Stair geometry limits under IRC 2021 (US Residential), with the governing clause for each and a worked 9-foot example. Then run your own numbers in the calculator.
IRC 2021 is the most widely adopted residential edition across US jurisdictions as of the mid-2020s. Its stair rules live in Section R311.7. The dimensional limits match both the 2018 and 2024 editions; the 2021 cycle is mainly distinguished by a clause renumber within the stairway section.
| Requirement | Limit | Clause |
|---|---|---|
| Max riser height | 7.75″ | R311.7.5.1 |
| Min tread depth | 10.0″ | R311.7.5.2 |
| Min stair width | 36.0″ | R311.7.1 |
| Min headroom | 80.0″ | R311.7.2 |
| Max rise between landings | 147.0″ | R311.7.3 |
For a 108-inch total rise under IRC 2021 (US Residential): dividing by the 7.75-inch maximum riser gives a minimum of 14 risers. Spreading 108 inches evenly across 14 risers yields 7.71 inches per riser — within the limit and uniform, as the code requires.
Run your stair against IRC 2021 (US Residential)
The dimensional limits — 7.75-inch maximum riser, 10-inch minimum tread, 36-inch width, 80-inch headroom, 147-inch maximum rise between landings — are unchanged from the 2018 edition. The notable housekeeping change is that the vertical-rise-between-landings provision was renumbered from R311.7.6 (2018) to R311.7.3 (2021). The walkline definition for winder treads at R311.7.4 was also clarified.
If you are moving from a 2018-based plan set, no stair dimension needs to change for the 2021 code — only the clause references on your drawings. The larger renumber to R318.7 comes in the 2024 edition.
Compare with IRC 2018, IRC 2024, IBC, UK Approved Document K, Australia NCC / BCA, Eurocode / EN.