How Much Paint Do I Need for a Room? Litres, Gallons and Coats

By the SAA Tool editor, an IT support engineer · Last reviewed 9 October 2026 · 7 min read

Working out paint takes four numbers: the area to paint, the number of coats, the coverage printed on the tin and the tin sizes you can buy. Most wrong estimates come from the coverage, which varies a lot between products and surfaces. This guide works through a 12 × 14 ft bedroom and a 3.6 × 4.2 m one, so you can check every figure.

The sum in one line

Paint = (wall area − doors − windows) × coats ÷ coverage

Keep to one system: square feet and US gallons, or square metres and litres.

1. Measure the walls

Measure each wall along the floor, and the height from floor to ceiling. Add the lengths (the room's perimeter) and multiply by the height. Behr's guide uses the same method.

• Imperial: a 12 × 14 ft room has a perimeter of 12 + 14 + 12 + 14 = 52 ft. With 8 ft walls, that's 52 × 8 = 416 sq ft.
• Metric: a 3.6 × 4.2 m room has a perimeter of 15.6 m. With 2.4 m walls, that's 15.6 × 2.4 = 37.44 m².

For the triangle of a gable wall, add base × height ÷ 2.

2. Take off doors and windows

Measure each opening and subtract it, as Dulux UK's guide suggests. Until you measure, the Paint Calculator uses 21 sq ft for a door (about 3 × 7 ft) and 15 sq ft for a window (about 3 × 5 ft), or 1.9 m² and 1.4 m².

With one door and two windows, the bedroom loses 21 + 15 + 15 = 51 sq ft, leaving 365 sq ft. In metric it loses 1.9 + 1.4 + 1.4 = 4.7 m², leaving 32.74 m².

Doors, frames and skirting boards (baseboards) usually take a harder-wearing trim paint. Dulux UK's rule of thumb is that a 750 ml tin does a door or the skirting in a small room.

3. Decide on coats

Two coats is the normal plan for an even colour, and Dulux Australia's calculator assumes two topcoats for most of its paints. One coat can do when you repaint in a similar colour with a one-coat paint; pale over a strong dark colour may need three. Multiply the area by the coats: 365 × 2 = 730 sq ft, or 32.74 × 2 = 65.48 m².

4. Use the coverage on your tin

Coverage (or spreading rate) is the area a gallon or litre covers in one coat. Some makers' figures:

That spread is why the figure on your own tin beats any rule of thumb. At 350 sq ft per gallon the bedroom walls need 730 ÷ 350 = 2.09 US gallons; at 400 they need 1.83. At 9 m² per litre they need 65.48 ÷ 9 = 7.3 litres, but at Dulux Australia's 16 m² per litre only about 4.1 litres.

Coverage figures assume a smooth, sound, previously painted wall. Rough and new surfaces take more for two reasons: texture adds real surface area and hollows to fill, and porous surfaces soak up the first coat before it forms a film. Behr warns that porous, rough, textured or unpainted surfaces may need more paint for extra coats, and Sherwin-Williams says the surface and how you apply the paint both change the amount. For textured or patchy walls, use the low end of the tin's range.

Primer, new plaster and old paint

New plaster. Dulux UK says plaster can take up to 4 weeks to dry and soaks up moisture fast, so the first coat should be a mist coat: emulsion thinned at about ten parts paint to one part water, which acts as the primer. A plaster sealer does the same job. Count it as an extra coat.

New drywall, bare wood, stains or a big colour change. Plan a primer coat and work it out on its own, because primer covers less. At 250 sq ft per gallon, the bedroom needs 365 ÷ 250 = 1.46 gallons: one gallon and two quarts. At 6 m² per litre, it needs 32.74 ÷ 6 = 5.46 litres.

Old paint. The US EPA says homes built before 1978 are more likely to have lead-based paint, and sanding it creates toxic dust. Other countries banned it at different dates, so test old paintwork before sanding.

Ceilings

A ceiling is length × width. The 12 × 14 ft ceiling is 168 sq ft, and two coats at 350 sq ft per gallon need 336 ÷ 350 = 0.96 gallons: one can. The 3.6 × 4.2 m ceiling is 15.12 m², and two coats at 9 m² per litre need 30.24 ÷ 9 = 3.36 litres: a 2.5 litre and a 1 litre tin. Ceiling paint has its own coverage figure, so read its tin. Walls and ceiling together come to about 3.05 US gallons, or 10.6 litres.

Litres and US gallons

• A US gallon is defined as 231 cubic inches, which is exactly 3.785411784 litres (NIST Handbook 44). One litre is about 0.264 US gallons, and a US quart is about 0.946 litres.
• A UK (imperial) gallon is about 4.546 litres, but UK paint is sold in litres anyway.
• To convert coverage, multiply sq ft per US gallon by 0.0245 to get m² per litre, or multiply m² per litre by 40.75 to get sq ft per gallon. So 350 sq ft per gallon is about 8.6 m² per litre.
• The US sells paint in quarts, gallons and 5-gallon buckets; the UK, Europe, Australia and many other countries sell it in litres.

Choosing tin sizes

Round up to whole tins. In US sizes the Paint Calculator rounds up to the next quart, using a gallon instead of three quarts and 5-gallon buckets for big jobs. In litres it picks from 5, 2.5 and 1 litre tins, weighing the paint left over against the number of tins: each extra tin counts as half a litre wasted. For the bedroom walls it suggests two 1-gallon cans and a quart (2.25 gallons), or a 5 litre and a 2.5 litre tin (7.5 litres).

So for the 5.46 litres of primer it suggests a 5 litre and a 1 litre tin (6 litres) rather than a 2.5 litre and three 1 litre tins: half a litre more paint, but two tins instead of four. Compare the cost per litre or gallon, and keep a little for touch-ups. If a mixed colour needs several tins, pour them into one bucket first so the shade is even.

How to do it with the Paint Calculator

1. Open the Paint Calculator. Under Units, choose Imperial (feet, inches) or Metric (metres, centimetres), then pick the Coats.
2. Enter the Wall length, Wall height and Number of walls. For walls of different lengths, type the total length and 1 wall: 52 ft, or 15.6 m, for the bedroom.
3. Enter the number of Doors and the Area of one door, then Windows and the Area of one window.
4. Type the Coverage on the tin, in sq ft per US gallon or m² per litre.
5. Press Calculate (results also update as you type). Read the You need about line, the Area to paint, Litres and US gallons, and the suggested tins in the Details table.
6. For a ceiling, enter its length and width as one wall with no doors or windows. For primer, use 1 coat and the primer's coverage.

It runs in your browser and covers walls and ceilings, not trim. To convert between square feet and square metres, use the Area Converter.

Quick answers

• How much paint for a 12 × 12 ft room? With 8 ft walls, one door, one window and two coats at 350 sq ft per gallon: 348 sq ft, so 1.99 gallons (two cans). These are the calculator's defaults.
• And for a 3.6 × 3.6 m room? With 2.4 m walls, one door and one window: 31.26 m². Two coats at 9 m² per litre need 6.9 litres: a 5 litre and two 1 litre tins.
• How far does a gallon or a litre go? 250 to 400 sq ft per gallon in one coat by the US makers' figures above (6.1 to 9.8 m² per litre), and about 16 m² per litre for Dulux Australia's paints. Read your tin.

This guide is general information for planning. Follow the maker's instructions for your product, and have damp, cracked or crumbling walls checked by a qualified professional before you paint.

Spotted a mistake or something out of date? Tell us and we'll fix it.