Paint Calculator (walls, coats, coverage)
Work out how much paint a room needs from its size, the number of coats, and the spread rate — with doors and windows deducted.
Openings are counted at standard sizes — a door 1.51 m², a window 1.44 m².
Paint for typical rooms
Two coats at 350 ft² per US gallon, with doors and windows deducted.
| Room | Size | Wall to paint | Paint needed | Gallons to buy |
|---|---|---|---|---|
| Small bedroom | 10 × 10 ft, 8 ft high | 288.3 ft² | 1.65 gal | 2 |
| Bedroom | 12 × 12 ft, 8 ft high | 336.8 ft² | 1.92 gal | 2 |
| Living room | 15 × 20 ft, 9 ft high | 551 ft² | 3.15 gal | 4 |
| Bathroom | 8 × 6 ft, 8 ft high | 192.3 ft² | 1.1 gal | 2 |
| Hallway | 20 × 4 ft, 8 ft high | 335.3 ft² | 1.92 gal | 2 |
Working out how much paint you need
Paint quantities come down to one number — the area you are actually covering — multiplied by the number of coats and divided by how far a tin goes. The area is the room's perimeter times the wall height, less the doors and windows you are not painting.
Wall area = 2 × (length + width) × height · Paint = (area − openings) × coats / spread rate
Worked example
A 12 × 12 ft room with 8 ft walls has a perimeter of 48 ft, giving 384 ft² of wall. Take off one door and two windows — about 47 ft² — and you are painting roughly 337 ft². Two coats makes 673 ft², and at 350 ft² per gallon that is 1.92 gallons. Buy two.
Spread rates vary more than you'd think
The figure on the tin assumes a smooth, sealed, previously painted surface. Bare plaster, new drywall, and anything textured drink far more — a heavily textured wall can take half again as much. Deep colours over pale ones, or the reverse, often need three coats rather than two. If you are unsure, round generously: a spare half-tin is useful for touch-ups later, and paint mixed to the same colour on a different day rarely matches exactly.
Should you deduct doors and windows?
For a normal room, yes — this calculator does it by default, and it is worth several square metres. But if the room has one small window and you would rather have extra paint than run out, leaving them in is a reasonable safety margin. Set both counts to zero to see the difference. What you should not deduct is furniture-height sections you think you can skip; you will paint them anyway.
Two coats is the usual answer
One coat is only enough when you are repainting the same colour on a sound surface. Anything else — a colour change, bare substrate, or a patch repair — wants two, and going from a dark colour to a pale one often wants a primer plus two. Primer is calculated the same way; just run the numbers again with its own spread rate.
Related
The ceiling figure above is simply length × width, and it is worth a separate tin because ceiling paint is usually a different product. For the geometry of an awkwardly shaped room, add the wall sections up with thearea calculator.