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How the math works
The 350 rule
Most interior latex paints claim about 350 square feet per gallon on a smooth, primed wall. That number comes from the can, measured under ideal conditions — so we treat it as the starting point, not the whole answer. Divide your paintable area by 350 for standard paint, and the result is how many gallons one coat takes. Everything else in this calculator is about adjusting that honest baseline to your actual room.
Why two coats
One coat colors the wall; the second coat is what makes it look professional. The first coat soaks into thirsty spots and dries unevenly — flashing, roller marks, and thin patches. The second coat evens out the color and the sheen so the finish looks the same from every angle. Two coats is the industry standard for a reason, which is why it's the default here. A third coat is rarely needed unless you're covering a dramatic color change.
Doors and windows come off the wall total
You don't paint the doorway or the glass, so we deduct them — but only from the wall surfaces, where they actually are. A standard interior door is 3 × 7 feet, or 21 square feet; an average window counts as 15. Ceilings and trim are computed on their own, with no openings deducted, because a window doesn't take area off your ceiling.
Why trim is a separate number
Baseboards, window casings, and crown molding are almost always painted with a different product than walls — typically a semi-gloss or gloss enamel that stands up to scuffs and wipes clean. Mixing trim into your wall-paint total would inflate the wrong can. So we estimate trim on its own (roughly your room's perimeter × 6 inches of trim height) and give it its own gallon figure, in its own paint.
Why premium paint covers more
Coverage comes down to solids: the pigment and resin left behind after the liquid evaporates. Premium paints pack more solids into each gallon, so each gallon stretches further — around 400 square feet instead of 350. On a big room that can mean a full gallon less to buy, which sometimes pays for the upgrade by itself.
Primer: when it pays for itself
New or unpainted drywall is absorbent — paint sinks in unevenly and you'll burn through extra coats trying to get even color. Primer seals the surface so your topcoat behaves, usually saving at least one coat of the expensive stuff. Primer also earns its keep over dark-to-light color changes and patched or repaired spots. Check the box if your drywall is new, and we'll add a one-coat primer figure (wall area ÷ 300 sq ft per gallon, rounded up to the quart).
Paint calculator FAQ
How much paint do I need for a 12×12 room?
A 12×12 room with 8-foot ceilings, one door, and two windows has about 333 square feet of wall (after deducting the openings) plus 144 square feet of ceiling — 477 square feet in total. Two coats of standard paint at 350 sq ft per gallon works out to 2.73 gallons, so you buy 2.75 gallons. Trim is figured separately, since it usually takes its own semi-gloss paint.
Should I include the ceiling in my paint calculation?
Only if you plan to paint it — but know that it adds a lot. A 12×12 ceiling is 144 square feet, nearly a third of the room's total paintable area. This calculator figures the ceiling as its own line so you can see exactly what it costs you. Most people use a flat ceiling paint, which is a different product from wall paint anyway, so buy it as a separate line item.
Do I really need primer?
For new or unpainted drywall, yes — bare drywall drinks paint, and primer seals it so your topcoat covers evenly in two coats instead of three or four. Primer also pays off when going from a dark color to a light one, or over patched and repaired spots where the texture differs. For a straightforward repaint in a similar color over sound paint, you can usually skip it and put the money toward a second finish coat.
Why does the calculator suggest more paint than the coverage number on the can?
The coverage figure on the can — often 350 to 400 sq ft per gallon — is measured under ideal lab conditions: a perfectly smooth, primed surface with one thin, even coat. Real rooms have texture, porous patches, and cut-in edges, and a good-looking job needs two coats. This calculator uses the can's coverage rate honestly, then multiplies by your number of coats and rounds up to the nearest quart. That rounding and the second coat are where the extra comes from — and that leftover paint is what you'll want for touch-ups later.
What if my walls are textured or rough?
Textured, stippled, or rough walls can need 10–20% more paint than smooth walls, because the texture adds real surface area that a flat length × height measurement misses. This calculator assumes smooth, rolled walls. If yours are textured, take the gallon figure and add roughly a quart per gallon as a buffer. Sprayers also use more paint than rollers — budget about 15% extra if you're spraying. And whatever you buy, keep one sealed quart for touch-ups.