How the plaster estimate works
Plaster is measured by the wet volume you apply — area times thickness — but dry cement and sand shrink and settle, so the industry multiplies by about 1.33 to get the dry volume you actually need to buy. That dry volume is then split in the ratio of cement to sand. A cement bag is taken as 50 kg with a density of 1,440 kg/m³, which is how the volume converts into bags. A 12 mm coat at 1:6 is the common wall plaster mix.
Frequently asked questions
What mix ratio should I use?
1:6 (cement:sand) is standard for 12 mm internal wall plaster; 1:4 is used for stronger external or ceiling plaster.
Why multiply by 1.33?
Dry materials occupy more volume than the finished wet plaster, and some is lost to voids and wastage. The 1.33 factor is the widely used allowance.
How thick should plaster be?
Internal walls are usually 12 mm; rough external walls 15–20 mm; ceilings around 6 mm.