Spiral Staircase Calculator

A spiral staircase wraps its treads around a central pole, which saves floor space but makes the geometry less obvious than a straight run. The three numbers that decide whether it is comfortable and legal are the riser height, the rotation between treads, and the tread depth measured at the walkline, the path your foot actually takes.

Start with the rise. Divide the total floor-to-floor height by the number of risers, which is one more than the number of treads because the last step lands on the upper floor. Spiral stairs are allowed steeper risers than ordinary stairs. The International Residential Code (IRC) permits up to 9.5 inches, though anything over about 7.75 starts to feel like a ladder. Next is the rotation. Spread the total turn, often 270 to 360 degrees, evenly across the treads to get the angle between each one.

Tread depth is where spiral stairs get strict. Because each tread is a wedge, it is narrow at the pole and wide at the rail, so depth is measured at the walkline, 12 inches in from the narrow edge. The code wants at least 6.75 inches there. This calculator works out the walkline depth from your diameter, pole size, and tread count, and flags it if it falls short. It also gives the depth at the outer edge and the clear length of each tread from pole to rail.

Two more spiral-specific numbers are worth knowing before you order anything. The clear walking width has to be at least 26 inches from the pole to the outer rail, which rules out most stairs under about 56 inches in overall diameter once you account for the pole and the rail. And headroom on a spiral is allowed down to 6 feet 6 inches rather than the 6 feet 8 inches a straight flight needs, so the headroom calculator reads 2 inches conservative if you point it at a spiral.

These figures match the common building-code limits for residential spiral stairs, but local rules vary, so confirm with your inspector before cutting anything. Enter the height, diameter, pole size, tread count, and total rotation to size the whole stair.