🎨 Decorating
How Much Paint for a 12×12 Room?
Wall area, minus openings, divided by the coverage on the tin — then multiplied by the number of coats. That is the whole calculation, and it is the one people get wrong by a factor of two.
How much paint does a 12 × 12 room need?
A 12 × 12 ft room with 8 ft ceilings has 384 sq ft of wall. Deduct one door (about 21 sq ft) and one window (about 15 sq ft) and you have 348 sq ft. At the usual 350 sq ft per gallon, that is almost exactly 1 US gallon, or 3.8 litres, per coat. Two coats need 2 gallons, roughly 7.6 litres.
Most people buy two gallons for a small bedroom and end up with half a tin in the garage, or buy one and run out on the second coat. The difference is almost always whether openings were deducted and whether the tin's coverage figure was taken at face value. Coverage is quoted for a smooth, primed, previously painted surface; bare plaster or a textured wall absorbs 10 to 20 percent more.
Paint for other room sizes
| Room | Wall area | 2 coats | Buy |
|---|---|---|---|
| 10 × 10 ft | 284 sq ft | 1.6 gal (6.1 L) | 2 gal / 7.5 L |
| 12 × 12 ft | 348 sq ft | 2.0 gal (7.5 L) | 2 gal / 7.5 L |
| 12 × 15 ft | 396 sq ft | 2.3 gal (8.6 L) | 2.5 gal / 10 L |
| 15 × 20 ft | 524 sq ft | 3.0 gal (11.3 L) | 3 gal / 11.5 L |
| 20 × 20 ft | 604 sq ft | 3.5 gal (13.1 L) | 4 gal / 15 L |
These assume 8 ft ceilings, one door and one window deducted. A room with a bank of windows or a wall of sliding doors can need 15 percent less; a room with a chimney breast or lots of trim needs more, because those surfaces are fiddly to cover and waste paint on the brush.
Litres or gallons? Coverage is not the same number
A US gallon is 3.785 litres and a UK gallon is 4.546, so "one gallon covers 350 square feet" is a US convention. UK tins quote coverage in square metres per litre, typically 10 to 14 m²/L, which is 8 to 11 m² per US gallon — noticeably better than the US figure because the two markets measure differently and thin their products differently.
Rather than convert, read the tin you are actually buying and put its number into the coverage field. If it says 12 m²/L, multiply by 10.764 to get 129 sq ft per litre, or enter the metric figure by switching the room dimensions to metres and reading the litres result directly.
One coat or two?
Two coats is the standard because the first coat soaks in and the second builds the film that gives the colour and the scrub resistance. A colour change from dark to light, or a switch between sheens, often needs three. Primer is a separate purchase: new plaster, bare wood and patched walls all need it, and it is cheaper to prime than to add a third finish coat.
Ceilings are usually quoted separately and covered in one coat of a flat white; a 12 × 12 ceiling is 144 sq ft, so under half a gallon. Trim and doors are best estimated by length rather than area — roughly 1 litre per 20 metres of skirting at two coats.