Stair Calculator
Measure the total rise from finished floor to finished floor. The calculator splits it into equal risers, checks them against the residential code, and gives the run and stringer length.
- Treads
- 14
- Total run
- 11 ft 8.0 in
- Stringer length (approx.)
- 14 ft 8.8 in
- Angle
- 37.6°
- 2 × riser + tread
- 24.4 in
- Code check
- Within IRC limits
How the stair is laid out
Risers = total rise ÷ target riser, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1 · Run = treads × tread depth
Stringer ≈ √(rise² + run²)
The top floor acts as the last step, so there is one tread fewer than risers. Example: a total rise of 108 in with a 7¾ in maximum riser needs 14 risers of 7.71 in and 13 treads. With 10 in treads the run is 130 in, and the stringer is about 14.1 ft before cutting.
Code limits (IRC R311.7)
The International Residential Code allows a maximum riser of 7¾ in and requires a minimum tread of 10 in. Risers within one flight may differ by no more than ⅜ in, and you need at least 6 ft 8 in of headroom. Many states and cities amend these rules, so check with your building department.
The 2 × riser + tread figure is an old comfort rule (Blondel). Stairs between 24 and 25 in feel natural for most people.
Buy stringer stock longer than the calculated length. The figure is the diagonal, and the bottom cut and top connection add to it.