How much paint do I need?
Area = (perimeter × height − openings + ceiling) × rooms
Paint (L) = area × coats ÷ spreading rate × (1 + wastage)
Paint (L) = area × coats ÷ spreading rate × (1 + wastage)
The spreading rate is printed on every paint tin, usually 8–12 m² per litre per coat for emulsion on smooth plaster. Rough or porous surfaces use more.
Worked example
A room with an 18 m perimeter and 3 m walls has 54 m² of wall. Deduct 5 m² of openings and add a 20 m² ceiling for 69 m². Two coats at 10 m²/L with 5% wastage needs 69 × 2 ÷ 10 × 1.05 = 14.5 litres, so one 18 L tin.
Typical spreading rates
| Product | m² per litre per coat |
|---|---|
| Water-based primer / sealer | 10–14 |
| Interior emulsion | 10–12 |
| Exterior textured paint | 3–6 |
| Enamel on doors and metal | 12–16 |
Frequently asked questions
How many coats of paint are needed?
One coat of primer on new plaster, then two finish coats. A strong colour change may need a third.
How do I find the room perimeter?
Add up the lengths of all walls. For a rectangular room, perimeter = 2 × (length + width).