How it works
Enter the room size, the number of doors and windows and how many coats you want. The calculator finds the paintable wall area and divides by the coverage on the can, then rounds up to whole gallons.
Most interior latex paints cover 350 to 400 square feet per gallon on smooth, primed drywall. Textured walls, bare drywall and dramatic color changes soak up more, so use 300 sq ft per gallon or plan on primer.
Two coats is the norm for even color and durability. A little left over is useful for touch-ups; store it sealed and labeled with the room.
Formula
Wall area = 2 × (length + width) × height − 20 sq ft per door − 15 sq ft per window. Gallons = (wall area + ceiling) × coats ÷ coverage per gallon.
Questions people ask
How much does a gallon of paint cover?
About 350 to 400 square feet for one coat on smooth, previously painted walls. Rough or porous surfaces may get only 250 to 300.
How much paint for a 12 × 12 room?
With 8 ft walls, one door and two windows there is about 334 sq ft of wall. Two coats at 350 sq ft per gallon is 1.9 gallons, so buy 2 gallons.
Do I need primer?
Prime new drywall, patched areas, stained surfaces and big color changes (dark to light). Many "paint and primer" products still need a separate primer on bare drywall.
Is a 5 gallon bucket cheaper?
Usually, per gallon, once you need 4 or more gallons of the same color. Five gallon pails also keep the color consistent across the job.