Tell me your room’s dimensions and I’ll tell you how many gallons of paint to buy, including a waste allowance so you don’t run short two-thirds of the way up a wall.
Paint calculator
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Estimate for planning. Coverage varies by paint, surface, and color change. Buy a little extra rather than risk a second trip.
How I calculate this
The math is simple, but the assumptions are where most calculators quietly differ, so here are mine in the open.
First I find your wall area: perimeter times height, or 2 x (length + width) x ceiling height. Then I subtract the openings you don’t paint. I use 21 square feet for a standard door (about 3 by 7 feet) and 15 square feet for a standard window (about 3 by 5 feet). If you tick “also painting the ceiling,” I add length x width on top.
I multiply the paintable area by your number of coats, then divide by paint coverage. I use 350 square feet per gallon per coat. Cans often claim 350 to 400, but coverage drops on porous, textured, or previously unpainted surfaces, and on a big color change, so I use the low end on purpose. Then I add 10 percent for waste, roller loss, and touch-ups, and round up to whole gallons because you can’t buy 3.1 gallons.
A worked example
Say your room is 14 by 16 feet with 9-foot ceilings, one door and two windows, two coats, ceiling not included. Walls come to 2 x (14 + 16) x 9 = 540 square feet. Subtract one door and two windows (21 + 30 = 51) and you’re left with 489 square feet of actual wall. Two coats makes 978 square feet. Divide by 350 and you get 2.79 gallons; add 10 percent and it’s 3.07. Round up and you need 4 gallons. That’s what the calculator shows, and you can check every step against your own numbers.
A few things I’d tell a friend
If this is your first time painting a room, I think the standard 10 percent waste is a little tight. Bump your own estimate to 15 percent if you’re new to a roller, because the learning tax is real. Buy your gallons in one batch with the same mix code so the color matches, and keep the lid label in case you need more. If you’re going from a dark wall to a light one, plan on the extra coat rather than hoping two will cover it; it usually won’t.
FAQ
How much does a gallon of paint cover? Around 350 to 400 square feet per coat on smooth, previously painted walls. I use 350 in the calculator because real surfaces rarely hit the top number.
Do I count doors and windows? No, subtract them, which the calculator does for you using standard door and window sizes. If yours are unusually large, knock the count down to match the area you’re actually leaving unpainted.
One coat or two? Two, almost always, unless you’re repainting the same color over a clean, sound surface. Color changes and bare drywall need two.
Does this include primer? No. Treat primer as its own coat with similar coverage. If you’re priming bare drywall, run the calculator again with coats set to 1 for the primer.
Built and maintained by Sam Walsh. Last updated June 2026. This is a planning estimate, not a guarantee of exact coverage.
Not sure how to measure the room first? See how to measure a room. Tiling or flooring the same space? Try the tile calculator.