The short answer
For ordinary interior wall paint on a prepared, smooth surface, a useful starting point is roughly 10–12 m² per litre per coat. In US measurements, many wall paints are commonly estimated near 350–400 sq ft per US gallon per coat. These are planning figures, not promises: the product label should always take priority.
PaintSimple starts at 11 m²/L for a smooth, previously painted wall. It then uses more conservative values for new, porous, rough or masonry surfaces and adds 10% for normal waste and touch-ups.
PaintSimple planning values
These values are deliberately simple. They help a beginner prepare a sensible estimate before choosing a product.
| Surface condition | Starting coverage | Why it changes |
|---|---|---|
| Smooth, previously painted | 11 m²/L/coat | A sealed, even surface usually absorbs less paint. |
| New or porous wall | 9 m²/L/coat | Bare plaster or skim coat may absorb more material. |
| Rough or textured wall | 7 m²/L/coat | Peaks, grooves and a larger real surface area require more paint. |
| Brick or masonry | 6 m²/L/coat | Joints, pores and uneven faces reduce practical spread. |
Special finishes can be lower still. For example, Nippon Paint Malaysia lists 4–6 m²/L per coat for one textured decorative finish, while several of its standard smooth interior wall products list 10–12 m²/L per coat. This is why a universal number cannot replace the label.
The calculation formula
“Paintable area” means the wall area after subtracting doors and windows. Ceiling paint and primer should be estimated separately because they may use different products.
Worked metric example
Imagine a room that is 4 m long, 3 m wide and 2.7 m high, with one standard door and one standard window:
- Gross wall area: 2 × (4 + 3) × 2.7 = 37.8 m².
- Subtract openings: 37.8 − 1.9 − 1.5 = 34.4 m².
- Two coats: 34.4 × 2 = 68.8 m² of coating work.
- At 11 m²/L: 68.8 ÷ 11 = 6.25 L.
- Add 10%: approximately 6.9 L to prepare.
A practical purchase could be one 5 L can plus two 1 L cans, if those sizes are available. Pack sizes vary by brand and country.
Worked US gallon example
For a 12 ft × 10 ft room with 8 ft walls, one standard door, two standard windows and two coats, the paintable wall area is about 299 sq ft. At 375 sq ft per gallon per coat, including 10% extra, the estimate is about 1.8 US gallons. Purchasing two 1-gallon cans would be a straightforward plan.
What changes real coverage?
- Surface condition: bare, chalky or repaired areas often absorb more.
- Texture: rough render, brick and decorative texture increase real surface area.
- Colour change: moving between very dark and very light colours may need primer or another coat.
- Application method: rollers, brushes and spray equipment leave different amounts on tools and surrounding protection.
- Product formulation: solids, recommended film thickness and dilution differ between products.
Litres and gallons are not the same
PaintSimple uses US gallons—not the larger Imperial gallon. Always check which gallon a retailer or product page means.
How to get the best estimate
- Measure the room rather than relying only on floor area.
- Choose the closest wall condition in the calculator.
- Use the number of coats recommended for the product and colour change.
- Open Advanced Settings and enter the label coverage when available.
- Keep a small amount from the same batch for future touch-ups.
Sources and limits
The reference figures above are supported by current manufacturer guidance, including Nippon Paint Malaysia EasyCoat, Nippon Paint Matex PRO ECO, Nippon Paint Momento Textured and the Sherwin-Williams paint calculator guide. Product specifications can change. PaintSimple is an independent estimating tool and is not affiliated with these manufacturers.