1. Calculate the square footage of the surfaces to be painted. Measure the length and width of the room and determine its perimeter, which is the distance all around the room. For example, if the room is 13 feet wide and 18 feet long, its perimeter is 62 feet (13 feet + 13 feet + 18 feet + 18 feet).
2. Multiply the perimeter by the room's height to get the square footage of wall space. If the room is 8 feet high, then its square footage is 496 square feet (62 feet x 8 feet).
3. Count the doors and windows in the room. Subtract from your wall area 21 square feet for each standard door and 15 square feet for each standard window. (If your room has large doors, such as a sliding patio door, or large windows, you can measure the width and height of each door and window, then figure the exact square footage of each. You don't need to be precise; round to the nearest square foot.)
4. To find the number of gallons of paint needed, divide the wall area by 300 -- the square footage easily covered by a gallon of interior paint.