How this calculator works
It measures your wall area (room perimeter × wall height), subtracts standard sizes for each door and window, multiplies by the number of coats, and divides by a gallon’s coverage of about 350 sq ft.
Worked example — a 12 ft × 12 ft room, 8 ft walls, 2 coats:
Perimeter 48 × 8 = 384 sq ft of wall. Minus 1 door (21) + 2 windows (30) = 333 sq ft. × 2 ÷ 350 = 1.9 gallons → buy 2 gallons.
Perimeter 48 × 8 = 384 sq ft of wall. Minus 1 door (21) + 2 windows (30) = 333 sq ft. × 2 ÷ 350 = 1.9 gallons → buy 2 gallons.
Standard openings used
Each door counts as 21 sq ft and each window as 15 sq ft. Adjust the counts if your openings are unusually large.
Frequently asked questions
How much paint for a 12×12 room?
About 2 gallons for two coats on 8-foot walls.
How many square feet does a gallon cover?
Roughly 350 sq ft per coat on a smooth, primed surface.