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The quick answer: one gallon of interior paint covers about 350 to 400 square feet per coat, and most jobs need two coats, so a gallon realistically finishes about 175 to 200 square feet of wall. To find your number: measure the perimeter of the room, multiply by the ceiling height to get total wall area, subtract about 20 square feet per door and 15 per window, then divide by 350 and multiply by the number of coats. A typical 12 by 12 bedroom needs about two gallons for the walls at two coats.
We do not test paint on our own walls; our product picks come from aggregated owner reviews and expert consensus, described in how we rank. The math below, though, is universal.
The coverage formula in plain terms
Every paint estimate comes down to one calculation. Get this right and the rest is arithmetic.
- Measure the perimeter. Add up the length of every wall you are painting. For a 12 by 15 room that is 12 + 15 + 12 + 15 = 54 feet.
- Multiply by ceiling height. Standard ceilings are 8 feet. 54 x 8 = 432 square feet of gross wall area.
- Subtract openings. Deduct about 20 square feet per standard door and 15 per average window. Two doors and two windows removes about 70 square feet, leaving 362.
- Divide by coverage. Use 350 square feet per gallon to stay safe: 362 divided by 350 is about 1.03 gallons per coat.
- Multiply by coats. Two coats needs about 2.1 gallons, so you buy 3 gallons or, more economically, 2 gallons plus a quart with a small buffer.
The single most common mistake is calculating for one coat. Always multiply by two unless you know you are doing a same-color refresh.
Why the can says 400 but you get less
Manufacturers print 350 to 400 square feet per gallon on the label, and that figure is honest, but it assumes a smooth, sealed, primed surface applied by an efficient roller. Real walls rarely match that. Coverage per gallon shifts with the surface and the paint.
| Surface | Coverage per gallon, per coat |
|---|---|
| Smooth, primed drywall | 375 to 400 sq ft |
| Previously painted, same sheen | 350 to 400 sq ft |
| Bare or patched drywall | 300 to 350 sq ft |
| Light texture (orange peel) | 300 to 350 sq ft |
| Heavy texture (knockdown) | 250 to 300 sq ft |
| Brick, block, or stucco | 150 to 250 sq ft |
When in doubt, calculate with 350 rather than 400. Buying a few extra dollars of paint is far cheaper than a second trip and a possible color-batch mismatch.
One coat or two: how to decide
Coat count drives your total more than any other factor, so decide it before you buy.
- Two coats (the default): any color change, painting over patched or bare drywall, going lighter over a darker color, or using a mid-grade paint.
- One coat (rare): repainting the exact same color over a clean, same-sheen surface with a premium paint.
- Three coats or a tinted primer: deep saturated colors (red, bright yellow, deep teal) or a big jump from dark to light.
Priming counts too. If you are covering bare drywall, repairs, or a drastic color shift, a coat of primer effectively adds a "coat" to your material list. See how to prime a wall before painting for when it is worth it, and how long to wait between coats so the second coat bonds properly.
Worked examples by room size
Here is how the formula lands for common rooms, walls only, at two coats, with typical doors and windows subtracted. Add a gallon whenever you also paint the ceiling.
| Room | Wall area (net) | Two-coat need | Buy |
|---|---|---|---|
| Bathroom, 5 x 8 | ~180 sq ft | ~360 sq ft | 1 gallon |
| Bedroom, 10 x 10 | ~290 sq ft | ~580 sq ft | 2 gallons |
| Bedroom, 12 x 12 | ~340 sq ft | ~680 sq ft | 2 gallons |
| Living room, 15 x 20 | ~500 sq ft | ~1000 sq ft | 3 gallons |
| Open great room, 20 x 25 | ~660 sq ft | ~1320 sq ft | 4 gallons |
These assume 8 foot ceilings. For 9 foot ceilings multiply wall area by 1.125, and for 10 foot ceilings by 1.25 before dividing by coverage.
Ceilings, trim, and doors
Walls are only part of most jobs. Budget separately for the other surfaces.
- Ceilings: length x width gives the area. A 12 x 12 ceiling is 144 square feet, needing about 0.8 gallons at two coats, so buy a gallon of dedicated flat ceiling paint.
- Trim and baseboards: a quart covers a small room's trim; a gallon handles the trim, doors, and windows of an average whole house floor. Trim paint is applied thin, so a little goes far. See how to paint trim and baseboards.
- Doors: figure about 20 square feet per side of a standard door. A quart covers roughly six door sides.
Because ceiling and trim paint differ in sheen and formula from wall paint, they are always a separate purchase, not part of your wall gallon count.
Deep and dramatic colors
Saturated colors are where paint budgets blow up. Deep reds, bright yellows, and some vivid blues use organic pigments that hide poorly, so they can need three or even four finish coats on their own. The professional fix is a gray-tinted primer, which the paint counter mixes to a shade that complements your topcoat. One coat of tinted primer plus two finish coats almost always beats four coats of finish paint, and it costs less.
If you are jumping from a dark wall to a pale color, the same principle applies in reverse: prime first so the old color does not ghost through. Choosing the color itself is its own project, covered in how to choose paint colors.
Buy a buffer, and buy it in one batch
Two habits save more grief than any calculation trick. First, round up to the next full gallon rather than the next quart. The price difference is small, and having leftover paint means future touch-ups blend perfectly. Second, buy all the paint for a room in a single tint batch. Paint tinted at different times, even to the same formula, can differ just enough to show on a wall in raking light.
Keep the leftover sealed and labeled with the room and date. A well-sealed can lasts years, and a matched touch-up is invisible in a way that a fresh re-tint never quite is.
The tools that affect how far paint goes
How you apply paint changes how much you use. A quality roller with the right nap loads and releases paint efficiently; a cheap roller sheds, skips, and wastes material. Sprayers cover fast but use noticeably more paint than a roller because of overspray, so add 10 to 20 percent to your estimate if you spray.
For most rooms a roller plus a brush is the most economical route. The right nap for your wall texture matters: see what nap roller for smooth walls, then pick a tool from best paint rollers. When you are ready to buy the paint itself, the ranked field is in best interior paint. Everything else lives at the paint and walls hub.