How to use Stair Calculator
The stair calculator computes the rise, run, number of steps and stringer length for any total rise height. It checks compliance with IRC residential building code (rise 4"–7.75", minimum run 10") and shows the cut stringer measurements for a standard open-riser stair.
- Enter the total rise (floor-to-floor height or deck height) in inches.
- Enter a target riser height (or leave blank to auto-calculate).
- Review number of steps, rise per step, run (tread depth) and total run.
- Check the IRC code compliance indicator.
Your data never leaves your device — 100% private processing.
IRC stair code and the 2R+R formula
IRC residential code requires: riser height between 4" and 7.75"; minimum tread depth (run) of 10"; maximum nosing projection 3/4"–1.25". The ergonomic comfort formula is 2R + R = 24"–25": twice the rise plus the run should equal 24 to 25 inches. A 7" rise × 11" run = 7×2 + 11 = 25" — comfortable and code-compliant.
| Rise per step | Run (tread) | 2R+R | Code status |
|---|---|---|---|
| 6.0" | 12.5" | 24.5" | Code — very comfortable |
| 7.0" | 11.0" | 25.0" | Code — most common |
| 7.5" | 10.0" | 25.0" | Code minimum run |
| 7.75" | 10.0" | 25.5" | Code maximum rise |
| 8.0" | 9.0" | 25.0" | Violates IRC — too steep |
Stringer length and material
Stringer length = √(total run² + total rise²). This is the hypotenuse of the right triangle formed by the stair. A 3×12 dimensional lumber is standard for cut stringers; 2×12 is marginal for runs over 10 steps. At least 3.5" of solid wood must remain below the deepest cut — less means a weak stringer. Use pressure-treated lumber for outdoor stairs.
Worked examples
9 ft floor-to-floor (108 in)
Inputs: 108 in ÷ 7.5 in ≈ 14.4 → 15 risers
Result: 15 risers × 7.2 in each (IRC-compliant)
Stringer length (15 risers, 11 in run)
Inputs: √(154² total run + 108² total rise)
Result: ≈ 188 in stringer (14 treads × 11 in)
2R+R comfort check
Inputs: 2 × 7.2 in rise + 11 in run
Result: 25.4 in — at upper end of 24–25 in comfort range
Glossary
- Riser
- The vertical face of a stair step; height from one tread to the next.
- Tread
- The horizontal surface of a step; where you actually step.
- Stringer
- The diagonal structural board supporting the treads and risers.
- Nosing
- The tread overhang beyond the riser face; adds depth and grip.
Related reading
Frequently Asked Questions
Why use Stair Calculator?
- Computes rise, run and stringer length for any total floor-to-floor height automatically
- Flags non-compliant riser heights and tread depths against IRC residential code limits
- Displays the 2R+R ergonomic comfort check alongside the code pass/fail result
- Calculates cut stringer measurements ready for direct layout on the lumber
Common use cases
- Design a deck stair with the correct rise and run before purchasing stringers
- Verify an existing stair meets IRC code when finishing a basement renovation
- Plan a floating interior staircase with a comfortable 7-inch rise and 11-inch tread
- Calculate stringer length and step count for a split-level floor-to-floor transition
- Check whether a proposed stair configuration is code-compliant before a permit inspection
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