How many coats do I need?

Two coats over an existing similar colour, three when going lighter over a dark wall or covering a strong colour. Bare plaster needs a thinned mist coat first, which behaves like an additional coat.

Should I subtract windows and doors?

Only large openings. Standard doors and small windows are usually left in the total because cutting in around them uses proportionally more paint than a flat wall.

How do I convert to gallons?

One US gallon is 3.785 litres and covers roughly 35 to 40 square feet per coat on a sealed surface. Divide the litre figure by 3.785 for US gallons.

Paint Calculator

Quick answer

Paint needed equals wall area multiplied by the number of coats, divided by the coverage rate on the tin. A typical emulsion covers 10–12 m² per litre per coat on a sealed surface.

How much paint do you need?

Wall area × coats ÷ coverage, plus a 10% waste allowance.

Wall area (m²)

Perimeter × height, minus large openings.

Number of coats

Coverage (m² per litre)

Check the tin. 10–12 on sealed walls, 6–8 on bare or textured.

Paint needed

8.0 litres

includes a 10% waste allowance

Without waste allowance

7.3 L

In US gallons

2.1 gal

Most people under-buy paint because they use the tin's coverage figure without adjusting for the surface. Coverage on the label is measured on a smooth, sealed, previously painted wall. Bare plaster, textured wallpaper and deep colour changes all consume noticeably more.

Buying one extra litre is cheap; running out mid-wall is not. A second batch can differ slightly in tint, and a join mid-wall shows under raking light.

Coverage rates that actually hold

Matt emulsion on a sealed wall covers roughly 10 to 12 square metres per litre per coat. Silk and soft sheen are similar. Bare plaster absorbs the first coat, so a mist coat thinned with water is standard practice and counts as an extra coat.

Textured surfaces, bare wood and masonry can drop coverage to 6 to 8 square metres per litre. Painting a pale colour over a dark one often needs three coats rather than two regardless of the product claim.

What to deduct, and what not to

Deduct large openings — a patio door, a wide window — but do not bother deducting a standard door or a small window. Decorators usually ignore anything under about two square metres because cutting in around an opening uses more paint per square metre than a flat run.

Ceilings are calculated separately from walls and are usually painted in a different product. Add ten per cent to whatever the arithmetic gives you as a waste allowance for roller loading, tray residue and touching up.

Frequently asked questions

How many coats do I need?: Two coats over an existing similar colour, three when going lighter over a dark wall or covering a strong colour. Bare plaster needs a thinned mist coat first, which behaves like an additional coat.
Should I subtract windows and doors?: Only large openings. Standard doors and small windows are usually left in the total because cutting in around them uses proportionally more paint than a flat wall.
How do I convert to gallons?: One US gallon is 3.785 litres and covers roughly 35 to 40 square feet per coat on a sealed surface. Divide the litre figure by 3.785 for US gallons.

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Updated 2026-09-20 · all calculators. Results are estimates for guidance, not professional advice.