Generally speaking, a gallon of paint will cover about 350 square feet. If this is the first time drywall is painted, it will soak up more paint. And remember to factor in whether your job will require one or two coats of paint. Dark and bright colors will require more coats of paint, as will painting a lighter color over an existing darker color. Divide the paintable wall surface by 350 to determine how many gallons of paint you will need to buy. If there’s an odd number with less than .5 leftover, round down, if it’s over .5 round up.