What this calculator does
The spiral staircase calculator estimates how many steps you need, the actual riser height, and the tread depth along the walkline, based on total rise, staircase diameter, and total rotation.
The formula
The calculator starts from a target maximum riser height of 7.5 inches (a common residential code maximum) to estimate a step count, then divides total rise by that count to get the actual, evenly-spaced riser height. Rotation per step is the total rotation divided by the number of steps. Tread depth is measured along the “walkline” — typically about two-thirds of the way out from the center pole — using the arc-length formula for a circle (2 x pi x radius x degrees / 360).
This is a general design estimate. Spiral staircases are subject to specific code requirements for minimum tread depth and headroom, so always verify the final design against your local building code.