Paint Calculator
Estimate gallons of paint from room size, ceiling height, doors and windows, and number of coats, at standard 350 sq ft per gallon coverage.
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
How to measure the room
You need three numbers: the room’s length, its width, and the ceiling height. Run a tape measure along the longest wall, then the shorter one, and measure floor to ceiling in a corner. Most US homes have 8-foot ceilings; newer builds often run 9 feet, so don’t assume.
Then count the openings you won’t paint. Walk the room and tally doors and windows. A standard 32-inch interior door is about 21 sq ft, and a typical double-hung window is about 15 sq ft. You don’t need exact dimensions for these unless you have something unusual like a sliding patio door (figure that one as two doors, roughly 40 sq ft).
If the room isn’t a simple rectangle, an L-shaped living room for example, split it into two rectangles, measure each, and add the wall lengths together. Closet interiors count too if you’re painting them.
Ceilings are a separate job with a separate number: just length times width, no height involved, and usually one coat of flat ceiling white.
The formula and what it assumes
The calculator works in two steps. First it finds your paintable wall area:
Wall area = 2 × (length + width) × ceiling height, minus door and window square footage.
That’s the room’s perimeter times its height. A 12 × 14 ft room with 8-foot ceilings has 2 × (12 + 14) × 8 = 416 sq ft of wall before subtracting openings.
Then it converts area to paint:
Gallons = (wall area × number of coats) ÷ 350 sq ft per gallon.
The 350 figure is the standard manufacturer coverage rating for smooth, primed, previously painted drywall. Real-world coverage depends on the surface. Fresh drywall, heavy texture like knockdown or orange peel, bare wood, and masonry all drink more paint, sometimes covering only 250-300 sq ft per gallon. If that’s your situation, bump the coats up or mentally shave the coverage down and buy accordingly.
The formula assumes vertical walls and a flat ceiling. Vaulted ceilings, stairwells, and gable walls add triangular area the basic formula won’t catch, so measure those sections separately.
Buying tips
US paint sells in quarts, gallons, and 5-gallon buckets. A quart covers roughly 85-90 sq ft, fine for a single accent wall or trim touch-ups. Gallons are the workhorse. Once a job calls for 4 or more gallons of the same color, a 5-gallon bucket usually costs less per gallon at Home Depot, Lowe’s, Sherwin-Williams, or Benjamin Moore dealers, and it guarantees every drop came from one batch.
Sheen matters as much as color. Flat or matte hides wall imperfections but scuffs easily, so it suits ceilings and low-traffic adult bedrooms. Eggshell and satin are the standard picks for living rooms, hallways, and kids’ rooms because they wipe clean. Semi-gloss goes on trim, doors, kitchens, and baths where moisture and fingerprints are constant. Higher sheen also reflects more light and shows roller marks more, so prep matters more as you go glossier.
Buy one more gallon than the math says if you’re on the borderline. Unopened, untinted cans return easily at the big-box stores. Custom-tinted paint is generally not returnable anywhere, which is exactly why you should get the quantity right before tinting rather than planning a second trip. If you do run short mid-job, bring the can back to the same store so they can match the formula from the label, and try to finish each wall from a single can to avoid visible batch lines.
One more habit worth keeping: label the leftover can with the room name and date. Two years from now, when a chair scrapes the hallway, you’ll be glad you did.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most US manufacturers rate one gallon at 350-400 sq ft on smooth, previously painted drywall. Our calculator uses 350 sq ft per gallon as a conservative standard. Textured, porous, or unprimed surfaces absorb more paint and may only get 250-300 sq ft per gallon.
Do I need two coats of paint?
Usually, yes. Two coats give even color and full hide, especially over a different color or patched drywall. One coat can work when you're repainting the same color in the same sheen with a premium paint-and-primer product.
Should I subtract doors and windows from the wall area?
Yes, if you want a tight estimate. A standard interior door covers about 21 sq ft and a typical window about 15 sq ft. In a room with one door and two windows, that's roughly 50 sq ft you won't paint, about a seventh of a gallon per coat.
How much extra paint should I buy?
Round up to the next quart or gallon and keep the extra. Leftover paint from the same can is the only guaranteed touch-up match, since a new can mixed later can vary slightly in tint. Store it sealed indoors, not in a freezing garage.