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Buy the right number of gallons the first time.
The difference between one gallon and three is whether you subtracted the windows and used the coverage printed on your can instead of a guess. This calculator nets out doors and windows, multiplies by your coat count, divides by real coverage — and separates primer, because new drywall drinks a different product than finish paint.
Weekend painters use it to buy once — the can coverage, coat count, and openings math decide whether it is a 2-gallon or 3-gallon job. Landlords refreshing units between tenants use the cost layer to compare DIY against a painter's day rate.
With 8-ft ceilings, 1 door, and 2 windows: about 340 sq ft of wall, so 2 coats at 350 sq ft/gal needs 2 gallons. Add the ceiling (168 sq ft) and you are at 3 gallons. Always round up — matching a touch-up batch later is nearly impossible.
On new drywall, yes — bare joint compound and paper face absorb paint unevenly and flash through the finish. Over a sound, similar-color existing paint, a quality paint-and-primer-in-one with two coats usually skips the separate primer step.