How to Calculate Stair Stringers
Step-by-step math for laying out a stair stringer: finding total rise, riser height, tread depth, and stringer length before you cut a single board.
A stair stringer is the diagonal support board that carries the risers and treads — get its length and notch layout wrong and the whole staircase is off. The math starts with one measurement you can't adjust: total rise.
Step 1: Measure total rise
Total rise is the exact vertical distance from the lower finished floor to the upper finished floor. Measure it directly with a level and tape or a laser level — don't estimate from framing drawings, since actual floor thickness and finish materials shift the real number.
Step 2: Solve for riser count and height
Divide total rise by your target riser height (commonly 7-7.5 inches) and round to the nearest whole number — that's your riser count. Divide total rise by that whole number to get the actual riser height every step will use; it should land at or under the 7¾" (196 mm) code maximum.
Step 3: Find total run and stringer length
Number of treads = risers − 1. Multiply by your tread depth (10" minimum by code) to get total run. Then apply the Pythagorean theorem: stringer length = √(total rise² + total run²). Add a few extra inches when buying stock to allow for the notch cuts at both ends.
The Stair Calculator runs this exact sequence for you and flags whether your riser height and tread depth meet typical residential code minimums.