Spiral Stairs Calculator
Calculate spiral staircase dimensions including tread depth, riser height, rotation, and verify building code compliance for your spiral stair design.
How to Use This Calculator
Design code-compliant spiral staircases:
- Enter floor-to-floor height in inches
- Enter overall staircase diameter (60 inch minimum for code)
- Select target riser height (7.5 inches typical)
- Choose total rotation (360 degrees = one full turn)
- Review tread depths, rotation per step, and code compliance
Why This Matters
Spiral staircases require precise calculations because tread dimensions vary from narrow at center to wide at outer edge. Building codes measure compliance at the walkline (12 inches from narrow end).
- Calculate number of treads for floor height
- Verify tread depth meets 7.5 inch minimum at walkline
- Check headroom through rotation
- Ensure IRC code compliance for all dimensions
Worked Examples
9-Foot Floor Height
108 inch rise, 60 inch diameter, 360 degree rotation
Calculation: 108 ÷ 7.5 = 14 risers, actual 7.71 inch riser height
Result: Walkline tread: 5.4 inch - FAILS code (need larger diameter or more rotation)
Standard Residential Spiral
96 inch rise, 66 inch diameter, 450 degree rotation
Calculation: 13 risers at 7.4 inches, more rotation = more tread depth
Result: Walkline tread: 7.8 inch - PASSES 7.5 inch minimum
Common Mistakes to Avoid
- Measuring tread depth at outer edge instead of walkline
- Forgetting headroom compounds with rotation angle
- Using standard 7.75 inch max riser (spirals allow 9.5 inch)
- Ignoring center column diameter in calculations
- Underestimating diameter needs (60 inch minimum for primary stairs)