The paint coverage formula
Start with the wall area, subtract openings, multiply by the number of coats, and divide by the coverage rate of the paint:
wall area = total wall length × height
paintable = wall area − (doors × 21) − (windows × 15)
gallons = ( paintable × coats ) ÷ coverage per gallon
A standard interior door is about 21 sq ft and an average window about 15 sq ft; one gallon typically covers 350 sq ft in a single coat, though that varies by paint and surface.
Worked example
40 ft of wall, 8 ft high, 2 coats, minus 1 door and 2 windows:
Wall area: 40 × 8 = 320 sq ft.
Minus openings: 320 − 21 − 30 = 269 sq ft.
Two coats: 269 × 2 = 538 sq ft of coverage.
Gallons: 538 ÷ 350 ≈ 1.54 → round up to 2 gallons.
Coats and coverage
Two coats is standard for an even finish, and more if you're going from a dark colour to a light one or painting bare drywall (which may need a primer coat first). Rough, porous or textured surfaces drink more paint, so they cover less than the rated 350 sq ft.
Tip: buy a little extra from the same batch for touch-ups — colour can vary slightly between batches, and matching later is hard.