Paint calculator
Enter the room size (or each wall), the doors and windows, and how many coats you'll apply. You get gallons and the gallon-and-quart cans to buy, with optional ceiling, trim and primer. The defaults are a 12 × 12 ft room with 8 ft walls, one door, one window and two coats at 350 ft² per gallon.
How the calculator works
- Wall area: for a rectangular room, 2 × (length + width) × wall height. In wall-list mode, each wall's width × height, added up.
- Openings: the number of doors and windows × the area of each, subtracted. Add the ceiling (length × width) if it gets the same paint.
- Paint: area × coats ÷ coverage per gallon.
- Cans: the gallons are rounded up to whole quarts, then split into gallons and leftover quarts (4 quarts to the gallon).
- Primer (optional): one coat over the same area ÷ the primer's coverage. Trim (optional): length × face height, × coats ÷ coverage.
Worked example
A 12 × 12 ft bedroom with 8 ft walls, one door and one window, two coats at 350 ft² per gallon:
- Walls: 2 × (12 + 12) × 8 = 384 ft². Openings: 20 + 15 = 35 ft². Area: 349 ft².
- Paint: 349 × 2 ÷ 350 = 1.99 gal, which rounds up to 8 quarts: 2 gallons.
- Add the ceiling (144 ft²) and it's 493 × 2 ÷ 350 = 2.82 gal, or 12 quarts: 3 gallons. In practice ceilings often get a dedicated ceiling paint, so run them separately.
- Primer, one coat over 493 ft² at 400 ft² per gallon: 1.23 gal, which is 1 gallon + 1 quart.
Coverage, coats and primer
- Coverage: Sherwin-Williams' data sheets for its main interior lines (SuperPaint, Emerald, Duration Home, Cashmere) all give 350–400 ft² per gallon, at the film thickness they specify. Rough, textured or porous surfaces take more, so the calculator defaults to the low end. The number on your can's label beats any default.
- Coats: two is the default because that's what Sherwin-Williams' SuperPaint data sheet specifies for every surface it lists, including drywall.
- Primer: for new drywall the same data sheet lists one coat of primer under two coats of paint, or two coats of the paint used as its own primer. Patched areas and big color changes usually benefit from primer too; the paint's label says what it recommends.
Assumptions and limits
- Door and window sizes are averages you can change. Large windows, sliding doors and built-ins can take a real bite out of the total, so measure them.
- The ceiling option is for a flat ceiling the size of the floor. For sloped or vaulted ceilings, use the wall list and add each ceiling plane as a "wall".
- There's no extra allowance for touch-ups or spills. Rounding up to whole quarts usually leaves a little over.
- Dark or bright colors may need more coats; tinted bases and colorants affect hiding. Check the label or ask the store.
Related: hanging new drywall first? The drywall calculator works out sheets, compound and tape for the same room, and the guide to fixing a hole in drywall covers patching before you paint, and how to remove wallpaper covers stripping paper before primer.
Ventilate the room while you paint and while it dries, and read the safety data on the can. If your home was built before 1978, the old paint may contain lead: sanding or scraping it can create hazardous dust, so test it first and follow the EPA's lead-safe practices. Use a stable ladder and don't overreach.
Sources
- Sherwin-Williams, SuperPaint Interior Latex product data sheet (PDF): 350–400 ft² per gallon at 4 mils wet, and the recommended systems for drywall (primer + 2 coats, or 2 coats self-priming).
- Sherwin-Williams product data sheets for Emerald Interior, Duration Home Interior and Cashmere Interior (PDFs): 350–400 ft² per gallon.
- Sherwin-Williams, Premium Wall & Wood Primer: 400 ft² per gallon at 4 mils wet.
- Sherwin-Williams, Paint calculator FAQ: a gallon typically covers about 350–400 ft², depending on texture.
- US EPA, Protect your family from sources of lead.