Hp is the height of the window sill, or the distance between the completed floor and the edge of the wall opening under the window sill. The hp also includes any raised area in front of the window such as a platform or parapet. The width of the room is measured from outer wall to outer wall, including doorways and passages. The height of the room is measured from floor to ceiling. The area of a room is calculated by multiplying the height and width of the room together. Then, dividing the result by 12 gives you the square footage of the room. For example, if the room is 72 inches high and 48 inches wide, its area is: 432/12 = 36 square feet. Some buildings have multiple floors separated by interior staircases. In this case, each floor is a separate room with its own area calculation. You will need to add up the areas of all the rooms you are considering when choosing an apartment. Most apartments include a parking space for your vehicle. When calculating the area of your apartment, there are two ways to account for the space: you can note the number of square feet or the linear feet available.