How to use this calculator
- Measure the room. Length, width, and ceiling height in feet. Walls = perimeter × height; ceiling = length × width.
- Subtract openings. A standard door is ~21 ft²; a standard window ~15 ft². Subtract these from the wall area for accuracy.
- Pick coats. Two coats is the norm. Going dark over light, or unprimed drywall, calls for primer + 2 coats.
- Read your result. You get gallons per coat plus a total. Round up — a partial gallon costs the same as a full one.
Formula
Gallons = (Wall ft² ÷ 350) × number of coats
Worked example
A 12 ft × 14 ft room with 8 ft ceilings has a wall perimeter of 52 ft × 8 ft = 416 ft². Subtract 1 door (21 ft²) and 2 windows (30 ft²) = 365 ft². Two coats = 730 ft² ÷ 350 = 2.1 gallons. Buy 3 gallons (one extra for touch-ups). Add 1 gallon for the ceiling.
Tips for accurate results
- Same color on ceiling and walls? Buy two extra quarts beyond the calculator number for cutting in.
- Paint dark colors over light primer-tinted to the topcoat — saves a coat.
- Trim is bought separately, usually in semi-gloss. A standard room takes ½–1 quart for trim.
- Eggshell hides imperfections better than satin or semi-gloss — best for walls in living spaces.