Plaster is a thin coat of cement-sand mortar applied to masonry walls and ceilings to give a smooth, level, protective surface ready for painting. Beyond appearance, it shields the brickwork from weather, improves water resistance and hides minor unevenness in the wall.
Plaster mix ratios are typically 1:4 to 1:6 (cement:sand) for internal walls and a richer 1:4 for external walls that face the weather. Thickness is usually 12 mm internally, 15–20 mm externally (often in two coats), and 6–8 mm on ceilings. A richer mix is stronger and more water-resistant but more prone to shrinkage cracks if overdone, so the ratio is matched to the job rather than simply maximised.
Estimating plaster uses the wall area and thickness to get a wet volume, then applies a mortar dry-volume factor of about 1.33 (not the 1.54 used for concrete, because mortar has no coarse aggregate and bulks up less) before splitting into cement and sand.