How Much Paint Do I Need?

What you need

A tape measure, a roller and tray, brushes for cutting in, drop cloths or old sheets, painter's tape, a sturdy step ladder, and the paint itself — plus primer if you are painting bare plaster or a dark colour. Buy the paint after you have measured, not before.

Step 1 — Measure the walls

Measure the perimeter of the room (all four walls added together) and multiply by the ceiling height. A room 4 m × 5 m with a 2.7 m ceiling has a perimeter of 18 m and a gross wall area of 48.6 m².

Step 2 — Subtract doors and windows

A standard door is 21 ft² (1.951 m²) and a window is 15 ft² (1.394 m²). Subtract them from the gross area. The room above, with 1 door and 2 windows, has a net wall area of 48.6 − 1.951 − 2 × 1.394 = 43.86 m².

Step 3 — Check the coverage

Coverage is printed on the tin — typically 10–12 m² per litre for emulsion, less for dark colours over light walls. Coverage varies by paint, so never assume one figure fits all paints. Enter the figure from your tin.

Step 4 — Allow for coats

Most walls need two coats. Multiply the paint for one coat by the number of coats. A dark colour over a light wall may need three — or a coat of primer first.

Worked example

Net wall area 43.86 m², coverage 11 m² per litre, 2 coats: 43.86 ÷ 11 × 2 = 7.97 litres. Paint is sold in whole tins, so buy 8 litres — or two 4-litre tins. The paint calculator does this in one step.

Common mistakes

• Painting the gross area. Doors and windows are not painted — subtract them, or you will buy too much.
• Forgetting the second coat. One coat is almost never enough. Always multiply by the coats.
• Assuming all paints cover the same. They do not. Read the tin and enter its coverage.

Safety

Open the windows while you paint and keep the room ventilated until the paint is dry — especially with oil-based paint. Never overreach from a ladder: climb down and move it instead. If your home was built before lead paint was banned — before 1978 in the US, before 1992 in the UK, and before the restrictions of the 1990s in Australia — get the old paint tested for lead before you sand or scrape it. Sanding lead paint creates dust that is hazardous, particularly to children. Keep paint and solvents away from children and pets, and never pour leftover paint down a drain.

When to hire a professional

Hire a painter for high ceilings and stairwells, for walls that need extensive repair before painting, or for a spray finish. If lead paint is present, hire a certified professional — in the US, firms working on pre-1978 homes must be lead-safe certified. A professional is also worth it when the job has a deadline you cannot miss.

Frequently asked questions

• Do I need to paint the ceiling too? Usually, yes — but measure it as a separate area, because ceilings take a different paint and are often painted first. Our guide to painting a ceiling without streaks covers the method.
• What is primer for? Primer seals bare plaster and stops a dark colour showing through a light one. If you are painting bare plaster or going from dark to light, prime first — the primer calculator works out how much you need.
• My tin says 9 m² per litre, not 11 — what changes? Nothing in the method changes: divide your net wall area by your tin's coverage figure instead of 11, then multiply by the coats. The worked example above scales the same way.
• How should I store leftover paint? Seal the tin, label it with the room and the date, and keep it somewhere cool and dry. Stir it well before reuse, and use it for touch-ups within a year or two.

Try these calculators

• Paint Calculator — litres, tins and cost for any room.
• Wall Paint Calculator — wall-by-wall paint quantities.
• Primer Calculator — primer for bare or dark walls.