Every project starts with a number.

Most interior latex paints cover 350–400 square feet per gallon per coat, per Sherwin-Williams’ Duration Home product data sheet. This calculator defaults to 350 — the conservative end — so you are less likely to run short. Porous surfaces, heavy texture, and drastic color changes all reduce real-world coverage.

Yes. The standard estimating practice (Hamilton & Hamilton estimating guide) subtracts 20 square feet per door and 15 square feet per average window. Some pros use 21 and 16 — the difference is small, and this calculator uses 20/15.

Double the one-coat number. Two coats is the standard for most repaints: the first coat builds color, the second evens it out. One coat is only realistic for same-color refreshes with a quality paint.

On new, unpainted drywall — yes, always. Drywall and joint compound are porous and will drink finish paint unevenly. Check the "new drywall" box and the calculator adds one coat of PVA primer at about 300 square feet per gallon (KILZ PVA drywall primer technical data sheet: 300–400). On previously painted walls in good condition, primer is usually unnecessary.

Yes — this calculator rounds up to the nearest quart automatically. Paint stores cannot mix a fraction of a quart, and a little extra covers touch-ups later. Running out mid-wall risks a visible color mismatch between batches.