How much paint do I need?
Measure the room, count the doors and windows, and this page tells you exactly what to carry to the register: gallons for the walls, plus ceiling, trim, and primer if you want them. The math is the standard estimator's method, planned at 350 sq ft per gallon.
How the math works
Wall area is the room perimeter times ceiling height: a 12x12 room with 8 ft ceilings has 2 × (12 + 12) × 8 = 384 sq ft of wall. Knock off 20 sq ft per door and 15 per window and you're at 334 sq ft of surface that actually takes paint. Multiply by coats, divide by coverage, done. Two coats there is 668 sq ft of spreading; at 350 sq ft per gallon that's 1.9 gallons, so you buy two.
Coverage is where estimates go wrong. The 400 sq ft figure printed on cans is a smooth-wall, thin-nap, second-coat number. Textured walls, 3/4-inch roller naps, and thirsty patched spots pull it down fast, which is why we plan at 350 and offer a textured-wall option at 300. Running out with one wall left costs you a trip; a leftover quart costs you nothing, because you'll want it for touch-ups anyway.
One honest caveat: this is estimating math, not a site visit. Vaulted ceilings, stairwells, and board-and-batten walls need their own measuring tape.
Quick reference: common room sizes
Walls only, 8 ft ceilings, one door, two windows, two coats at 350 sq ft per gallon. Each size links to a full page with 9 and 10 ft ceiling tables.
Questions people ask
How much paint do I need for a 12x12 room?
Two gallons, for two coats on the walls of a 12x12 room with 8 ft ceilings, one door, and two windows. That room has about 334 sq ft of paintable wall; two coats is 668 sq ft of spreading, and at 350 sq ft per gallon you need 1.9 gallons. Buy two.
Do I subtract doors and windows?
Yes. Standard estimating practice knocks off 20 sq ft per door and 15 sq ft per interior window. The calculator does this for you. Skipping the deduction on a small room can push you into a gallon you never open.
How many square feet does a gallon of paint cover?
The can says 350 to 400 sq ft per coat on smooth, previously painted drywall. We plan at 350, the conservative end, because rollers with heavy nap, textured walls, and first coats over patches all drink more. Rough or textured surfaces can drop real coverage below 300.
Is paint-and-primer-in-one really one coat?
Over a similar color on clean walls, sometimes. Over a color change, bare drywall, or patched spots, no. Self-priming paint is thicker paint, not a coat of primer; plan two coats for any color change and you will not be disappointed at 5 pm.
Should I buy three quarts or a gallon?
The gallon. At big-box pricing three quarts costs more than a gallon of the same line, so the calculator rounds three quarts up automatically. Leftover paint keeps for touch-ups if you seal the can well.