Stair Calculator
Enter a floor-to-floor rise to size the flight — risers, riser height, tread run, total run, and pitch, checked against IRC R311.7.
The finished floor-to-floor height the stair has to climb.
- 16 risers at 6.81" with 15 treads at 11" (riser ≤ 7¾" R311.7.5.1, tread ≥ 10" R311.7.5.2).
- 2R + T = 24.6" is within the comfortable 24–25" range (rule of thumb).
- 4 or more risers — at least one handrail required, 34–38" high (R311.7.8).
- Provide ≥ 6'-8" headroom (R311.7.2) and ≥ 36" stair width (R311.7.1).
How the stair is sized
Given the total rise (the finished floor-to-floor height), the calculator targets a comfortable ~7" riser, then divides the rise into equal risers and adds one if needed so each stays within the 7¾" maximum (IRC R311.7.5.1). A flight always has one more riser than tread, so the total run is the tread depth times the number of treads.
The tread depth (run per step) must be at least 10" (R311.7.5.2). As a comfort check, the old carpenter's rule 2 × riser + tread ≈ 24–25" (the Blondel rule) flags stairs that will feel too steep or too shallow — it's a rule of thumb, not code.
Other R311.7 limits to keep in mind: at least 6'-8" of headroom (R311.7.2), a minimum 36" stair width (R311.7.1), a handrail 34–38" high where there are 4 or more risers (R311.7.8), and no single flight rising more than 151" between landings (R311.7.3). These are residential (IRC) values — commercial stairs follow the IBC, and local amendments vary.