The key numbers (IRC)
| Requirement | Residential (IRC) |
|---|---|
| Maximum riser height | 7¾ in |
| Minimum tread depth | 10 in |
| Minimum width | 36 in |
| Minimum headroom | 6 ft 8 in |
| Max difference between risers (or treads) in a flight | ⅜ in |
| Handrail height | 34–38 in above the nosing |
| Handrail required | Flights with 4 or more risers |
| Guard required | Open sides more than 30 in above the floor below |
These are the International Residential Code model figures used across most of the US. Local jurisdictions can amend them, so confirm with your building department before you build.
Designing a stair that passes
Start with the total rise — floor to floor, measured to finished surfaces. Divide by the maximum riser height and round up to get the number of risers; divide the total rise by that number for the exact riser height. For a 108 in rise, that gives 14 risers of 7.71 in.
There's one fewer tread than risers, so the total run is treads × tread depth. Check that run fits the space, and that headroom stays above 6 ft 8 in along the whole flight — headroom is the most common problem in basements.
Comfort, not just compliance
A code-legal stair isn't necessarily comfortable. The classic rule of thumb is that two risers plus one tread should equal about 24–25 in. A 7½ in riser with a 10 in tread meets it exactly; shallower risers with deeper treads feel more relaxed and are easier for children and older adults.
Build tips
- Cut stringers from 2×12 lumber, leaving at least 3½–5 in of solid wood below the notches.
- Drop the bottom of the stringer by one tread thickness so the first riser matches the rest.
- Keep riser heights within ⅜ in of each other — uneven steps are a leading cause of falls.
- For outdoor steps, use pressure-treated lumber or pour concrete steps on a solid footing.