How the calculator works
Wall area is the room perimeter times ceiling height, minus standard openings of 21 sq ft per door and 15 sq ft per average window:
Gallons = (wall area × coats) ÷ 350
350 sq ft per gallon is the realistic mid-point of manufacturer claims (which run 250–400). The result is rounded up to the quarter gallon, because paint is sold in quarts and gallons and because leftover paint from the same batch is your only perfect color match for touch-ups.
When one coat is a lie
"One-coat" paints work when you're repainting a similar mid-tone color on smooth walls. Plan on two coats when: changing color noticeably, covering red/yellow/deep tones, painting over patches or new drywall (prime first), or using deep accent colors, which have less hiding pigment. Dramatic color changes over dark walls can take a tinted primer plus two finish coats.
Don't forget
- Trim and doors: a typical room's trim takes about a quart of enamel; each door side about ⅛ quart.
- Primer: new drywall drinks its first coat, so figure 300 sq ft/gal for primer.
- Sheen strategy: flat/matte hides wall flaws (bedrooms, ceilings); eggshell/satin wipes clean (halls, kids' rooms); semi-gloss for trim, baths, kitchens.