How much paint do you need? The square footage of the area and the spread rate of your paint are all you need to know.
Two factors go into figuring out how much paint you will need for a given project: the square footage of the area and the spread rate of the paint. (A gallon of paint usually covers about 400 square feet, but this information should be on the can.)
To calculate square footage, multiply the width by the height of each wall and then add the figures together. Next, calculate the area of all surfaces that will not be painted, such as windows, fireplaces, and the like. Again, multiply width times height and add the figures together. Then, subtract this number from the first number. Finally, add 10% so you will have some paint left over for touch-ups or repairs in the future.
To calculate how many cans of paint you will need, divide your final number by the spread rate of the paint. To calculate the amount of paint for the trim, figure it at about one-quarter the amount of wall paint.