Work it out in four steps
- Add up the length of every wall and multiply by the ceiling height.
- Subtract about 20 square feet for each door and 15 for each window.
- Divide by the coverage on the can to get gallons per coat.
- Multiply by the number of coats and round up.
A worked example
A 12 by 12 foot room has 48 feet of wall. At 8 feet tall, that's 384 square feet. Subtract one door and one window and you have about 349 square feet, or roughly 1 gallon per coat at 350 square feet per gallon. Two coats means about 2 gallons.
When you'll need more
- Big color changes, especially light over dark or the reverse.
- New drywall or patched areas that need primer first.
- Rough or textured walls, which soak up more paint.
- Touch-ups later: keep a little extra from the same batch.