How much paint you need depends on four things: the real area, the number of coats, the product’s coverage and a small margin. Here is how to work it out.
1. Work out the wall area
In a rectangular room the perimeter is 2 × (length + width). Multiply it by the height:
A 4 × 3 m room (about 13 × 10 ft) with 2.5 m (8.2 ft) ceilings has 2 × 7 × 2.5 = 35 m² (≈ 377 ft²) of wall. If you paint the ceiling, add length × width.
2. Subtract doors and windows
Measure each opening or use an approximate value. With a 1.6 m² door and a 1.5 m² window, the net area is 35 − 3.1 = 31.9 m².
3. Multiply by coats and divide by coverage
The tin gives coverage per litre (or per gallon) per coat. With two coats and 10 m²/L (≈ 400 ft²/gal):
4. Add a margin and round up to whole tins
10% covers touch-ups, uneven absorption and what stays in the tray: 6.38 × 1.1 ≈ 7.02 L. With 4 L tins you need 2; with 1-gallon cans, 2 as well. Always round up.
What changes the result
- New plaster or drywall: absorbs more; a primer reduces how much topcoat you need.
- Strong colour change: may need an extra coat.
- Textured surfaces: real coverage drops below the figure on the tin.
To turn the quantity into a budget with labour and supplies, use the house painting cost calculator.