Spiral Staircase Calculator
Plan a spiral staircase: risers, total rotation and tread depth at the walkline.
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What this tool does
Plan a spiral staircase: risers, total rotation and tread depth at the walkline. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
How to use the Spiral Staircase Calculator
- Enter or select total floor-to-floor rise in in.
- Enter or select maximum riser height in in.
- Enter or select rotation per step (degrees).
- Enter or select stair diameter in ft.
- Read the calculated result; change any measurement to compare alternatives.
Formula
risers = ceil(rise / max riser); treads = risers − 1; total rotation = treads × rotation per step; walkline tread depth = 2π × (radius − 12 in) × rotation/360
- rise
- Total floor-to-floor rise (in, before metric conversion)
- maxRise
- Maximum riser height (in, before metric conversion)
- rotation
- Rotation per step (degrees)
- diameter
- Stair diameter (ft, before metric conversion)
The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
Worked example
For spiral staircase calculator, the following measurements illustrate the exact method: Total floor-to-floor rise: 108; Maximum riser height: 7.75; Rotation per step (degrees): 30; Stair diameter: 5.
Inputs
- Total floor-to-floor rise108 in
- Maximum riser height7.75 in
- Rotation per step (degrees)30
- Stair diameter5 ft
Result
- Risers14
- Individual rise7.71 in
- Treads13
- Total rotation (degrees)390
- Tread depth at walkline9.42 in
- Tread depth at outside edge15.71 in
Results explained
- Risers
- Risers from the formula above. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
- Individual rise
- Individual rise from the formula above, expressed in in before metric conversion. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
- Treads
- Treads from the formula above. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
- Total rotation (degrees)
- Total rotation (degrees) from the formula above. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
- Tread depth at walkline
- Tread depth at walkline from the formula above, expressed in in before metric conversion. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
- Tread depth at outside edge
- Tread depth at outside edge from the formula above, expressed in in before metric conversion. The walkline is taken 12 in in from the outer edge (the usual checking line for spiral tread depth). Many codes cap riser height and set minimum walkline tread depth and headroom for spiral stairs — verify locally; spiral stairs are often restricted to secondary access.
Frequently asked questions
Divide the floor-to-floor rise by the maximum riser height and round up: 108 in at 7.75 in max needs 14 risers (13 treads) at an actual rise of 7.71 in each.
About 30° per step is standard — 12 steps per full turn. More rotation per step makes the stair steeper and the walkline treads shallower.
At the walkline, conventionally 12 in from the outer (handrail) edge — not at the narrow centre pole, where treads taper to almost nothing.
Residential kits commonly run 3 ft 6 in to 6 ft diameter. Bigger diameters give deeper walkline treads and gentler climbing; check the well opening and headroom too.
Often not — many codes allow them only as secondary stairs or for small areas (some allow them as the primary stair under conditions in recent IRC editions). Verify with your building department.