It’s not a magic formula, but you should be aware of some basic facts whenever you purchase paint for a room. Follow my guide and you’ll do fine.
First, I always tell people to buy a reliable paint from a national manufacturer and remember to purchase one of its higher grades.
Second, you must to buy enough paint to actually finish the job in one project. What I mean is, if you run out of paint and then jump in the car to buy more, you have a half-finished wall. The problem you’ll encounter is that even if the color match is perfect, the overlap will show.
The standard for purchasing is to remember that a gallon of paint will be good for 400 to 450 square feet. Simply calculate the number of square feet by measuring the total footage around the perimeter of the room and multiplying by the ceiling height in feet.
(Don’t subtract for windows, doors or other openings unless they’re huge.)
What about the ceiling area? Just calculate the width times the depth of the room.