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The 2006 International Building Code calls out the tolerance for how consistent each stair tread and riser needs to be in a set of stairs. The code refers to it as dimensional uniformity. What the IBC states on stair tolerance is that the difference between the largest and smallest riser height and the largest and smallest tread depth can't be greater than 3/8" within a flight of stairs.

With spiral stairs the 3/8" is measured at the 12" walk line.

1. New Definitions of Stair and Stairway Under 2009 IRC
2. Commercial Stair Design – Guidelines, Criteria and Dimensions
3. Residential Stair Design – Guidelines, Criteria and Dimensions
4. Residential Spiral Stairs – Guidelines, Criteria and Dimensions
5. Residential Stairs Type in the 2009 IRC
6. Handrail Extensions for Commercial Stairs
7. Slope on Stair Treads and Landings
8. Steep Stairs Are Allowed in Theater Assembly Aisles