Formula
Wet volume = area × thickness. Dry volume = wet × 1.33, split by the mix ratio; one cement bag = 0.035 m³. Wastage is applied to the cement and sand quantities.
Standard / source
Standard site-estimating practice for plaster quantities, using the conventional 1.33 dry-to-wet factor. Typical thicknesses are about 12 mm internal and 15–20 mm external, but the specification for the job governs.
Worked example
10 m² at 12 mm, 1:4 mix, 5% wastage: wet volume = 10 × 0.012 = 0.12 m³; dry volume = 0.12 × 1.33 ≈ 0.160 m³. Cement ≈ 0.160/5/0.035 × 1.05 ≈ 0.96 bags; sand ≈ 0.160 × 4/5 × 1.05 ≈ 0.134 m³.
Safety note
Ordering estimate only. Uneven walls, dubbing-out coats, openings and site losses change quantities. No prices are hard-coded — apply your local rates.