A worked example: 5m × 3m wall, 12mm plaster, 1:4 mix
A 5m long, 3m high wall plastered 12mm thick, using a 1:4 (cement:sand) mix with 5% wastage, needs a plaster volume of 0.18 m³, which works out to 1.4 cement bags (50kg each) and 0.2011 m³ of sand.
The formula: wet volume, dry volume, then the mix split
Plaster (wet) volume = length × height × thickness. For the 5m × 3m wall at 12mm (0.012m), that's 5 × 3 × 0.012 = 0.18 m³. Because dry cement and sand settle into a smaller volume once mixed with water, the dry volume needed is larger: dry volume = wet volume × (1 + wastage%) × 1.33, where 1.33 is the standard dry-volume factor for mortar (plaster has no coarse aggregate, unlike concrete's 1.54 factor). That dry volume is then split by the mix ratio's parts — for 1:4, one part cement to four parts sand out of five total parts — and the cement share is converted to 50kg bags using 0.0347 m³ per bag.
A bigger wall: 8m × 3.2m, 15mm plaster, 1:6 mix
An 8m by 3.2m wall plastered 15mm thick with a leaner 1:6 mix and 10% wastage needs 0.384 m³ of wet plaster volume, which comes out to 2.3 cement bags and 0.4815 m³ of sand — a leaner mix ratio and thinner wall both reduce cement demand relative to sand, even though this wall is larger overall than the first example.
Why the mix ratio changes the cement count so much
The mix ratio determines what fraction of the dry volume is cement rather than sand — a 1:3 mix devotes one part in four to cement, while a 1:6 mix devotes only one part in seven, so the same wall area plastered with a richer 1:3 mix (typical for external or higher-strength work) needs noticeably more cement bags per square meter than the same job done with a 1:6 internal-plaster mix, even before wastage or thickness differences are factored in.