The amount of paint required to paint a room will depend on whether or not the room has been previously painted. Primed, textured or unfinished walls will absorb more paint than a wall that already has been coated with paint. Also, the ceiling height will need to be accounted for in your calculation. As a rule of thumb, one gallon will cover about 350 square feet. If you run out and have your paint can, the formula for mixing another can will be on the can and another batch can be mixed to match.