How many gallons your room actually needs — doors and windows deducted, second coat included, no leftover-gallon graveyard in the garage.
We take your four walls (2 × (length + width) × height), knock off about 21 sq ft per door and 15 per window, then multiply by coats. The 350 sq ft/gallon default is for smooth walls — textured or never-painted drywall drinks more, so drop coverage to 250–300 and recalculate.
One honest warning: going from a dark wall to a light color usually takes an extra coat (or a tinted primer). If that's you, bump coats to 3 — it's cheaper than discovering the old navy blue ghosting through your "warm white" at 9pm on a Sunday.
The quart tip matters because paint stores round against you: quarts cover ~87 sq ft, and three quarts cost more than a gallon. Buy the combo we suggest, and keep whatever's left for touch-ups.
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A gallon of wall paint covers roughly 350–400 square feet in one coat. To size a job, add up your wall area (perimeter × ceiling height), subtract the big openings — doors and windows — and then multiply by the number of coats. Most color changes and fresh drywall need two coats for an even finish, so a room that's 400 sq ft of wall really means about 800 sq ft of painting.
A 12 ft × 12 ft room with 8-ft ceilings has a 48-ft perimeter × 8 ft = 384 sq ft of wall. Take out a door and two windows (about 60 sq ft) and you're near 324 sq ft per coat. Two coats is ~650 sq ft, so two gallons covers it comfortably with a little to spare for touch-ups. Ceilings, trim, and doors are figured separately since they often use different paint.
One coat or two?
Plan on two for color changes, bold colors, or new drywall. One coat only works when repainting a similar color over a sealed, previously painted wall.
Do I really subtract doors and windows?
For a tighter estimate, yes — but leaving a small buffer means you won't run out mid-wall, which is worth more than saving a half-gallon.
What about primer?
Bare drywall, patches, and drastic color changes need primer, which is essentially an extra coat to budget for.