What is the standard plaster thickness for internal and external walls in India?

For internal walls, single-coat cement plaster is most commonly 12 mm with a 1:6 cement:sand ratio. For external walls, 15–20 mm at a stronger 1:3 or 1:4 ratio is standard for weather resistance. IS 1542:1992 (paraphrased) describes a two-coat system for better-quality work: a base rendering coat of 12–15 mm (1:3 to 1:6) followed by a finishing coat of 6–8 mm (1:4 to 1:6), totalling 18–20 mm. The Speak Arch PLASTER_WORK.xlsx uses 6 mm at 1:3 for scratch coats and 12–15 mm at 1:6 for single-coat internal work.

When should I use 12 mm plaster versus 20 mm plaster?

12 mm is the standard for internal walls in residential construction — it provides sufficient coverage for machine-made or well-laid brickwork and uses less material. 20 mm (or 15+6 mm two-coat) is used for external walls exposed to weather, for rough or irregular masonry backing, or where a stronger waterproof finish is required. External plaster at 1:3 or 1:4 ratio with 15–20 mm thickness provides better resistance to rain and temperature cycles. CPWD analysis of rates specifies 15 mm for external and 12 mm for internal as standard schedule items.

What is the dry-volume factor for plaster mortar — 1.33 or 1.60?

Both factors appear in Indian QS practice. This calculator uses 1.33 — confirmed in PLASTER_WORK.xlsx (Speak Arch source) and BRICK_WORK.xlsx, consistent with CPWD analysis of rates: V_dry = V_wet × 1.33. The 1.60 convention (used in some QS textbooks) adds 50% for voids and 10% for surface unevenness in one combined factor. The difference is significant: using 1.60 produces approximately 20% more material than 1.33. When in doubt, add 5–10% wastage to the 1.33 result for site losses — which is already provided as a chip in this calculator.

How are doors and windows deducted from plastered area?

IS 1200:Part 14 (method of measurement for plastering, paraphrased from CPWD practice) specifies: openings below 0.5 m² — no deduction; openings 0.5–3 m² — deduct one face of the opening; openings above 3 m² — deduct both faces. Most residential doors (approximately 1.89 m², i.e. 2.1×0.9 m) and windows (approximately 1.80 m², i.e. 1.5×1.2 m) fall in the 0.5–3 m² range — deduct one face from the gross wall area. This calculator provides door and window preset chips and deducts from one face by default.

Does this plaster calculator include waterproofing, skim coat, putty, or paint?

No. This calculator covers cement–sand plaster mortar only. Waterproofing compounds (SBR slurry, crystalline coatings) are applied as additional treatments measured separately in m². Wall putty (skim coat) is priced and measured separately. Paint and primer quantity is a different calculation — use the Speak Arch Paint and Tile Estimator for paint litres. For multi-material finish systems, calculate each layer separately.

How many cement bags are needed per square metre of 12 mm plaster (1:6)?

Calculation for 1 m² of 12 mm plaster at 1:6 ratio (no wastage): V_wet = 1 × 0.012 = 0.012 m³; V_dry = 0.012 × 1.33 = 0.01596 m³; cement volume = 0.01596 ÷ 7 = 0.00228 m³; cement mass = 0.00228 × 1440 = 3.28 kg; fractional bags = 3.28 ÷ 50 = 0.066 bags/m². For 100 m²: 6.56 → 7 bags (rounded up). Sand: 0.01596 × 6/7 = 0.01368 m³ = 13.68 litres per m². Always round bags up — partial bags cannot be returned.

Plaster Quantity Calculator India

Estimate cement and sand for wall plaster — room dimensions or net area, single or two-coat (12 mm + 6 mm), IS 1542 mix practice.

Educational BOQ only. Excludes waterproofing, putty, and paint litres. Two-coat uses independent ratios per coat.

How to use this calculator

Mode A: direct net area, or Mode B: room L×W×H.
Single or two-coat; thickness presets 6–20 mm.
Door/window deductions and optional ceiling.
Calculate bags, sand, per-m² reference.

After plaster, estimate paint with the paint & tile estimator.

Coat system

Wastage

Coat — thickness & mix ratio

Thickness (mm)

Mix ratio (cement:sand)

Enter the net plaster area after deducting openings. For room-based calculation with deductions, use Mode B.

Net plaster area *

Net surface area to be plastered, after deducting doors and windows.

Coat breakdown

Coat	Thickness	Ratio	V_wet (m³)	V_dry (m³)	Bags	Sand (m³)

Cement (bags, rounded up)

—

Sand

—

Per m² of plaster — —

Full breakdown

Item	Value	Unit

Constants & formula references

Frequently asked questions

Standard plaster thickness — internal vs external?

Internal walls: typically 12 mm at 1:6 for residential. External walls: 15–20 mm at 1:3 or 1:4 for weather resistance. IS 1542:1992 (paraphrased) describes a two-coat system: 12–15 mm base + 6 mm finish = 18–20 mm total. CPWD analysis of rates specifies 15 mm external and 12 mm internal as schedule defaults.

12 mm vs 20 mm — when to use each?

12 mm suits well-laid internal brickwork in protected locations. 20 mm (or 15+6 mm two-coat) is used for external surfaces, rough or irregular masonry, and where improved weather resistance or waterproofing is required. A thicker base coat also compensates for uneven backing without sagging — IS 1542 recommends two coats for total thickness above 20 mm.

Dry factor 1.33 or 1.60 — which is correct?

This calculator uses 1.33 — confirmed in PLASTER_WORK.xlsx and BRICK_WORK.xlsx (Speak Arch sources) and consistent with CPWD analysis of rates. The 1.60 convention (50% voids + 10% unevenness) appears in some QS textbooks and gives ~20% more material. To be conservative with 1.33, add 5–10% wastage (provided as a chip above).

How are doors and windows deducted?

IS 1200:Part 14 (paraphrased): openings below 0.5 m² — no deduction; 0.5–3 m² — deduct one face; above 3 m² — deduct both faces. Standard door ≈ 1.89 m² (2.1×0.9 m); standard window ≈ 1.80 m² (1.5×1.2 m) — both fall in the deduct-one-face range. This calculator deducts user-entered opening areas from one face.

Plaster calculator vs Cement–Sand calculator (Tool 8)?

Tool 8 is a general mortar estimator for any application (bedding, pointing, screeds). This tool is specific to plastering — it adds room-geometry input (2(L+W)×H formula), two-coat system with independent ratios per coat, IS 1542 thickness presets, and per-m² reference. For brick wall BOQ including brick count, use the Brick Quantity Calculator.

Does this include waterproofing, putty, or paint?

No. Cement–sand plaster only. Waterproofing (SBR, crystalline) is applied separately in m². Wall putty (skim coat) is measured separately. For paint litres, use the Speak Arch Paint & Tile Estimator.

How many bags per m² of 12 mm plaster at 1:6?

Per m²: V_wet = 0.012 m³; V_dry = 0.01596 m³; cement = 0.01596/7×1440 = 3.28 kg = 0.066 bags/m². For 100 m²: 6.56 → 7 bags (rounded up). Sand: 13.7 L/m². Always round bags up — partial bags cannot be returned. Use the calculator above for exact values per your area and wastage.