Plastering quantities start from the simplest of volumes — the wall area times the coat thickness — but that thin wet film hides two adjustments. Mixing cement and sand with water fills the voids, so the loose dry materials you buy take up more room than the mortar on the wall; and plastering wastes material to droppings and uneven backgrounds. A single dry/wastage factor of about 1.30 covers both, after which the dry volume splits by the cement:sand ratio into bags of cement and cubic metres of sand.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: cement-sand mortar volume-batching practice.
The plaster equations
Convert the thickness to metres (12 mm = 0.012 m) so the wet volume comes out in cubic metres. The dry factor is a single multiplier — 1.30 here, though crews use anywhere from 1.27 to 1.35 depending on how rough the background is. Cement weight follows from its 1,440 kg/m³ density and 50 kg bag, while sand stays a volume because it is sold and batched by the cubic metre.
Worked example — plastering 100 m²
Scenario: 100 m² of internal wall, 12 mm thick, in a 1:6 cement-sand mix.
The 100 m² wall needs about 6.4 bags of cement and 1.34 m³ of sand for a 1:6 internal plaster. Choose a stronger 1:4 mix for an external coat and the cement rises to about 9 bags for the same area, while the sand drops to ~1.25 m³ — leaner mixes use less cement but more sand. Round bags up and keep a little spare for the inevitable droppings.
Frequently Asked Questions
wet = area × thickness, dry = wet × ~1.30, then split by the ratio. 100 m² at 12 mm 1:4 → ~9 bags cement, 1.25 m³ sand.
1:4 for strong/external coats; 1:5–1:6 for internal walls and ceilings where workability matters more than strength.
~12 mm internal walls, ~6 mm ceilings, 15–20 mm external/rough. Material scales directly with thickness.
Mixing fills voids and plastering loses droppings, so dry materials exceed wet mortar. ~1.27–1.35 covers it.
At 12 mm: ~6.4 bags (1:6) or ~9 bags (1:4), plus sand. Round up and allow for wastage.