Gallons of paint for a room (2 coats).
Gallons needed = (sq ft × coats) / coverage rate per gallon. Typical coverage 300–400 sq ft/gal for smooth walls.
Round up to the nearest whole gallon and account for touch-ups.
Coverage rate varies more than people expect based on wall texture and paint type — a textured or porous surface can soak up noticeably more paint per square foot than a smooth, previously painted wall.
400 sq ft × 2 coats ÷ 350 sq ft/gal = 2.29 gallons — buy 3 to have a touch-up reserve.
Smooth interior walls: 350–400. Textured or first coat on new drywall: 250–300. Check the can.
Yes over bare drywall, previous dark colors, or stained surfaces. Primer coverage is separate.
For a single door or window, usually not worth the trouble — but for a room with several large windows or doors, subtracting them gives a meaningfully more accurate paint estimate.