Stair Calculator
Design code-compliant stairs in seconds. Enter total rise and desired step count to get rise per step, run per step, total run, stringer length, and whether it meets IRC code.
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.
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
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Why use Stair Calculator?
- Accurate material estimates reduce waste and over-ordering
- Supports both metric and imperial measurements
- Covers common materials: concrete, lumber, flooring, paint
- Results include waste factor buffers for professional accuracy
Common use cases
- Calculate concrete needed for a driveway
- Estimate flooring tiles for a room renovation
- Work out paint quantity for interior walls
- Calculate lumber needs for a deck project
- Estimate roofing material for a pitched roof
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