Stair Calculator
Calculate stair rise, run, total stringer length and angle to design a code-compliant staircase between two floors.
How to use the Stair Calculator
- Enter your inputs into the Stair Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The stair geometry
Number of risers = ceiling height ÷ ideal riser · · · Total run = (risers − 1) × tread depth · · · Stringer = √(rise² + run²)
IRC residential code: max riser 7.75", min tread 10". The 2R + T = 24–25" rule produces comfortable stairs. Total rise / number of risers must yield a consistent riser height for all steps.
Worked example
For a 108-inch floor-to-floor rise with 7" risers: 108/7 = 15.4, round to 15 risers at 7.2" each. With 11" treads: total run = 14 × 11 = 154 inches. Stringer length ≈ √(108² + 154²) ≈ 188 inches.
Frequently asked questions
What's the code for residential stairs?
IRC 2021: max riser height 7.75", min tread depth 10", min headroom 6'8", min handrail height 34–38". State codes may vary. Always verify with local building department.
Why are all risers the same height?
For safety. A single different-height riser disrupts the rhythm of climbing and is the #1 cause of stair falls. Codes require uniformity within 3/8" across all risers in a flight.
What is the ideal rise-to-run ratio?
2 × rise + run = 24–25" produces a comfortable stair (7" rise + 11" run = 25" works well). Steeper stairs feel uncomfortable; shallower take more floor space.