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Free calculatorNo signupShort answer: One gallon covers about 350 to 400 sq ft per coat. Wall area is 2 × (length + width) × height, minus about 20 sq ft per door and 15 per window. A 12×12 room with 8-ft walls, one door and two windows is 334 sq ft, so two coats take 668 ÷ 350 = 1.9, about 2 gallons.
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Wall area = 2 × (length + width) × height − 20 sq ft per door − 15 sq ft per window
Gallons = wall area × coats ÷ coverage per gallon
Ceiling gallons = length × width × coats ÷ coverage
Most paint makers list 350–400 sq ft per gallon on smooth walls. Use 300 or less for textured, porous or new drywall, or when you are covering a dark color.
12 × 12 room, 8-ft walls, 1 door, 2 windows, 2 coats, 350 sq ft per gallon:
Paint, drywall, flooring, deck boards and tile — exact shopping lists. Save jobs, brand your quotes and stop re-typing numbers.
Two coats is standard for a color change or fresh drywall. One coat can be enough when repainting the same color over a sound finish.
Prime new drywall, patched areas, stains and big color changes. Primer often covers less area than finish paint on porous surfaces, so check the coverage on the can.
About a quart covers the trim, doors and baseboards in a typical bedroom.