Spiral Staircase Calculator

Your details

Switch between inches and millimetres. All inputs and outputs update together.
Vertical distance from finished floor to finished floor.
in
The full outer width of the staircase, measured from outside stringer to outside stringer.
in
Diameter of the center pole or open core. Typically 4-6 inches for steel columns.
in
360 degrees for one full turn, 420 for a one-and-sixth turn, 540 for one-and-a-half turns. More rotation improves tread depth but needs more floor area.
deg
Preferred step height. The calculator finds the nearest whole-step count that stays at or below this value. IBC code maximum is 9.5 in (241 mm).
in
Nominal thickness of each tread board. Used for headroom calculation.
in
Distance from the inside stringer to the point where tread depth is measured. IBC sets the walk-line at 12 in (305 mm) from the narrow end.
in
Number of stepsSteep but code-compliant
14

Whole step count that keeps each riser at or below your target

Actual riser height7.714
Tread angle30deg
Tread depth at walk-line7.506
Inner arc length (per tread)1.309
Outer arc length (per tread)15.708
Stair angle (slope)45.5deg
Inner stringer length109.544
Outer stringer length245
Handrail length151.531
Headroom clearance91.07
Inner radius2.5
Outer radius30
Walk-line radius14.5
_unitin
Inner stringer109.544
Handrail (walk-line)151.531
Outer stringer245

14 steps, riser 7.71 in, tread depth 7.51 in at walk-line.

  • Your staircase needs 14 steps with an actual riser height of 7.714 in.
  • The riser height of 7.714 in is within the IBC maximum of 9.5 in.
  • Tread depth at the walk-line is 7.506 in, meeting the IBC minimum of 6.75 in.

Next stepThe stair slope exceeds 40 degrees, which can feel steep. Increase staircase diameter or rotation angle to reduce the pitch.

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