How many coats of paint do I need?

Two is standard. Add a third over a big colour change or bare plaster, and prime new or patched surfaces first.

Does primer count toward the paint quantity?

Estimate it separately with its own coverage. You only need primer on new, patched, stained or very dark walls.

How much paint for the ceiling?

Use the floor area: a 4 × 3 m ceiling is 12 m², so about 1.2 litres per coat at 11 m²/L.

How Much Paint Do You Need to Paint a Room?

Published 12/9/2025 · 2 min read · Everyday calculators

Marco Bianchi — Home, DIY & motoring writer at Allin

Renovation · Materials

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In short

To estimate paint, find the wall area (perimeter × height), subtract doors and windows, then divide by the coverage on the tin — usually 10–12 m² per litre per coat. A typical 4 × 3 m room with 2.5 m walls needs about 8–9 litres for two coats. Add around 10% for touch-ups and buy in one batch so the colour matches.

A paint tin, brushes and a dust sheet ready for decorating. — Blue Bird · Pexels · Pexels

Estimate paint by wall area, coats and the coverage on the tin — with a worked example and the 10% rule so you buy once and match the colour.

Start with the wall area

Add up the width of every wall to get the perimeter, then multiply by the ceiling height. A 4 × 3 m room has a perimeter of 14 m; at 2.5 m high that is 35 m² of gross wall before you take anything out.

Subtract doors, windows and count the coats

Take out roughly 1.8 m² for each door and 1.5 m² for each window. One door and two windows trim our 35 m² to about 30 m². Most jobs need two coats, so double it: 60 m² of paintable surface for the example room.

Coverage: how far a litre goes

A litre typically covers 10–12 m² per coat, but porous, rough or bare plaster drinks more, and a strong colour change may need an extra coat. Divide 60 m² by 11 m²/L and you land near 5.5 litres — plus a primer coat on new or patched walls, estimated the same way.

Buy smart: batches and leftovers

Add about 10% for touch-ups and round up to whole tins — around 8–9 litres here once primer is included. Buy it all in one batch so the dye lot matches, and keep the leftover tin, labelled with the room, for chips and scuffs later.

Worked with our own calculator

Paint calculator

Given

Surface area (m²): 36
Coats: 1
Coverage (m²/L): 5

Result

Paint needed (L): 7.2
2.5 L cans: 3

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

How many coats of paint do I need?: Two is standard. Add a third over a big colour change or bare plaster, and prime new or patched surfaces first.
Does primer count toward the paint quantity?: Estimate it separately with its own coverage. You only need primer on new, patched, stained or very dark walls.
How much paint for the ceiling?: Use the floor area: a 4 × 3 m ceiling is 12 m², so about 1.2 litres per coat at 11 m²/L.

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