Paint Coverage Calculator

Painting a room comes down to a simple question: how many square feet of wall are you actually covering, and how many gallons does that take? This calculator gets there by first finding the total wall area, then subtracting the parts you won't be painting, then dividing by how much a gallon of paint actually covers.

Wall area starts with the room's perimeter — twice the length plus the width — multiplied by the wall height, which gives you the total square footage of all four walls combined. From there, the calculator subtracts space for doors and windows, since paint doesn't go on glass or open doorways. It assumes a standard interior door takes up about 21 square feet and a standard window about 15 square feet; if your room has an oversized picture window or a double door, your actual paintable area will be a bit smaller than the estimate.

Once the paintable area is set, the calculator multiplies it by your number of coats, since a second coat means painting the same square footage all over again. Most walls look best with two coats, especially with a color change or a lighter color going over a darker one, though a single coat of a quality paint-and-primer product can sometimes get the job done.

Finally, that total is divided by 350 square feet, which is the coverage a single gallon of paint typically provides per coat on primed drywall — the number printed on most paint cans. The result is rounded up to a whole gallon, since paint is sold in full cans and you can't buy a fraction of one. Textured walls, unprimed surfaces, or very porous drywall can reduce actual coverage, so it's worth buying slightly more than the estimate, or keeping the receipt in case you need another can partway through.