Pacific Roll LLC
Spiral Stair Calculator
Enter any known dimensions — radius, rise, treads, rotation — and solve for helix angle, developed stringer length, equivalent bend radius, and more.
Units (length)
Direction of Rotation
Known Values
Enter whatever dimensions you have — the calculator solves for the rest.
To the inner face of the inner stringer — enter this or Inside Radius of Outer Stringer (plus Tread Width) and the other will be solved
Section depth of inner stringer — used to compute CL developed length (optional)
To the inner face of the outer stringer — enter this or Inside Radius of Inner Stringer (plus Tread Width) and the other will be solved
Section depth of outer stringer — used to compute CL developed length (optional)
Radial depth between inner faces (outer inside radius minus inner inside radius)
Vertical height of full stair
Total tread count
Arc length at the inside face of the outer stringer per tread — requires inside radius of outer stringer to be entered first
Full rotation of stair in degrees (e.g. 360 = one full turn)
Vertical rise per tread
How to Use
- 1.
Select your unit of measurement.
- 2.
Enter any dimensions you know — inside radius, outside radius, tread width, total rise, number of treads, degrees per tread, total rotation, or riser height.
- 3.
You need at least: one radius value (inside or outside), one rise value (total rise or riser height), and two of the three rotation values (treads, degrees/tread, total rotation).
- 4.
Results appear instantly — helix angle, developed lengths for inside/centerline/outside stringers, equivalent bend radii, and chord per tread.
- 5.
Use Print / Save PDF to export a results sheet for your shop.
Definitions
Inside / Outside Radius
Horizontal radius measured to the inner or outer face of the stringer.
Tread Width
Radial depth of each tread — equal to outside radius minus inside radius.
Total Rise
Vertical height from bottom to top of the stair.
Riser Height
Vertical rise per individual tread.
Run of Tread (Outside)
Arc length at the outside radius per tread. Enter this instead of degrees per tread — requires outside radius to be entered first.
Degrees per Tread
Horizontal rotation swept by each tread. Solved automatically from run of tread and outside radius.
Total Rotation
Total horizontal rotation of the stair. 360 = one full turn.
Helix Angle
Angle of the stringer from horizontal — steeper stairs have a larger helix angle.
Inside Radius of Inner Stringer
Distance from the center of the stair to the inner face of the inner stringer.
Depth of Inner Stringer
Section depth of the inner stringer (e.g. 6 for a W6). When entered, the CL developed length is computed at ir + depth/2.
Inside Radius of Outer Stringer
Distance from the center of the stair to the inner face of the outer stringer.
Depth of Outer Stringer
Section depth of the outer stringer. When entered, the CL developed length is computed at outer inside radius + depth/2, and the outside face radius is used for run-of-tread.
Developed Length
Actual curved length of the stringer — what you order from the mill.
Equiv. Bend Radius
The radius to set on the rolls. A helix bends tighter than a flat arc at the same horizontal radius — this accounts for the pitch.
Chord per Tread
Straight-line 3D distance across one tread step, useful for layout.
Equivalent bend radius accounts for helix pitch. For tight-radius spiral stairs, contact our team — springback and section distortion affect final dimensions.
Also try
Arc Length Calculator →For standard circular bends — angle, arc, chord, and all three radii.