How much can stair risers vary in height?
Within a stair flight, the difference between the tallest and the shortest riser cannot exceed 3/8 inch, and the same 3/8-inch limit applies to tread depths (2018 IRC §R311.7.5.1 and §R311.7.5.2). Uneven risers are a trip hazard, so measure each one after framing — not just the design height.
Source: 2018 IRC §R311.7.5.1 · Verify against your AHJ and the adopted code edition.
Every step in a flight should feel the same underfoot — your body locks onto the first riser height and expects it the whole way up. The IRC enforces that:
- Risers: the greatest riser height minus the smallest cannot exceed 3/8 inch in a flight (§R311.7.5.1).
- Treads: the same 3/8-inch limit applies to tread-depth variation (§R311.7.5.2).
The classic mistake is forgetting the finished floor. If you frame every riser equal but then add tile at the bottom and carpet at the top, the first and last steps change height — often enough to bust the 3/8-inch limit. Account for finishes when you set the total rise.
That’s why an even riser height matters: divide the total rise by a whole number of risers so they all come out the same. Editions and amendments vary — verify with your AHJ.
Work it out: the stair calculator divides the total rise into equal risers and flags when a step would exceed the 7¾-inch max, so the flight stays uniform.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R311.7.5.1, §R311.7.5.2