Stair Calculator (Stringer, Rise, Run)
Calculate the number of stairs, individual rise, run, and stringer length for any total height.
What this calculates
Building stairs requires getting the rise and run exactly right — code-compliant, comfortable, and consistent step-to-step. This calculator gives the number of treads, the rise per step, the run per step, and the diagonal stringer length given the total height. Most US codes: max 7.75 inch rise, min 10 inch run.
Formula & how it works
Number of risers = round(total_rise ÷ ideal_rise). Each rise = total_rise ÷ risers. Treads = risers − 1 (top floor is the last 'tread'). Each run typically 10-11 inches. Stringer length (diagonal) = √(total_rise² + total_run²) where total_run = treads × run_per_step.
Worked example
Floor to floor: 108 inches (typical 9-foot ceiling + joist). Target rise ~7.5 inches. Risers = round(108/7.5) = 14. Each rise = 108/14 = 7.71 inches (within code). Treads = 13. With 10.5 inch run: total run = 13 × 10.5 = 136.5 inches (11 ft 4.5 in). Stringer ≈ √(108² + 136.5²) ≈ 174 inches.
Frequently asked questions
Code limits?
US IRC: max rise 7.75 in, min run 10 in (with nosing) or 11 in (no nosing). Variance between steps ≤ 0.375 in. Headroom 6'8". Local codes can be stricter. Always check before building.
What's a stringer?
The diagonal board on each side that supports the treads. 2×12 dimension lumber is standard. Cut the rise and run pattern into the top edge. Typically 3 stringers for stairs up to 36 in wide; more for wider.
Are stair calculators useful for spiral stairs?
Not really. Spiral and curved stairs have different geometry. They're usually engineered or kit-purchased rather than built from raw lumber.
Why does rise consistency matter?
Inconsistent rises cause tripping. Brain memorizes the rhythm by the third step; a single off step is a frequent fall site. Building inspectors usually measure all rises before passing.