Wall Area: The total surface area of all four walls before any deductions. This is the perimeter of the room multiplied by the ceiling height.
Paintable Area: The wall area minus standard deductions for doors (21 sq ft each) and windows (15 sq ft each). This is the actual surface you need to cover with paint.
Gallons (1 coat): The amount of paint needed for a single coat, assuming 350 sq ft of coverage per gallon. Most paint jobs over a similar color can get by with one coat of premium paint.
Gallons (2 coats): Double the one-coat amount, which is typical when painting over a darker color, switching to a drastically different shade, or using lower-coverage paint. Two coats generally provide the best finish.
How This Calculator Works
You enter the room length, width, and ceiling height in feet along with the number of doors and windows. The tool calculates the perimeter, multiplies by height for total wall area, subtracts 21 sq ft per door and 15 sq ft per window, then divides by a standard 350 sq ft per gallon coverage rate. It does not include the ceiling, trim, or accent walls, and assumes smooth to lightly textured walls.
Quick Questions
Why does the calculator use 350 sq ft per gallon?
Most interior latex paints cover 350–400 sq ft per gallon on smooth, primed surfaces. We use 350 as a slightly conservative estimate to account for typical wall texture and application losses from rollers and brushes.
Should I always plan for two coats?
Two coats are recommended when making a dramatic color change, painting over stains, or using a lighter color over a darker one. If you are refreshing a wall with a similar shade using premium paint, one coat may suffice.
Does this include the ceiling?
No. This calculator estimates wall paint only. Ceiling paint is typically sold separately and covers at a slightly higher rate (about 400 sq ft per gallon) since ceilings are usually smooth and painted in flat white.
What if my room is not rectangular?
For L-shaped or irregular rooms, divide the space into rectangular sections, calculate each one separately, and add the totals. Alternatively, measure the full perimeter of the room in feet and multiply by the ceiling height manually.
How much extra paint should I buy for touch-ups?
Keep at least a quart of each color for future touch-ups. Paint stores can also store your color formula for easy reorder, though dye lots may vary slightly over time.