How much paint do I need? A practical guide

Every paint calculation comes down to the same three numbers: the area you paint, how many times you paint it, and how much area one litre covers. Multiply the first two, divide by the third, and you have the litres you need.

The difficulty is never the arithmetic — it's getting each of the three numbers right. A wall measured carelessly, a coverage figure taken from the most optimistic line of the data sheet or a forgotten primer coat can easily put you 30% off.

Don't bother subtracting sockets, switches or small radiators; the paint lost in the tray, on the roller and in the brush is about the same order of magnitude.

1. Measure the length and width of the room at floor level and add them up twice: that's the perimeter.
2. Measure the wall height from the floor (or the top of the skirting) to the ceiling.
3. Multiply perimeter × height to get the gross wall area.
4. Measure each door and window (width × height) and subtract them. A standard interior door is close to 1.6–1.8 m² and a typical window 1.2–2 m², but measure yours.
5. If you paint the ceiling, add length × width as a separate item — ceiling paint often has a different coverage.

Manufacturers state the coverage (sometimes called the spreading rate) in square metres per litre for one coat. For interior emulsion, 8–14 m²/L is typical; in imperial units that is roughly 325–570 ft² per US gallon.

The figure assumes a smooth, sealed, evenly absorbent surface. On real walls you will usually get less:

• new plaster and fresh filler absorb a lot of the first coat;
• textured plaster, wood chip wallpaper and bare blockwork have a much larger real surface area;
• applying the paint thinner than recommended stretches the litres but often costs you an extra coat.

Two coats are the standard for an even finish, even with a paint that claims one-coat coverage. A third coat is often needed when you move from a strong, dark colour to a light one, or when you use a deep shade over white — deep colours frequently have lower hiding power.

Primer is not a coat of paint. On new plaster, porous surfaces or stains, apply a suitable primer or a diluted mist coat first, as recommended by the paint manufacturer, and calculate it separately with the primer's own coverage.

A 4 × 3 m bedroom with 2.6 m walls has a perimeter of 14 m and 36.4 m² of wall. After deducting a door and a window (3 m² in total) there are 33.4 m² to paint. Two coats make 66.8 m². With a paint that covers 10 m² per litre, you need 6.7 litres.

If you also paint the ceiling (12 m², two coats, a ceiling paint covering 8 m²/L), add another 3 litres. In practice: one 10-litre tin of wall paint and a 5-litre tin of ceiling paint — or one larger tin if you use the same product for both.

• Round up to the tin sizes available; a slightly larger tin is usually cheaper per litre than two small ones.
• If the colour is mixed in the store, buy everything at once — a second mix can be a shade off. For ready-mixed paint, check that the batch number is the same on all tins.
• Keep a small amount for touch-ups. Close the tin tightly and store it frost-free; water-based paint that has frozen is usually unusable.
• Write the room, the colour code and the date on the lid — it saves a lot of guesswork two years later.

• Calculating floor area instead of wall area — a 12 m² room has over 30 m² of walls.
• Using the best-case coverage from the data sheet on a rough or absorbent wall.
• Forgetting the primer, or treating it as the first coat.
• Not counting the ceiling, built-in wardrobes or a staircase wall in an open-plan space.