Spiral Staircase Calculator

Enter floor-to-floor height, an illustrative maximum rise per interval, radial tread width measured from the pole surface to the outer edge, and pole diameter. All inputs are millimeters. The model uses at least two rise intervals and exactly one turn; the upper floor may act as the final landing rather than an additional manufactured tread.

Rise intervals = ceil(height ÷ entered rise) (at least two intervals required). Actual rise = height ÷ intervals. A one-turn illustration gives angle per interval = 360° ÷ intervals. Radius at the radial midpoint = pole diameter ÷ 2 + tread width ÷ 2; one-turn midpoint path = 2π × radius.

Example: height 2,800 mm, rise input 180 mm, tread width 800 mm, and pole diameter 100 mm give 16 rise intervals, 175 mm actual rise, 22.5° per interval, and a 0.45 m midpoint radius. The one-turn midpoint path is about 2.83 m.

Equal angular sectors and a circular tread envelope are assumed. The midpoint path is geometric; it does not establish usable tread depth or a safe walking route. A different number of turns, pole shape, landing arrangement, or tread profile changes the result.

Use these values to compare early geometry sketches and check unit conversions. They are not a material estimate or approval for construction.

The 2021 International Residential Code §R311.7.10.1 is one example of a spiral-stair provision: it specifies clear width, walkline tread depth, rise, and headroom. This calculator does not check those conditions or determine whether that edition applies to your project. Source edition: 2021; checked 2026-09-29.