Carpenters lay out a staircase's form on a framing member called a stringer. Viewed from profile, the stringer is flat on its lower edge, resembles a zigzag pattern on its upper edge and connects to opposite floor surfaces at both its top and bottom. To calculate the length of the stringer and the size of individual treads and risers, carpenters must measure the overall rise of the staircase and calculate the size of each step. Equipped with this basic information, carpenters can calculate the length of a stringer and transfer the values onto framing lumber.
Things You'll Need
 Tape measure
 Calculator or pencil and paper

Draw a tape measure straight up and down between the floors that the staircase will span; the distance between the floors is the staircase's total rise. Divide the total rise by the rise of an individual stair to determine the total amount of treads. Carpenters generally choose 7 inches as a stair height because a rise of roughly 7 inches and run of roughly 11 inches (run referring to the distance the stair travels) places the staircase at a comfortable 30degreeto35degree angle.

Round the result of the calculation up or down to achieve a whole number of stairs. For example, a total rise of 36 inches divided by 7 equals approximately 5.14 stairs, which rounds down to 5 treads. Multiply the number of treads by 11 inches to determine the total run (distance traveled) of the staircase. For example, 5 treads multiplied by 11 inches equals 55 inches.

Divide the total rise of the staircase by the revised number of treads to determine the final rise of each riser. For example, a total rise of 36 inches divided by 5 steps equals 7.2 inches rise per step, or roughly 7 13/64 inches on a standard tape measure. Use the determined values of each stair's rise and run to lay out cut marks on stringer framing lumber.
Tips & Warnings
 Use a carpenter's framing square equipped with stair stops or "buttons" to accurately lay out a stringer.
 Remember to account for differences in the specific design of your staircase, such as if the top step rests flush or below the staircase's upper landing.
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