Paint math is simple once you know the one number everything hangs on: a gallon of wall paint covers roughly 350 to 400 square feet in a single coat on smooth, previously painted drywall. Use 350 for planning. It builds in a margin for roller loading, textured walls, and the fact that no wall drinks paint in perfectly even coats.
Add up the wall lengths. Measure each wall and total them — that's the room's perimeter. A 12×12 room is 48 feet around. For an L-shaped room, just measure and add every wall segment; the method doesn't care about the shape.
Multiply by the ceiling height. Perimeter × height = gross wall area. The 12×12 room with 8-foot ceilings: 48 × 8 = 384 square feet.
Subtract doors and windows. Use 21 square feet for a standard door (3 ft × 7 ft) and about 15 square feet for an average window. One door and one window: 384 − 21 − 15 = 348 square feet. Small windows under about 10 square feet aren't worth subtracting — the paint you'd "save" is your margin for error.
Divide by 350, then multiply by coats. 348 ÷ 350 ≈ 1 gallon per coat. Two coats is the standard for even color and full coverage, so this room needs 2 gallons of finish paint. Round your final number up to the next quart, never down.
| Room | Wall Area After Deductions | Paint Per Coat | Buy for 2 Coats |
|---|---|---|---|
| 10×10 ft, 8-ft ceiling, 1 door, 1 window | ~284 sq ft | ~0.8 gal | 2 gal (1 gal + 1 qt gets it done) |
| 12×12 ft, 8-ft ceiling, 1 door, 1 window | ~348 sq ft | ~1 gal | 2 gal |
| 12×16 ft, 8-ft ceiling, 1 door, 2 windows | ~397 sq ft | ~1.1 gal | 2 gal + 1 qt |
| 16×20 ft, 9-ft ceiling, 2 doors, 3 windows | ~561 sq ft | ~1.6 gal | 3 gal + 1 qt |
Notice the leftovers. They're not waste — a partial gallon in the garage is your touch-up supply for the next several years of scuffs and nail holes, in the exact same batch. Keep the lid sealed tight and store it where it won't freeze.
Ceilings are easy: length × width, no deductions. Ceiling paint covers about the same 350 square feet per gallon, and most ceilings need one coat over an existing white, two over stains or bare drywall.
Trim and doors barely dent the budget. A quart of trim paint usually handles one average room's baseboards, window frames, and one side of a door; a full gallon of trim paint covers roughly 400 square feet and lasts most people several rooms. This is also where finish choice matters more than quantity — see our paint sheen comparison before you buy trim paint in the same flat finish as the walls.
Primer spreads thinner than finish paint on hungry surfaces. Over bare drywall or patched areas, plan on 200 to 300 square feet per gallon. New drywall genuinely needs it — skip the primer and the first finish coat just becomes an expensive primer, which is one of the classic reasons paint ends up peeling a year later. Over sound existing paint in a similar color, you can usually skip primer entirely.
Bump your estimate by about 20 percent if the walls are heavily textured, brick, or rough stucco — porous surfaces can drop coverage toward 250 to 300 square feet per gallon. Add a third coat's worth if you're covering a dark color with a light one, or going to a deep red or navy, which are notoriously thin-hiding colors. A tinted primer under deep colors often saves that third coat. And if the walls are freshly patched in several spots, spot-prime the patches first; unprimed compound shows through finish paint as dull blotches no matter how much you buy.
Buying one gallon for a two-coat job, then having the second gallon mixed days later in a visibly different batch. Measuring only two walls and doubling, in a room that isn't actually square. Forgetting the closet interior and the wall space above cabinets. And the mirror-image mistake — buying three gallons "to be safe" for a one-gallon room. Most stores will take back unopened, untinted stock, but custom-tinted paint is usually yours forever. If you're new to this, following a start-to-finish walkthrough like our guide on how to paint a room will flag the spots people forget to count.
Measure the perimeter, multiply by height, subtract 21 per door and 15 per window, divide by 350, and multiply by two coats. That five-minute calculation gets you within a quart of the right amount for almost any room in the house.
With 8-foot ceilings, one door, and one window, about 348 square feet of wall — one gallon per coat. Buy 2 gallons to cover the standard two coats, plus a quart of trim paint if you're doing the baseboards and door too.
One coat of it, in a small-to-average bedroom, yes. It will not cover two coats in anything bigger than a small nursery or office, and it won't stretch to a living room at all. Plan per coat, not per room.
Almost always. One coat over a similar color can look acceptable in flat paint under soft light, but two coats give you the color on the chip, even sheen, and washability. Over bare drywall, primer, or a big color change, two finish coats is the minimum, not the deluxe option.
A standard door is about 21 square feet per side; with the frame, call it 25. A quart paints roughly two door sides with trim paint, with some left over. Doors drink a little more on the first coat if they're raw or previously oil-painted and being converted to latex over primer.
Unopened cans can usually be returned if they were never tinted; tinted paint typically can't. Extra paint isn't a disaster, though — a sealed leftover gallon stored indoors keeps for years and is the only way to get an invisible touch-up later, since a fresh mix rarely matches a wall that's been sun-fading for two years.