Plaster is a volume problem: wall area times coat thickness gives the wet volume, and the mix ratio splits that into cement bags and sand. The thickness matters more than any other number — a couple of extra millimetres across a whole wall adds up fast.
The method
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.
Mix ratio
| Mix (cement:sand) | Use |
|---|---|
| 1:5 or 1:6 | Internal walls |
| 1:4 | External and wet areas — stronger, denser coat |
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.
Plaster Calculator
Enter wall area, coat thickness and mix ratio for the cement bags and sand to buy.
How thick?
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).
Coverage per bag
At a 1:5 mix and ½ in (12 mm), 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). That sensitivity is why an accurate average thickness matters more than any other input.
Why you used more than calculated
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.