Plaster Calculator

Volume = area × thickness, plus waste; then split by the mix. Plastering 430 sq ft (40 m²) at ½ in (12 mm) with 10% waste is about 19 ft³ (0.53 m³) of wet plaster — roughly 24 ft³ (0.69 m³) of dry material once bulking is counted. At a 1:5 mix that is about 3.3 bags of 94 lb (50 kg) cement and 0.75 yd³ (0.57 m³) of plaster sand. The calculator does the arithmetic for any area and mix.

1:5 or 1:6 (cement:sand) suits internal walls; 1:4 is common outside and in wet areas where a stronger, denser coat matters. Richer than 1:4 tends to crack; leaner than 1:6 gets weak and dusty. Use clean, well-graded plaster sand — the sand quality shows in the finish.

⅜–⅝ in (10–15 mm) in a single coat is the norm for masonry walls, with ½ in (12 mm) a good target. Don’t exceed about ⅝ in (15 mm) in one coat — thick builds slump and crack; where walls are badly out of true, plaster in two coats and let the first set. External work is often two coats totalling ⅝–¾ in (15–20 mm).

At a 1:5 mix and ½ in (12 mm) thickness, a 94 lb (50 kg) bag of cement plasters roughly 110–130 sq ft (10–12 m²). It falls fast with thickness — the same bag covers only about 85 sq ft (8 m²) at ⅝ in (15 mm) — which is why an accurate average thickness matters more than any other input.

The wall was probably less true than assumed — plaster fills the hollows, so the real average thickness beats the nominal figure. Raked joints, porous brick sucking moisture, and droppage add more. The waste allowance covers normal jobs; allow extra on rough stone or badly built walls.