How we calculate
Wall area = perimeter × ceiling height, minus 21 sq ft per standard door and 15 sq ft per average window. One gallon covers about 350 sq ft per coat on smooth walls (rough or unpainted surfaces cover less, 250–300). Primer is counted as one coat over the same area. Round up — running out mid-wall with a custom-mixed color is worse than a spare quart.
Frequently asked questions
How much paint for a 12×12 room?
Walls only, 8 ft ceilings, 2 coats: about 2 gallons. Add roughly half a gallon if the ceiling is included. The calculator above gives your exact room.
Do I really need primer?
On new drywall, bare wood, stains, or a dramatic color change (dark to light), yes — primer is cheaper than a third coat of finish paint and it makes the color true.
Why does professional painting cost so much more than the paint?
You are buying preparation and labor: masking, patching, sanding and two careful coats. Materials are often under a fifth of a pro invoice.