Plastering Calculator

Enter each wall or ceiling (length and height), subtract doors and windows, set the plaster thickness and mix ratio, and the plastering calculator computes wet mortar volume, dry volume (wet x 1.33), then splits it into cement and sand. Example: both sides of a 5 m x 3.5 m wall at 12 mm with a 1:3 mix need 201.1 kg of cement (4.02 bags, buy 5 bags) and 0.419 m³ of sand.

At 12 mm thickness with a 1:4 mix and 10% wastage, 100 sq ft of plastering needs 103.6 lb of cement, which is 0.94 bags (buy 1 x 50-kg bag), plus 4.61 ft³ of sand. In metric that is 47 kg of cement and 0.13 m³ of sand.

A 1:4 plaster ratio means 1 part cement to 4 parts sand by volume. It is the standard recommendation for external walls in normal climates, ceiling plastering and plastering over concrete walls or columns. For reference, 1 m³ of wet mortar at 1:4 becomes 1.33 m³ of dry mortar, needing 383 kg of cement (7.66 bags) and 1.064 m³ of sand.

Four steps: 1) net area = length x height minus doors and windows; 2) wet mortar = area x thickness; 3) dry mortar = wet x 1.33 (the dry volume of mortar factor); 4) split by ratio: cement = dry / (1 + sand parts) x 1440 kg/m³, sand = dry x sand parts / (1 + sand parts). Worked example: 5 m x 3.5 m, both sides, 12 mm, 1:3 gives 0.42 m³ wet, 0.5586 m³ dry, 201.1 kg cement, 0.419 m³ sand.

Usually because of the dry-volume factor: some use 1.33, others 1.54. The 1.33 factor is the standard for cement-sand mortar (it adds ~33% for voids in the sand). The 1.54 factor belongs to concrete (it also covers voids in coarse aggregate), and some sites use ~1.54 because they quietly bundle ~20% wastage into the factor. On the same job (35 m², 12 mm, 1:3) that is 201.1 kg of cement at 1.33 versus 232.8 kg at 1.54. This calculator defaults to the honest 1.33 and applies wastage separately, so you can see both numbers.

The dry volume of mortar is the volume of the loose dry ingredients (cement + sand) needed to make a given volume of wet, mixed mortar. Because sand has about 33% voids, dry volume = wet volume x 1.33. So 1 m³ of wet mortar needs 1.33 m³ of dry ingredients; at a 1:6 mix that splits into exactly 0.19 m³ of cement (273.6 kg, 5.47 bags) and 1.14 m³ of sand.

Ceiling plaster thickness is usually 6-12 mm, with 8 mm the common recommendation (thicker coats risk debonding overhead). Use a 1:4 mix for ceilings. Example: a 10 m x 10 m ceiling at 8 mm and 1:4 needs 306.4 kg of cement (6.13 bags) and 0.851 m³ of sand.

The sand required for plastering 100 sqm at 12 mm is therefore 1.368 m³ of sand (1.505 m³ with 10% wastage). To answer how much sand for plastering any other area, scale these figures by area. With the richer 1:4 mix the same area needs 1.277 m³ of sand, because more of the dry volume is cement. For reference, the 1:6 plaster mix cement bags figure for the same 100 m² is 6.57 bags (buy 7).

For the classic cement bags for 100 sq ft plaster question, the short answer is one 50-kg bag. At 12 mm with a 1:4 mix and no wastage, 100 sq ft of plastering needs 42.7 kg of cement, which is 0.85 bags of 50-kg bags — so you buy 1 bag. Add 10% wastage and it becomes 0.94 bags, still 1 bag to buy, with 4.61 ft³ of sand.

The water required for plastering is easy to estimate once the cement is known: a practical rule of thumb is about 0.5 litres of water per kg of cement (a water-cement ratio near 0.5), added gradually until the mortar is workable — sand moisture varies, so treat it as an estimate. Example: 100 m² of wall at 12 mm with a 1:6 mix uses 328.3 kg of cement, so allow about 164.2 L of water.

External walls are often plastered in two coats: a 12 mm backing coat plus a 6 mm finishing coat. This two-coat plaster mode calculates each coat separately and adds them. Use the two coat plaster calculator mode above (tick Two-coat on any surface card) for the same breakdown on your own walls. Example: 20 m² of external wall, 12 mm backing + 6 mm finish, both 1:4 gives 137.9 kg of cement (2.76 bags), 0.383 m³ of sand and about 68.9 L of water.

Measure the full wall (length x height), then subtract each opening. Example: a 5 m x 3 m wall (15 m²) with one 0.9 m x 2.1 m door (1.89 m²) and two 1.2 m x 1.2 m windows (2.88 m²) has a net plastering area of 10.23 m². The surface builder above does this automatically for every wall and ceiling you add.