer on new concrete blocks and bonding primer on dusty or weak old surfaces. Allow 2–4 hours drying time.

Before applying plaster: remove old loose plaster and grease or dust contamination; fill large voids with mortar or foam; apply reinforcing mesh at junctions of different materials (e.g. concrete–brick); prime with an appropriate product; dampen highly absorbent substrates before applying cement plaster.

Gypsum plaster: dry to the touch after 2–4 hours, ready to skim after 24–48 hours, ready to paint after 7–14 days. Cement-lime plaster: dries much slower, can be painted after 28 days (full carbonation). Do not accelerate drying with heat in the first 24 hours — it causes cracking.

Approximate prices per 25 kg bag: gypsum plaster 18–35 PLN; cement-lime plaster 12–25 PLN; machine gypsum plaster 20–40 PLN; external render 25–60 PLN. Labour: 35–70 PLN/m² for machine plastering, 50–90 PLN/m² for hand plastering. Add primer, mesh and corner beads.

Hand plastering of small areas (reveals, patches) is manageable for a DIYer. Plastering an entire room requires skill — keeping an even thickness and a true vertical is hard without experience. Machine plastering is about 5 times faster and gives a better finish, but requires specialist equipment. For large areas, hiring a professional is usually more cost-effective.