Paint Calculator

Start with the wall area. For a rectangular room, that is the perimeter times the wall height: 2 × (length + width) × height. Subtract the parts you will not paint. The calculator uses about 20 sq ft per door and 15 sq ft per window, which matches a standard 3 × 6.7 ft interior door and a 3 × 5 ft window. Multiply the remaining area by the number of coats and divide by the coverage printed on the can.

Worked example: a 12 × 14 ft bedroom with 8 ft walls has 2 × 26 × 8 = 416 sq ft of wall. One door and two windows remove 50 sq ft, leaving 366 sq ft. Two coats at 350 sq ft per gallon need 732 ÷ 350 = 2.09 gallons, so you buy 3 gallons, or 2 gallons plus a quart if the math is tight.

Most interior latex paints list 350–400 square feet per gallon on smooth, primed drywall. The calculator defaults to 350, the conservative end, because real walls absorb more than a test panel. Use the lower end, or reduce the number further, for:

• Textured walls, popcorn ceilings, brick and rough plaster (often 250–300 sq ft per gallon)
• New unprimed drywall, which soaks up the first coat
• Deep reds, yellows and other saturated colors that need an extra coat to look even

Self-priming or "paint and primer" products still usually need two coats when you change color significantly.