How to Calculate How Much Paint a Room Needs (Litres, Coats and Waste)

Buying too little paint means a second trip and a shade mismatch. Buying too much wastes money. The calculation is short once you know the wall area.

Quick answer

Wall area = perimeter × height − doors and windows. Divide by the coverage of one litre, multiply by the number of coats, and add 10 percent.

Step by step

1. Perimeter = 2 × (length + width). For a 4 m × 3.5 m room, that is 15 m.
2. Wall area = perimeter × height = 15 × 2.8 = 42 m².
3. Subtract openings: a door is about 1.9 m² and a window about 1.5 m². With one door and two windows: 42 − 1.9 − 3 = 37.1 m².
4. Coverage: most emulsions cover 10 to 12 m² per litre per coat.
5. Two coats: 37.1 × 2 ÷ 11 = 6.7 litres.
6. Waste allowance: add 10 percent for touch-ups: about 7.4 litres.

Buy a 10-litre tin or 5 + 2.5 litre tins depending on prices.

Using the calculator

Enter the room length, width and height, the number of doors and windows, the coats and the coverage in m² per litre. The tool gives the wall area and litres.

Ceilings and trims

• Ceiling area = length × width. Use ceiling paint separately.
• Skirting boards, doors and window frames use gloss or satin paint and much less of it.

Tips

• Dark or bold colours often need an extra coat; use a primer.
• Rough or porous walls absorb more paint.
• Keep a small leftover tin for repairs; write the colour code on the lid.
• Check the coverage printed on the tin.