CostBuildrPlaster Quantity Calculation Formula
The plaster quantity formula is straightforward once you understand the three steps: calculate wet volume, convert to dry, and split by ratio. This guide provides the formula, worked examples, and a per-sq-ft reference table so you can estimate plaster materials in seconds.
The Three-Step Formula
Step 1 — Wet volume: Area (sq ft) × thickness (ft). For 12 mm thick plaster on 1,000 sq ft: 1,000 × 0.04 = 40 cft. Step 2 — Dry volume: multiply by 1.33 (the plaster dry-to-wet factor, lower than concrete's 1.54 because plaster mortar compacts less): 40 × 1.33 = 53.2 cft.
Step 3 — Split by ratio. For 1:4 (5 total parts): cement = 53.2/5 = 10.64 cft = 10.64/1.25 = 8.5 bags. Sand = 53.2 × 4/5 = 42.6 cft. For 1:6 (7 total parts): cement = 53.2/7/1.25 = 6.1 bags. Sand = 53.2 × 6/7 = 45.6 cft. The 1:6 ratio saves about 28% cement but uses slightly more sand.
Per-Square-Foot Reference Table
For quick site estimation—cement bags per 100 sq ft: 12 mm at 1:3 = 1.13 bags; 12 mm at 1:4 = 0.85 bags; 12 mm at 1:6 = 0.61 bags; 20 mm at 1:4 = 1.42 bags; 20 mm at 1:6 = 1.01 bags. Sand per 100 sq ft: 12 mm at 1:4 = 4.3 cft; 20 mm at 1:4 = 7.1 cft.
Print this table and keep it on site for instant reference. When your mason reports completing 200 sq ft of plastering today, you can quickly verify: at 1:4 and 12 mm, he should have used about 1.7 bags of cement and 8.6 cft of sand. Significant deviations indicate inconsistent mixing or incorrect thickness.
Accounting for Waste and Unevenness
Brick walls are rarely perfectly plumb, so actual plaster thickness varies from 8 mm to 25 mm across a wall. The nominal 12 mm is an average. Add 15–20% to your calculated material quantities to account for this variation and for drops/spillage during application.
For ceiling plaster, waste is higher (20–25%) because plaster falls during overhead application. Exterior walls with exposed brick faces (rougher surface) consume 10–15% more mortar than interior walls with neat brick joints. Factor these into your estimate and adjust based on experience with your specific masons and wall quality.
أسئلة شائعة
What is the dry volume factor for plaster?
1.33 (compared to 1.54 for concrete). Plaster mortar has only two ingredients (cement and sand) with finer grading and smaller voids, resulting in less volume expansion from dry to wet state.
How much cement for plastering a 10 marla house?
A two-story 10 marla house has roughly 10,000–12,000 sq ft of plaster area (interior + exterior). At 12 mm thick with 1:4 ratio: 85–102 bags of cement. With 20 mm exterior coat on 3,000 sq ft: add another 42 bags. Total: approximately 130–145 bags for all plastering.
Why is the plaster dry volume factor 1.33, not 1.54?
Concrete has three ingredients with coarse aggregate creating large voids. Plaster mortar has only cement and fine sand—the particles pack more tightly, so less dry material is needed per unit of wet mortar. The exact factor varies from 1.30 to 1.35; 1.33 is the standard site estimate.