By Zencalcy Editorial TeamLast reviewed About us
A spiral stair fits a small footprint but has to satisfy the same comfort rules as a straight stair: risers between 7 and 7¾ inches and a walking line with adequate tread depth. Enter the floor-to-floor height and the outer diameter, and this calculator works out the risers, rotation per step and the tread depth at the walk line.
Riser count, tread angles and clear walking line for a spiral stair.
Risers = ⌈height ÷ 7.75⌉ riser = height ÷ risers walk line = centre column radius + 12 in
The walk line is taken 12 inches from the centre column, where code tread depth is measured. A full turn is 360°; rotation per step = 360° ÷ risers. The clear tread depth at the walk line is the arc length of that step at the walk-line radius.
| Symbol | Meaning |
|---|---|
| H | Floor-to-floor height. |
| N | Number of risers. |
| θ | Rotation per tread. |
108 inches ÷ 7.75 = 13.9, so 14 risers at 7.71 inches each. Each tread turns 25.7°, giving a walk-line depth of about 6.3 inches — tight even by the 6¾ inch spiral-stair allowance, which is why 6 ft diameters are common.
Continue with the Gravel Calculator, Sand Calculator, Rock Calculator, or the River Rock Calculator — or browse more materials & quantities and all construction calculators.
Five feet is the usual minimum for a code-compliant spiral; 6 ft is comfortable. Smaller diameters fail the walk-line depth requirement.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Estimate decking boards by length, screws, hidden fasteners and waste
Calculate concrete blocks needed for wall construction
Calculate full deck materials list including boards, joists, beams, posts, and cost estimates
Price hardwood the way the lumber yard does — by board footage.
Turn measured dimensions into the cubic yards concrete and gravel are sold in.
How many gallons it takes to cover an area at a given depth.
Content updated: September 2026