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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