How much paint a room really needs - adjusted for your wall surface, coats, doors and windows, with primer worked out separately.
Each door removes about 20 sq ft.
Each window removes about 15 sq ft.
Most paint calculators use one coverage number: about 400 sq ft per gallon. That is a laboratory figure measured on smooth, sealed walls. On new drywall, plaster or any porous surface, the first coat soaks in and real coverage drops to around 250 sq ft per gallon - which is why so many rooms run dry halfway through the second coat.
This calculator asks about your surface. Smooth or already-painted walls are worked out at 375 sq ft per gallon, new drywall and porous surfaces at 250, and rough or textured walls at 300 - so the number you take to the store matches the room you actually have.
Wall area is the room perimeter multiplied by ceiling height (for a 12 x 10 ft room with 8 ft ceilings: 44 x 8 = 352 sq ft), minus 20 sq ft per door and 15 per window. Multiply by your coats, divide by the coverage for your surface, and you have gallons. Primer is calculated separately at its own coverage, because primer and paint are different products that soak in differently - bundling them into one number is the second-biggest cause of under-buying.
Rounding rule: results are rounded up to whole gallons, and under a gallon you will see quarts. Keep a little extra for touch-ups - the same colour mixed later never matches exactly.
A typical 12 x 10 ft room with 8 ft ceilings needs about 2 gallons for two coats on smooth walls - more on new drywall, where primer matters too.
On new drywall, bare plaster, stains or a big colour change, yes - primer seals the surface so the paint coats evenly. On a sound, already-painted wall in a similar colour, usually not.
About 375 sq ft per coat on smooth, painted walls; around 250 on new or porous surfaces and about 300 on rough texture. The figure on the can (350-400) assumes ideal smooth walls.
Round up to the next whole gallon or quart rather than down - running out mid-wall shows, and a later batch of the same colour never matches exactly. Unopened cans can usually be returned.
Renovating the whole room? See the Tile Calculator, Area Converter, Mulch Calculator and all Home Building Calculators.