Plaster Calculator
Cement and sand for wall and ceiling plaster in 1:3 to 1:6 mix, 6 to 20 mm thick.
Assumptions and rates
Free tool by EngineeringNepal.com.np · Results are for guidance; check critical figures with a qualified engineer.
Plaster quantity: cement and sand
Plaster thickness and mix are usually given in the drawings or specification. In Nepali houses, inside walls typically get 12 mm plaster in 1:4 or 1:6, outside walls 15 to 20 mm in 1:4, and ceilings about 6 to 12 mm in 1:3 or 1:4. This calculator converts the wall area into the cement bags and sand you need to order.
Formula
- Wet volume = plaster area × thickness, plus an allowance for uneven brick surfaces and joints (10% by default).
- Dry volume = wet volume × 1.33, to account for voids and bulking.
- Cement = dry volume ÷ (1 + sand parts); cement bags = cement volume × 1440 ÷ 50.
- Sand = dry volume × sand parts ÷ (1 + sand parts).
Example: 100 m² of 12 mm plaster in 1:4 needs a wet volume of 1.2 m³ + 10% = 1.32 m³, dry volume 1.76 m³, cement 0.35 m³ (about 10 bags) and sand 1.40 m³ (about 50 cft).
Frequently asked questions
Why is the dry factor 1.33 and not 1.54?
Plaster mortar has no coarse aggregate, so it has fewer voids than concrete. 1.27 to 1.33 is the range used in most estimating textbooks; we use 1.33 and let you change it.
Do I deduct doors and windows?
Yes, deduct the openings, but remember the sides (jambs) of openings are also plastered. Many estimators ignore small openings under 0.5 m² for this reason.
How to use the calculator
- Choose Wall size to enter length and height, or Known area if you already have the plaster area from a drawing or estimate.
- For an internal wall that is plastered on both sides, pick Both sides. For a whole room, enter the room perimeter as the length.
- Enter the openings to deduct in the same unit (sq ft or m²). For doors and windows between 0.5 and 3 m², the usual rule (IS 1200 Part 12) is to deduct them from one face only; openings up to 0.5 m² are not deducted.
- Pick the thickness and mix from your drawing. Open Assumptions and rates to change the dry volume factor, the extra for uneven surfaces or to add prices.
Worked examples (results from the calculator above)
Example 1: walls of a 12 × 14 ft bedroom
Perimeter 2 × (12 + 14) = 52 ft, height 9.5 ft, one side. Deduct half the 3 × 7 ft door (10.5 sq ft, the other half comes off the next room) and half the 5 × 4 ft window (10 sq ft, the other half comes off the outside face): 20.5 sq ft. With 12 mm in 1:6 at the default settings, the tool gives 473.5 sq ft (43.99 m²), 3.2 bags of cement and 23.4 cft of sand.
Example 2: a 115 mm internal wall, both faces
Length 40 ft, height 9.5 ft, both sides, two 3 × 7 ft doors deducted once each (21 sq ft). Area 739 sq ft (68.66 m²). In 12 mm 1:6: 5.0 bags and 36.5 cft of sand.
Example 3: outside walls of a two-storey house
Known area 313.4 m² (the outside plaster of the house in our whole-house plaster guide), 20 mm in 1:4. At the default settings the tool gives 52.8 bags and 259.1 cft. Change the dry factor to 1.27 and the extra to 0 and it gives 45.9 bags and 224.9 cft, which is exactly the figure in that guide. The difference (about 15%) is the default allowance: 10% extra for uneven brickwork and a dry factor of 1.33 instead of 1.27.
Which settings should you use?
Use the defaults (1.33 and 10%) for ordering material for rough brick walls, where the mason fills hollows and joints. Use 1.27 and 0% when you want to compare with a textbook or with our 1 m² plaster post, then add your own wastage. For block walls or concrete surfaces that are straight, 5% extra is often enough. Whatever you choose, keep it the same for the whole estimate.
Cement and sand per 100 sq ft at the default settings
| Thickness | 1:3 cement (bags) / sand (cft) | 1:4 | 1:6 |
|---|---|---|---|
| 6 mm | 0.59 / 2.2 | 0.47 / 2.3 | 0.34 / 2.5 |
| 12 mm | 1.17 / 4.3 | 0.94 / 4.6 | 0.67 / 4.9 |
| 15 mm | 1.47 / 5.4 | 1.17 / 5.8 | 0.84 / 6.2 |
| 20 mm | 1.96 / 7.2 | 1.57 / 7.7 | 1.12 / 8.2 |
Basis: dry factor 1.33, 10% extra, cement 1,440 kg/m³, 50 kg bags. Multiply by your area in hundreds of sq ft.
Cost per m² from a municipal rate analysis
If you need the cost of the finished work (material, labour and scaffolding), use a rate per m². The Suryabinayak Municipality rate analysis for 2081/82 gives, including 15% contractor overhead:
| Plaster | 1:3 | 1:4 | 1:6 |
|---|---|---|---|
| 12.5 mm | Rs 563.55 | Rs 472.19 | Rs 452.42 |
| 20 mm | Rs 613.70 | Rs 599.84 | Rs 568.74 |
At Rs 452.42 per m², the bedroom walls in example 1 (43.99 m²) cost about Rs 19,902. Rates are revised each fiscal year and differ by municipality, so check your local rate analysis or get quotes.
Common mistakes
- Plastering internal walls on one side only in the estimate. Select both sides for walls inside the house.
- Deducting the full opening from both faces. Mid-size doors and windows come off one face only, because the jambs and sills are also plastered.
- One thickness for everything. Outside 20 mm, inside 12 mm and ceilings 6 to 12 mm use very different amounts of cement. Run each separately.
- Ignoring bulking of damp sand. Moist sand takes up more volume than dry sand, so a tractor or truck load measured in cft gives less sand than it looks. Order a little extra or buy by weight where you can.
- Thick plaster to correct crooked walls. Fixing line and level in the brickwork is cheaper than filling 30 mm of plaster later.
More questions
How many bags of cement for 1,000 sq ft of 12 mm plaster in 1:4?
About 9.4 bags at the default settings (0.94 per 100 sq ft), or about 8.2 bags with a 1.27 dry factor and no extra.
What mix should I use for ceiling plaster?
1:3 or 1:4 is common for ceilings, usually 6 to 12 mm thick. Follow your drawing or engineer's specification.
How long should plaster be cured?
Keep it damp for about 7 days. Start the next day, and do not let fresh outside plaster dry in direct sun.
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