Spiral Stairway Tread Minimum
The spiral stairway
tread minimum is 7 1/2 inches. However, this measurement isn't taken
from the smallest point on the stairway.
Measure 12 inches from the inside radius of the stair tread, toward the center of the stairway and then measure the actual tread run. The stair run measurement must be taken at a 90° angle or square off of the front of the stair tread ( see picture below).
This might not be the actual tread size. If your stair tread has a 1 inch overhang, you will need to deduct 1 inch, to obtain the actual tread run.