How far a gallon of paint actually goes
Every can in the store says 350 to 400 sq ft. Both numbers are true, in the way a car's highway mileage is true: achieved under conditions your project may not have.
Where the can's number comes from
The printed rate assumes smooth, sealed, previously painted drywall, a 3/8-inch roller nap, and a painter who isn't mashing the roller dry. Sherwin-Williams and Behr both publish 350 to 400 sq ft per gallon for their interior lines under those conditions, and a second coat over the first one really does hit the top of that range, because the wall is freshly sealed and slick.
Our calculators plan at 350 across the board. The bottom of the honest range costs you at most a leftover quart; the top of it costs you a 6 pm trip in painting clothes.
What each surface really delivers
| Surface | Plan on | Why |
|---|---|---|
| Smooth wall, repaint | 350 to 400 | The can's own conditions |
| Bare drywall, first coat | 275 to 300 | Fresh mud and paper soak up the first pass |
| Orange peel / knockdown | 300 | Texture adds surface the tape measure can't see |
| Popcorn ceiling | 250 | Heavy texture nearly doubles the true surface |
| Rough-sawn wood, shingles | 200 to 250 | End grain and shadow gaps drink paint |
| Stucco, brick, block | 200 to 250 | Porous and rough at once, the worst case |
The quiet coverage thieves
Nap size matters more than people think: a 3/4-inch cover built for texture carries about a third more paint per pass than a 3/8-inch, which is the point, but it means the texture rates above already assume you switched. Color plays a part too. Bright yellows, oranges, and reds are notoriously weak hiders, and organic reds especially can act like a two-coat color that needs three. And sprayers throw 20 to 30% of the can into the air as overspray; if you spray, buy for it.
The good news: none of this compounds secretly. Pick the right row from the table, use the matching calculator setting, and the math holds. The room calculator has the texture option built in; so does the exterior one.