What is the "walking line" for a spiral stair?

Building codes measure the usable tread depth at the walking line, typically 12 inches (300 mm) from the outer handrail. The arc length at that radius must meet the minimum run requirement (≥ 7.5 in per IBC, ≥ 6.75 in per IRC for residential).

How many treads does a full 360° spiral need?

A standard residential spiral uses 11–13 treads for one full revolution, plus a landing platform at the top. This keeps the rotation-per-step between 25° and 33°, which is comfortable to walk.

What total rotation angle should I aim for?

Most codes require at least 270° of total rotation so users cannot fall straight back down. A 360° (or 540°) rotation gives more comfortable geometry and allows headroom clearance above the entry opening.

How is rise per step calculated?

Rise per step = floor-to-floor height ÷ number of treads. If a landing platform counts as the final "riser," use number of treads for the divisor. Most codes limit rise to ≤ 9.5 inches (241 mm) per step.

What is tread arc length and why does it matter?

Arc length is the curved distance along the walking line on each tread. Because spiral treads are wedge-shaped, the depth varies from zero at the center column to a maximum at the outer edge. Arc length at the walking line is what building codes measure for minimum run compliance.

Can I use this calculator for metric dimensions?

Yes — switch the unit toggle to metric and enter heights in millimeters. All outputs convert accordingly.

Spiral Stair Calculator

Name: Spiral Stair Calculator
Availability: InStock
Author: Nham Vu

Enter floor height, tread count, and column diameter to get rise, run, tread arc length, and rotation for each step of a spiral staircase.

Staircase Parameters

Imperial Metric

Floor-to-Floor Height

Number of Treads

Center Column Diameter

Overall Staircase Diameter

Total Rotation (degrees)

Per-Step Breakdown

Step	Cumul. Rise	Cumul. Rotation	Arc Run	Inner Depth	Outer Depth

Enter your staircase parameters and click Calculate.

Copied to clipboard

Summary

Enter floor height, tread count, and column diameter to get rise, run, tread arc length, and rotation for each step of a spiral staircase.

How it works

Enter the floor-to-floor height (the total vertical rise the staircase must cover).
Enter the number of treads — typically 10 to 16 for a standard 360° spiral.
Enter the center column diameter and the overall staircase diameter.
The calculator divides total rise equally across all treads to find rise per step.
It computes the rotation angle per step, then the arc length at the walking line (outer radius minus ~12 in / 300 mm).
All key dimensions are displayed in a per-step table plus a summary panel.

Use cases

Designing a residential spiral staircase to fit a fixed floor-to-floor height.
Confirming that step rise and run meet building code minimums (typically rise ≤ 9.5 in, run ≥ 7.5 in at walking line).
Planning tread count to achieve a comfortable stair angle (30°–45° recommended).
Ordering prefabricated spiral stair kits by knowing total rotation and per-step dimensions.
Calculating arc-length tread depth for custom-cut wood treads.
Estimating material for a 360° or 270° spiral between two floors.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu