Estimate how much paint you need for walls and ceilings, including coats.
Paint is sold by volume but used by area, so the calculation runs area first. The walls of a rectangular room are the perimeter — twice length plus width — times the ceiling height, which is why a long thin room takes more paint than a square one of the same floor area. Doors and windows come off at roughly 20 sq ft each, and the ceiling goes on only if you are painting it. Divide by the spread rate on the can and multiply by the coats. Two coats is the normal answer rather than a luxury: one coat over a different base colour leaves a patchy, translucent finish. Under-buying is the expensive mistake, because a second trip usually means a different batch and a visible seam where the two meet.
Room Paint Needed
Wall area = 2 x (length + width) x height, plus ceiling if painted, less 20 sq ft per opening; gallons = area x coats / spread rate
About 1.5 gallons for two coats on the walls with three openings deducted, so buy two cans. Add roughly another half gallon per coat if you are doing the ceiling.
On bare drywall, over a dark colour or on a repaired surface, yes — primer is what stops the topcoat soaking in unevenly. Over sound, similar-coloured paint you can usually skip it.