How plaster is estimated
Plaster wet volume is the area to be plastered multiplied by the thickness. Because mortar packs denser when dry than the finished applied layer, a 1.33 dry-volume factor (not the 1.54 used for concrete) converts wet volume to the dry material you must buy, which is then split by the cement:sand ratio. For a 10 m² wall at 12 mm in a 1:4 mix, that comes to roughly one 50 kg bag of cement and about 0.13 m³ of sand.
Choosing thickness and ratio
Plaster thickness and mix richness depend on the surface and exposure. A richer mix (more cement) is stronger and more water-resistant but also more prone to shrinkage cracks if overdone, so the ratio is chosen to match the job rather than simply maximised.
- Internal walls: 12 mm thick, 1:4 to 1:6 mix
- External walls: 15–20 mm, 1:4 (stronger, weather-facing)
- Ceilings: 6–8 mm, 1:3 to 1:4
- Rough/single-coat: often done in two coats for thicker external work
How to measure the area
For a single wall, area is length × height. For a whole room or house, total all the wall faces (and ceilings, if plastering them), then deduct large openings like doors and big windows — small openings are often left in as a built-in wastage margin. Enter the combined area here in one go to get the total cement and sand.
Why mortar uses 1.33, not 1.54
Concrete contains coarse aggregate with large voids between stones, so its dry ingredients bulk up by about 54%. Mortar is only cement and sand — finer particles, smaller voids — so it bulks up less, about 33%. Using the wrong factor is a common estimating error that leaves you short of, or oversupplied with, cement.