Formula and input notes
Risers = ceil(total rise ÷ target riser); walking-line run = average radius × total rotation in radians
| Input | Practical note |
|---|---|
| Target riser height | This is a planning target used only to choose a whole riser count; the calculator then recomputes the equal actual riser height. |
| Inner radius | Measure from the stair center to the inner edge of the walking annulus used by this approximation. |
| Outer radius | This must exceed the inner radius; their average becomes the planning walking radius in the displayed model. |
| Total rotation | Enter the total angular sweep of the spiral flight, not the angle of one tread. The calculator divides it by the whole tread count. |
Worked spiral staircase example
Spiral Staircase Calculator example: ceil(126.4 in ÷ 7.25 in) = 18 treads at 7.02 in actual rise. Over 360°, each tread turns 20° and the average-radius walking line runs 105.24 in.
Interpret tread rotation and walking-line run
Use angle per tread and actual riser to explore geometry before checking a complete manufacturer or engineered stair layout.
Walking-line, headroom, and structural checks
Real spiral stairs use code-defined walklines, tread shapes, center columns, landings, headroom, clear width, handrails, and proprietary connections.
Questions specific to this calculation
How is the spiral walking line estimated?
The tool uses the average of the entered inner and outer radii as a planning walking radius, multiplies it by total rotation in radians for plan run, and combines that run with total rise for a helix length.
Why can the actual walking line differ?
Codes and manufacturers may define the walking line at a specific offset from the narrow end rather than at the average radius. Center post size, tread support, clear width, landings, headroom, and overlap also affect usable geometry.
Does this calculator design a spiral stair?
No. It provides a geometric screening estimate only. Use a code-compliant manufacturer or qualified designer for tread shape, structure, guards, handrails, landings, and connections.