Spiral Staircase Tread Calculator
A spiral staircase climbs a fixed floor-to-floor height in even steps while turning around a central post. To plan it you divide the height by a target rise to get the number of risers, recompute the exact rise so every step is equal, and divide the total rotation by the treads to get the turn angle between them. This calculator does all three. Always confirm the maximum riser height against the building code adopted where you build before finalising.
Spiral staircase formula
risers = round(total height / target rise)
actual rise = total height / risers
treads = risers - 1
rotation per tread = total rotation / treads
Rounding the height-to-rise ratio gives a whole number of equal risers. Dividing the rotation by the treads (risers minus one) gives the consistent angle between treads.
Worked example
A 108 inch height with a 9 inch target rise gives round(108 / 9) = 12 risers, and actual rise = 108 / 12 = 9.00 inches. With 11 treads and a 360 degree spiral, rotation per tread = 360 / 11 = 32.73 degrees. Confirm 9 inches is within your local riser limit before building.
Spiral staircase treads: frequently asked questions
How many treads does a spiral staircase need?
The number of treads equals the total floor-to-floor height divided by the target rise per tread, rounded to a whole number. The rise per tread is then the total height divided by that number of risers. Building codes set a maximum riser height, so the rise you target must respect your local code.
What rise per tread is allowed?
Maximum riser height is set by the building code in force where you build. For spiral stairs many US jurisdictions following the International Residential Code permit a riser up to about 9.5 inches, but the exact limit depends on your adopted code and edition, so confirm with your local authority. Enter your target rise and the calculator checks how many treads result.
How is the rotation angle between treads found?
Divide the total rotation of the spiral (for example 360 degrees for one full turn) by the number of treads. That gives the angle each tread turns relative to the one below it. Consistent angular spacing keeps the walkline even and the stair comfortable.
Is the number of treads one less than risers?
A flight has one more riser than treads when the top landing serves as the final step, so treads equal risers minus one in that arrangement. This calculator reports the number of risers from the height and rise; subtract one for treads if your top step lands on the upper floor.
Sources and method
- The tread and rotation figures are geometric divisions of height and angle; they are arithmetic, not sourced figures. Maximum riser height is set by your adopted building code.
- U.S. Access Board: accessibility standards for stairs and ramps. Confirm riser limits with your local building authority.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.