How the paint calculation works
Paint is sold by volume but used by area, so the job is to find the area you will cover and divide it by what one gallon or litre covers.
- Wall area. Add the four wall lengths to get the perimeter, then multiply by the wall height.
- Remove openings. Subtract about 20 sq ft (1.9 m²) for each standard door and 15 sq ft (1.4 m²) for each window.
- Add the ceiling if you are painting it: length × width.
- Multiply by coats, then divide by the coverage printed on the can.
gallons = ((2 × (L + W) × H) − openings + ceiling) × coats ÷ coverage
Worked example
A 14 × 12 ft bedroom with 8 ft walls has a perimeter of 52 ft, so 416 sq ft of wall. One door and two windows remove 50 sq ft, leaving 366 sq ft. Two coats make 732 sq ft. At 350 sq ft per gallon that is 2.1 gallons: buy 2 gallons and a quart, or a third gallon if you want paint left for touch-ups.
How many coats do you need?
| Situation | Coats |
|---|---|
| Same or similar colour, wall in good shape | 1 |
| Most repaints and colour changes | 2 |
| Light colour over a dark one | Primer + 2 |
| New, unpainted drywall or plaster | Primer + 2 |
| Deep reds, yellows and oranges | Tinted primer + 2 or 3 |
Work out primer as a separate job. Its coverage is often lower than paint on bare, porous surfaces, so read its own label.
Coverage depends on the surface
The figure on the can assumes smooth, primed walls. In the US and Canada most interior paints list about 350 to 400 sq ft per gallon. In Australia, tins usually state coverage in m² per litre, often somewhere between 10 and 16. Textured walls, bare plaster, brick and old flat paint all soak up more, so lower the coverage figure in the calculator by 15 to 25 percent for those surfaces.
Before you buy
- Buy all the paint for one room at the same time so the tint matches.
- Box the paint: pour every can into one bucket and mix, so colour stays consistent across walls.
- Trim, doors and skirting boards usually take a different sheen. Measure them separately.
- Keep a little leftover, labelled with the room and date, for later touch-ups.
Frequently asked questions
How much does one gallon of paint cover?
Most interior paint covers about 350 to 400 square feet per gallon for one coat on smooth, primed walls. That is roughly 9 to 10 square metres per litre. Rough or porous surfaces get noticeably less.
How much paint do I need for a 12 × 12 room?
With 8 ft walls, a 12 × 12 room has 384 sq ft of wall. Remove one door and one window and you have about 349 sq ft. Two coats come to roughly 700 sq ft, which is about 2 gallons of paint.
Should I subtract doors and windows?
Yes, if you want an accurate figure. In a room with few openings the difference is small, but in a room with patio doors or many windows it can save you a gallon.
Is it better to buy a bit too much paint?
A small surplus is useful for touch-ups and for a second coat on patchy areas. Running out mid-wall is worse, because a new batch can look slightly different.
Last updated November 1, 2026.