Calculate exactly how many litres of paint you need for any room, wall, ceiling, or exterior surface. Deduct doors and windows, choose your coat count, and account for surface texture — all in one calculation.
For built-in wardrobes, tiled splashbacks, etc.
Coverage varies by brand and surface porosity. Check your paint tin.
2 coats standard. New plaster or colour change: 3 coats.
The formula is: Litres = (Net Area × Coats) ÷ Coverage Rate × (1 + Wastage). Worked example for a typical 4m × 3m room with 2.7m ceiling height: Wall area = 2 × (4+3) × 2.7 = 37.8 m². Ceiling = 4 × 3 = 12.0 m². Total gross = 49.8 m². Subtract 2 doors (3.6 m²) and 2 windows (2.4 m²) = 43.8 m² net. At 10 m²/litre with 2 coats: 43.8 ÷ 10 × 2 = 8.76 litres. Add 5% wastage = 9.2 litres. Buy 2 × 5L tins (10L total), giving you 0.8L spare for touch-ups.
| Paint Type | Coverage (m²/L) | Typical Use |
|---|---|---|
| Standard emulsion | 10–12 | Interior walls |
| Premium/silk emulsion | 12–14 | Feature walls |
| Masonry paint | 6–9 | Exterior brick/render |
| Gloss/satin | 12–16 | Woodwork, trim |
| Undercoat/primer | 8–10 | New surfaces |
| Textured coatings | 4–7 | Feature/rough walls |
1 coat is only sufficient for maintenance repaints using the same colour on a well-prepared surface. 2 coats is the standard for new work and most repaints — the first coat seals the surface and the second provides full coverage and uniform finish. 3 coats are needed when painting new plaster (which is highly porous), making a drastic colour change (especially dark to light), or painting exterior surfaces exposed to harsh weather and UV.
A standard internal door measures approximately 2.0m × 0.9m = 1.8 m². A standard window is roughly 1.2m × 1.0m = 1.2 m². In a small room with multiple openings, deductions can total 10–20% of the wall area, saving you an entire tin. As a rule, always deduct openings if they exceed 10% of total wall area. For rooms with minimal openings (e.g. one door), the difference is marginal and the extra paint serves as touch-up reserve.
Exterior paint is formulated to resist UV radiation, rain, temperature fluctuations and fungal growth. It has lower coverage (6–9 m²/L) because it forms a thicker, more protective film. Interior paint has lower VOC (volatile organic compounds), better washability, and higher coverage (10–14 m²/L). Never use interior paint outdoors — it will chalk, peel, and fail within one season. Conversely, exterior paint indoors is safe but unnecessarily expensive and has stronger odour during application.
1. Prepare surfaces properly — fill cracks and sand smooth before painting to improve coverage. 2. Use the right roller nap — short nap (5–10mm) for smooth walls, long nap (15–25mm) for textured. 3. Maintain consistent technique — use W-pattern rolling and maintain a wet edge. 4. Don’t overload your brush — dip only one-third of bristle length. 5. Seal leftover paint — strain back into the tin, seal tightly, store upside-down to create an airtight seal for touch-ups.