Multiply the length of the perimeter of the room by the wall height to get the wall area. Multiply length times width for the area of a rectangular shape such as a ceiling or floor. Usually, you are interested in square feet, so the measurements need to be expressed in terms of feet.

Example

Suppose you have a room that is 10' by 12' with a ceiling that is 7' 10" above the floor. The perimeter of the room is 10' + 12' + 10' + 12' = 2(10' + 12') = 2*22' = 44'. The height in feet is 7 10/12 ft = 7 5/6 ft. (An inch is 1/12 ft.) The total wall area is

(44')(7 5/6') = (44*7 + 44*5/6) ft^{2} = (308 + 36 2/3) ft^{2} = 344 2/3 ft^{2}

The ceiling or floor area of the rectangular room is the product of length and width,

(10')(12') = 120 ft^{2} The total surface area of the room is the sum of the areas of ceiling, walls, and floors:

120 ft^{2} + 344 2/3 ft^{2} + 120 ft^{2} = 584 2/3 ft^{2}.

Of course, if you are figuring for paint, you just add up the areas that will be painted.