Practical tools for measured work
Spiral stair dimension planner
Calculate spiral diameter, riser height, angular pitch, walking-line arc/chord and tread elevation table.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
The plan projects all turns onto one plane. Multiple turns can overlap; use the angle and elevation table to distinguish them.
Spiral tread positions
| Tread | Start angle | End angle | Elevation (mm) |
|---|---|---|---|
| 1 | 0 | 24 | 175 |
| 2 | 24 | 48 | 350 |
| 3 | 48 | 72 | 525 |
| 4 | 72 | 96 | 700 |
| 5 | 96 | 120 | 875 |
| 6 | 120 | 144 | 1050 |
| 7 | 144 | 168 | 1225 |
| 8 | 168 | 192 | 1400 |
| 9 | 192 | 216 | 1575 |
| 10 | 216 | 240 | 1750 |
| 11 | 240 | 264 | 1925 |
| 12 | 264 | 288 | 2100 |
| 13 | 288 | 312 | 2275 |
| 14 | 312 | 336 | 2450 |
| 15 | 336 | 360 | 2625 |
How it works
The upper floor is the final platform, so individual spiral treads equal risers minus one. Angular pitch is the entered total rotation divided by that tread count. Arc going is radius × angle in radians; the straight chord is 2 × radius × sin(angle/2). At 2800 mm rise, 16 risers and 360° rotation, there are 15 treads, 175 mm risers and 24° per tread.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Total floor-to-floor rise
- 2800 mm
- Total riser count
- 16
- Rotation from first tread to upper floor
- 360 °
- Inner tread radius
- 100 mm
- Radial tread width
- 800 mm
- Walking-line offset from inner edge
- 500 mm
- Tread thickness
- 40 mm
Primary result: Outside stair diameter: 1800 mm.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
The green circle is the user-entered walking line, not a code-selected measurement line. Rise per revolution minus tread thickness is only a geometric separation and does not certify headroom: floor opening, landings, tread shape and supports matter. No structural design, handrail or guard clearance, minimum going, maximum rise or legal stair-width check is performed. Rotation is limited to 720°. Exports are plan schematics.
Frequently asked questions
What should I verify before using this result?
Recheck the physical measurements, confirm that every input uses the displayed unit, and compare the result with the applicable drawing, product specification, code requirement or test piece.
Why might the finished result differ?
Real work can include thickness, kerf, seams, tolerances, hardware allowances, installation clearances or site conditions that are outside the calculation. The limitations section identifies the main exclusions for this tool.
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