Paint Calculator
Find out how many gallons of paint to buy for a room — accounting for doors, windows, ceilings, and the number of coats — with a cost estimate.
Enter length, width & ceiling height
Assumes ~21 sq ft per door and ~15 sq ft per window. Adjust counts to match your room.
Estimate only. These figures help you plan how much to buy — they are not engineering or structural advice. Actual needs vary with site conditions, waste, and installation method, so order a little extra and confirm final quantities with your supplier or contractor before you buy or start the job.
Popular room sizes
Jump to a ready-made answer for a common room.
How the paint calculation works
Paint is sold by coverage area, so the math is paintable area × coats ÷ coverage per gallon.
2 × (length + width) × ceiling height. Add length × width if you're painting the ceiling too.
Take out about 21 sq ft per door and 15 sq ft per window to get the paintable area.
Paintable area × coats ÷ coverage (≈ 350 sq ft/gallon), rounded up to whole gallons.
Prime bare drywall, patches, and big color changes. Count primer as an extra coat at similar coverage.
Sources & assumptions
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 ft × 12 ft room with 8 ft ceilings has about 384 sq ft of wall. Subtract a door (~21 sq ft) and a window (~15 sq ft) and you have ~348 sq ft paintable. Two coats at 350 sq ft/gallon needs about 2 gallons.
How many square feet does a gallon of paint cover?
One gallon of interior wall paint covers about 350–400 square feet in a single coat. Rough or porous surfaces and deep color changes cover less and may need an extra coat or primer.
How do I calculate paint for a room?
Find the wall area: add the lengths of all walls and multiply by ceiling height (for a rectangular room that's 2 × (length + width) × height). Subtract doors and windows, multiply by the number of coats, then divide by the paint's coverage per gallon.
Should I use one or two coats?
Two coats is standard for an even, durable finish, especially when changing colors. One coat may be enough for minor refreshes of the same color. Prime bare drywall, patches, and stains first.
Do I need to subtract doors and windows?
For a more accurate estimate, yes. This calculator subtracts about 21 sq ft per door and 15 sq ft per window. If you have unusually large windows, lower the count or switch to the wall-area mode and enter your own number.