What is the minimum stair tread depth?
The minimum tread depth (the run of each step) for stairs in a dwelling is 10 inches under the 2018 IRC (§R311.7.5.2), measured horizontally from nosing to nosing. As with risers, treads must be uniform — the largest and smallest in a flight can differ by no more than 3/8 inch.
Source: 2018 IRC §R311.7.5.2 · Verify against your AHJ and the adopted code edition.
The tread depth (or run) is how far your foot travels forward on each step. The IRC sets a residential minimum of 10 inches, measured nosing to nosing — not counting the overhang of the nosing itself.
Treads follow the same 3/8-inch uniformity rule as risers: every tread in a flight should be the same depth so the stair has a steady rhythm.
A useful sanity check beyond code is the “2R + T” comfort rule (two risers plus one tread ≈ 24–25 inches). It’s a rule of thumb, not a requirement, but a stair that satisfies both the code limits and 2R + T feels natural to climb.
Commercial stairs are stricter. The IBC requires an 11-inch minimum tread (§1011.5.2). Confirm the depth for your occupancy and adopted code with your AHJ.
Work it out: the stair calculator takes a rise and a tread depth and returns the riser height, total run, pitch, and the 2R + T comfort check — flagging anything outside the IRC limits.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R311.7.5.2