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How the Paint Calculator Works
This calculator takes your surface area — entered either directly in m² or derived from room dimensions — and applies coverage rate, coat count, and wastage to give you a precise litre quantity and the number of tins to buy. Both 4 L and 10 L tin counts are shown so you can choose whichever pack size suits your project and budget.
Coverage Rate: What It Means
Coverage rate (m²/L) is the area one litre of paint covers in a single coat under normal application conditions. The figure varies significantly by surface texture and paint product. Smooth POP or drywall surfaces achieve 12–14 m²/L because the surface is non-porous and paint sits on top without being absorbed. Rough cement plaster — the default finish on most Indian interior walls — gives 10–12 m²/L. Exterior rough plaster drops to 8–10 m²/L, and heavily textured or Lappam finishes can fall as low as 6–8 m²/L because the increased surface area absorbs far more paint. The default in this calculator is 12 m²/L, suitable for smooth interior walls. For any other surface, check the coverage rate printed on the tin label of brands such as Asian Paints Royale, Berger Silk, Dulux Velvet Touch, or Nerolac Excel — coverage varies not only by surface but by product line.
Door and Window Deductions
When you select room dimension mode, the calculator deducts painted area for doors and windows before computing paint quantity. A standard Indian interior door measures 0.9 m × 2.0 m = 1.8 m², and a standard window measures 1.2 m × 1.0 m = 1.2 m². These deductions are applied per opening you specify. Doors and windows themselves are typically painted in a contrasting colour or enamel, so excluding them from the wall emulsion count is both accurate and practical. If your openings differ significantly from these standard dimensions — for example, large floor-to-ceiling glass panels or oversized double doors — use the direct area input mode instead and manually subtract the actual door and window areas before entering the net paintable area.
Number of Coats and Primer
Two coats is the minimum recommendation from every major Indian paint manufacturer for acceptable coverage and durability on previously painted walls. Three coats are required when changing from a dark to a light colour (dark pigments bleed through fewer coats), on new plaster that has not been primed, or on high-traffic areas where you want maximum film build. This calculator covers only topcoat emulsion paint. Primer is not included in this calculation — budget separately for approximately 1 coat of primer at 10–12 m²/L (products such as Asian Paints Wall Primer or Berger Primer Seal). On well-maintained previously painted surfaces, primer is often unnecessary; on bare new plaster or repaired patches, it is essential for adhesion and to prevent efflorescence peeling.
Paint Quantity Formula
The formula calculates net paintable area first, then applies coverage rate and coat count to arrive at the volume of paint needed, adjusted upward by the wastage percentage to ensure you have enough. Tin counts are always rounded up — you cannot buy a fraction of a tin.
Room mode — Wall area:
Total area (m²) = 2 × (Length + Width) × Height − (Doors × 1.8) − (Windows × 1.2)
Paint volume:
Paint (L) = Total area × Coats ÷ Coverage rate (m²/L) × (1 + Wastage / 100)
Tins (4 L) = ⌈Paint volume ÷ 4⌉
- Total area = net paintable wall surface in m²
- Coats = number of paint coats applied (1, 2, or 3)
- Coverage rate = area covered per litre per coat in m²/L (from tin label)
- Wastage = percentage added for edges, absorption, and touch-ups
- ⌈ ⌉ = ceiling function (round up to next whole number)
Worked example: Room 4 m × 3 m × 2.8 m high, 1 door, 2 windows, 2 coats, 12 m²/L coverage, 10% wastage.
Wall area = 2 × (4 + 3) × 2.8 − (1 × 1.8) − (2 × 1.2) = 39.2 − 1.8 − 2.4 = 35.0 m²
Paint = 35.0 × 2 ÷ 12 × 1.10 = 5.833 × 1.10 = 6.42 L
4 L tins = ⌈6.42 ÷ 4⌉ = ⌈1.605⌉ = 2 tins (8 L)
The calculator handles this automatically — the formula is shown here for transparency.
