Stair Stringer Calculator
A stair stringer calculator finds the length of the diagonal support board and the cut marks for each step. From your total rise and run, it computes the stringer length with the Pythagorean theorem and lays out the rise and run notches. Enter your total rise and chosen run below to get the stringer layout.
Enter your total rise (floor-to-floor height) to see the layout.
Estimate only. Confirm against your local building code: stairs, framing, rebar, roofing and retaining walls are structural and code-dependent. This site does not provide engineering judgment.
How Do You Calculate a Stair Stringer?
To calculate a stringer, first find the number of steps and the rise and run per step from your total rise, then mark the stringer board with a framing square: each step is one rise up and one run across. The overall stringer length is the diagonal, found by the square root of (total rise squared plus total run squared). For 14 steps at 7.71 inch rise and 10.5 inch run, the stringer spans about 177 inches. The calculator gives every cut mark.
How Many Stringers Does a Staircase Need?
A staircase needs at least two stringers (one on each side), and a third in the middle when treads are wider than about 36 inches or the material is flexible, to stop the steps from bowing. Stair width and tread material drive the count. The calculator sizes a single stringer; multiply the board stock by the number of stringers your width requires.
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Frequently asked questions
- How long should my stair stringer be?
- The stringer length is the diagonal: the square root of (total rise squared plus total run squared). A 108 by 140 inch staircase needs a stringer about 177 inches long.
- How many stringers do I need for stairs?
- At least two, one per side, plus a middle stringer when treads exceed about 36 inches wide or the material flexes.