How the paint estimate works
Wall area = room perimeter × wall height. Subtract about 20 sq ft per standard door and 15 sq ft per average window. Multiply by the number of coats and divide by the coverage on the can—most interior latex paints state 350–400 sq ft per gallon on smooth surfaces.
Example: a 12 × 12 ft bedroom has a 48 ft perimeter. At 8 ft high: 384 sq ft − 20 (door) − 30 (two windows) = 334 sq ft. Two coats = 668 sq ft ÷ 350 ≈ 1.9 → buy 2 gallons.
When to lower the coverage number
- Bare drywall or fresh plaster: use primer first, or assume ~250–300 sq ft/gal.
- Textured or rough walls absorb more paint.
- Big color changes (dark to light) often need a third coat.
Ceilings and trim
Ceiling area is simply length × width. Trim usually needs well under a quart for a single room.