Paint Coverage Calculator
Buy too little paint and you are back at the shop mid-job hunting for a matching batch. This calculator works out the paintable area of a room, deducts the doors and windows, applies your coat count, and converts the result into the quantity you actually need to buy.
Formula
Paint needed = (wall area − openings) × coats ÷ coverage per litre
- Wall area = perimeter × height
- Coverage is typically 10–12 m² per litre
A 4 × 3 m room, 2.4 m high: 33.6 m² × 2 coats ÷ 11 = 6.1 litres
How the Area Is Worked Out
You do not need to measure each wall. Take the room's perimeter and multiply by the ceiling height:
Wall Area = 2 × (Length + Width) × Height
A 4 m × 3 m room with a 2.4 m ceiling has 14 × 2.4 = 33.6 m² of wall.
Standard allowances of roughly 1.9 m² per door and 1.4 m² per window are then deducted.
Coats and Coverage
Coverage figures on a tin are quoted per coat, and most jobs need two, so the area to cover doubles. Typical coverage per litre, per coat:
- Matt emulsion: 12–14 m²
- Vinyl silk: 12–14 m²
- Gloss or satinwood: 12–16 m²
- Primer or undercoat: 10–12 m²
- Masonry paint: 6–8 m²
Rough, textured and porous surfaces drink paint, so take the lower end whenever the wall is not smooth.
When You Need a Third Coat
Budget for an extra coat when going from a dark colour to a light one, when using a strong saturated colour such as a bright red or yellow, or when covering stains. New plaster needs a thinned mist coat first, which counts as one of your coats.
Quick Reference
| Room size | Wall area | Walls only | Walls and ceiling |
|---|---|---|---|
| 3 × 3 m | 28.8 m² | 5.2 L | 6.9 L |
| 4 × 3 m | 33.6 m² | 6.1 L | 8.3 L |
| 4 × 4 m | 38.4 m² | 7.0 L | 9.9 L |
| 5 × 4 m | 43.2 m² | 7.9 L | 11.5 L |
Frequently Asked Questions
- How much paint do I need for one room?
- A typical 4 m × 3 m room with a 2.4 m ceiling needs roughly 5 litres for two coats on the walls, and about 2 litres for the ceiling. Measure rather than assume if the room is unusual.
- How much does a litre of paint cover?
- Around 12 m² per coat for standard emulsion, but always use the figure on your tin. Masonry paint can be as low as 6 m² per litre.
- Do I deduct doors and windows?
- Yes, at roughly 1.9 m² per door and 1.4 m² per window. Measure large windows or patio doors properly, since standard allowances will be badly out.
- What is a mist coat?
- Emulsion thinned with water, usually about 70:30, applied to bare plaster so it soaks in and seals the surface. Without one, normal paint sits on top and peels later.