Draw a diagram of your home's floor space and include all of the rooms and halls that you plan to carpet.
Divide the floor space into smaller rectangles. For example, a single room or a hallway may occupy a rectangular area. An L-shaped room can be divided into two rectangles, one for the base of the L and one for the stem.
Measure the width and length of each rectangle with the measuring tape. Round the measurements up to the nearest whole foot.
Compute the area of each rectangle by multiplying the width and length.
Add the areas of all of the rectangles to find the total square footage.
Multiply the cost of carpet per square foot by the total square footage. For example, suppose the cost of carpeting is $1.25 per square foot and the total square footage (just the floor space you plan to carpet) is 340. Then the total cost of carpet is ($1.25)(340) = $425.