Stair Geometry Code Check (IBC 1011 / IRC R318)
Whether a stair's riser, tread, and clear width meet the adopted code, the check every stair permit turns on.
Example
You enter
- Occupancy / code commercial
- Proposed riser height (in) 7
- Proposed tread run, no nosing (in) 11
- Proposed clear stair width (in) 44
You get
- Max riser 7
- Tread depth ok (min 11 in)
- Stair width ok (min 44 in)
- Riser ok Yes
- Tread ok Yes
- Width ok Yes
- 2R + T comfort 25.00 in (in the 24-25 in band)
- Verdict Yes
Checked against published worked examples from: American Wood Council.
Details, formula, and sources
ScopeIBC (commercial) caps the riser at 7 in, floors the tread at 11 in, and needs 44 in of width (36 in where the occupant load is under 50); IRC (residential) allows a 7-3/4 in riser and a 10 in tread at 36 in. A 7-1/2 in riser is a legal residential stair and an illegal commercial one - the single most common tenant-improvement stair red-tag. Reports each dimension pass/fail against the selected code plus the 2R + T comfort read (24-25 in). Uniformity, nosing, landings, and winders are separate checks; the egress width from occupant load is separate. A design aid, not a code-official determination; the AHJ governs.
Formula or tableIBC (commercial): riser 4-7 in, tread >= 11 in, width >= 44 in (36 in where the occupant load is under 50). IRC (residential): riser <= 7.75 in, tread >= 10 in, width >= 36 in. Each measured value is compared to its limit; 2R + T (comfort 24-25 in) is reported, not enforced.
Source and editionIBC 2027 (International Building Code) §1011.5.2 (riser/tread) and §1011.2 (width), and IRC 2024 (International Residential Code) R318.7.5 [R311.7.5 in IRC 2021] (riser/tread) and R318.7.1 [R311.7.1 in IRC 2021] (width), by section; dimensional minimums only, not reproduced.
Free accessRead-only at codes.iccsafe.org. The dimensional stair limits are facts; the AHJ-adopted code and edition govern.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesoccupancy, riser_height_in, tread_depth_in, stair_width_in, max_riser, min_tread, min_width, riser_ok, tread_ok, width_ok, two_r_plus_t, all_pass
- Thresholds IBC commercial riser 4-7 in, tread 11 in min, width 44 in (36 in where the occupant load is under 50); IRC residential riser 7-3/4 in max, tread 10 in min, width 36 inIBC 1011.5.2 / 1011.2 / IRC R318.7.5 [R311.7.5 in IRC 2021] / R318.7.1 [R311.7.1 in IRC 2021]
- Comfort read 2R + T between 24 and 25 in is a design rule of thumb reported for reference, not a code pass/fail requirementtrade practice
- Separate checks the 3/8 in flight uniformity limit, nosing, landings, winders, and the occupant-load egress width (the egress-capacity tile) are not checked here; the AHJ governsIBC 1011 / IRC R318 [R311 in IRC 2021]