Paint Coverage Rate by Surface Type
Coverage rate is the single most important input in this calculation. Using 12 m²/L on a rough exterior wall instead of the correct 8–10 m²/L will underestimate paint by 20–33%, leaving you short mid-project. The table below covers the most common interior and exterior surfaces found in Indian construction. For premium or textured specialty paints, always verify the specific product's coverage rate on the tin label — it will override the general guidance below.
| Surface Type | Coverage Rate | Notes |
|---|---|---|
| Smooth POP / drywall | 12–14 m²/L | Best coverage; non-porous surface |
| Cement plaster (smooth finish) | 10–12 m²/L | Standard Indian interior wall |
| Cement plaster (rough / sand finish) | 8–10 m²/L | Common on exterior walls |
| Textured / Lappam finish | 6–8 m²/L | Requires significantly more paint |
| New unpainted plaster (no primer) | 8–10 m²/L | Add separate primer coat at 10–12 m²/L |
Frequently Asked Questions
Calculate the total wall area using 2 × (Length + Width) × Height, then deduct door and window areas (1.8 m² per door, 1.2 m² per window). Multiply the net area by the number of coats, divide by the coverage rate in m²/L, and add 10% for wastage. For a 4 m × 3 m room at 2.8 m height with 1 door and 2 windows, 2 coats at 12 m²/L with 10% wastage, you need approximately 6.4 litres — two 4 L tins.
Coverage rate and spreading rate are the same thing — the area one litre of paint covers in a single coat, expressed in m²/L. Paint manufacturers in India often print this on the tin label as "spreading rate". Asian Paints Royale, for example, specifies 12–14 m²/L on smooth surfaces; exterior emulsions typically state 8–10 m²/L. Always use the manufacturer's published figure for the specific product you are buying rather than the calculator's default value for the most accurate estimate.
A 12 × 12 ft room (3.66 m × 3.66 m) at a standard 10 ft (3.05 m) ceiling height has a wall area of about 44.7 m². Deducting one door (1.8 m²) and one window (1.2 m²) gives 41.7 m². At 12 m²/L and 2 coats with 10% wastage, you need approximately 7.6 litres — two 4 L tins. Budget for a separate primer coat of about 3.5 litres if the walls are bare new plaster.
Yes, primer is strongly recommended on new plaster, bare cement, or repaired surfaces. Primer seals the porous surface and improves adhesion, which means the emulsion topcoat covers more uniformly and lasts longer. This calculator does not include primer — budget approximately 1 coat of primer at 10–12 m²/L (products like Asian Paints Wall Primer or Berger WeatherCoat Primer) separately. On previously painted walls in good condition, primer can often be skipped.
Use 8–10 m²/L for rough cement plaster exterior walls and 6–8 m²/L for textured or Lappam-finish surfaces. The default 12 m²/L is suitable only for smooth interior POP or drywall. Using the wrong coverage rate will significantly underestimate the paint quantity needed. Check the tin label of your specific exterior paint product — Nerolac Excel Total, Berger WeatherCoat, and Asian Paints Apex all publish product-specific spreading rates that should be used in place of the default value.
The calculator is accurate within ±5–10% when you use the correct coverage rate from the manufacturer's tin label. Real-world consumption varies with wall texture, painter technique, temperature, and paint lost at edges and in the tray. The 10% wastage default already accounts for typical losses. For very rough or porous surfaces, select 15% wastage. Always buy one extra tin of the same batch number to ensure colour consistency for touch-ups, since colour can vary slightly between batches.
For a ceiling, the area equals room length × room width. Use the direct area input mode in this calculator and enter the ceiling area in m². Ceiling paint — typically flat white emulsion such as Asian Paints Tractor Emulsion or Berger Bison — usually achieves 12–14 m²/L on smooth surfaces. Two coats are standard. A 4 m × 3 m ceiling (12 m²) at 12 m²/L for 2 coats needs approximately 2.2 litres including 10% wastage — one 4 L tin is sufficient.
Each coat multiplies paint consumption proportionally — two coats use exactly twice the paint of one coat; three coats use three times as much. Two coats is the minimum recommended by all Indian paint manufacturers for adequate coverage and durability. Three coats are needed when painting a light colour over a dark one, or on brand-new plaster that has not been primed. On a typical 35 m² room, going from 2 to 3 coats at 12 m²/L adds approximately 3.2 extra litres of paint (including 10% wastage).