The question of how much paint to buy is a tricky question…On the paint label there is a prediction of square foot coverage and as mentioned earlier it may or may not be accurate. Good guideline is a small bedroom (10x10x 8 feet high) will need a gallon of paint if a room is bigger and has taller walls then chances are you will need a second gallon. I always assess and if I think I need 5 gallons of paint I buy 3 and see how it goes. Considering you cannot take back tinted paint you may as well start small.