Measure the bottom stair, in inches, to determine the length of the stair runner. Begin at floor level, and measure the height of the riser from the floor to the front edge of the first stair. The riser is the board that elevates each step from the previous step. Record this figure. Then measure the depth of the stair from front to back. Record this measurement. Add these two measurements together. The sum represents the length of carpet required to cover one stair.
Multiply the length of carpet required to cover one stair in inches by the number of stairs in the stairway. This measurement is the total length of carpet required to cover the entire stairway.
Determine the width of the stair runner in relation to the width of the stairs. Decide if you want the carpet to cover all of the area of each step or only the center part, with exposed wood on either side. Record the desired width in inches.
Multiply the width of the stair runner in inches, determined in Step 3, by the total length of the carpet runner to cover the entire stairway in inches, determined in Step 2. The answer is the total number of square inches of carpeting required for the stair runner. To convert this figure into square feet, divide the total by 144 (the number of inches in a square foot). The resulting figure is the number of linear square feet of carpeting required for the stair runner.