Gallons of paint and primer, with cost estimate
Enter your total paintable area, the number of coats, and your paint's coverage rate, and this calculator tells you exactly how many gallons to buy. It also prices an optional primer coat. Most interior latex covers 350–400 ft² per gallon per coat; textured, porous, or bare surfaces can drop that to 250–300 ft².
A 12 ft × 14 ft bedroom with 8 ft ceilings: perimeter = 52 ft, walls = 52 × 8 = 416 ft², ceiling = 168 ft², minus one door (20 ft²) and two windows (30 ft²) = 534 ft². Two coats at 400 ft²/gal: 534 × 2 ÷ 400 = 2.7 → buy 3 gallons. With primer: one more gallon of primer.
Do I always need two coats? For color changes, dark colors, or patch repairs, yes. Touch-ups over a similar color can sometimes get away with one.
Should I prime first? Prime new drywall, repaired areas, stains, or when painting light over dark. Tinted primer dramatically improves coverage on big color changes.
How much does trim paint differ? Trim, doors, and baseboards use far less paint — figure about 1 gallon per 400 linear feet of trim per coat.
Why round up? You can't buy 0.7 of a gallon, and leftover paint is valuable for touch-ups. Always round to the next gallon.