Stair Rise and Run Calculator — IBC and IRC check
Riser height, tread depth, total run and stringer length from a total rise, checked against IBC 1011.5 and IRC R311.7 limits, which are not the same.
Stair Rise & Run Calculator
Enter the total floor-to-floor rise. Most calculators give you a riser height and stop — this one tells you whether that riser is legal, and under which code, because IBC and IRC do not agree.
| Risers | — |
|---|---|
| Riser height | — |
| Treads | — |
| Total run | — |
| Stringer length | — |
| Angle from horizontal | — |
| 2R + T (comfort rule) | — |
Both limits live in the same section. IBC 1011.5.2 sets a 7" maximum riser and an 11" minimum tread. Exception 3, same section, gives back 7‑3/4" and 10". It applies only in Group R‑3, within dwelling units in Group R‑2, and in Group U accessory to either, and only with a nosing of not less than 3/4" where a solid riser meets a tread under 11". Occupancy decides, not preference. The stair inside an apartment gets Exception 3; the common stair serving that same building does not. That one is a Group R‑2 exit: 7" and 11".
The penalty is run, not riser. On a 108" floor‑to‑floor, 7"/11" forces 16 risers and 15 treads. That is 165" of run at the minimum tread. Under 7‑3/4"/10" the same rise takes 14 risers and 13 treads: 130". Thirty‑five inches more floor, plus the longer well opening that 80" of headroom off the nosing line then demands (IBC 1011.3). Settle which code governs before the opening is framed.
Uniformity fails at the finish, not the framing. IBC 1011.5.4 and IRC R311.7.5.1 both cap the spread between the largest and smallest riser in a flight at 3/8", and IRC R311.7.5 takes those dimensions exclusive of carpets, rugs or runners. Carpet costs you nothing. A finish floor does. Cut the stringers subfloor to subfloor and the error lands in the two risers nobody equalises: a 3/4" hardwood floor upstairs makes the top riser 3/4" taller, and the same floor downstairs takes 3/4" off the bottom one. Either alone is double the tolerance; both give 1‑1/2" of spread, four times the limit, out of one nominal flooring thickness. Take the rise finish floor to finish floor before you divide it.
The exception itself gets amended. Baltimore adopts the IBC but rewrites Exception 3 to an 8‑1/4" riser and a 9" tread (Building, Fire and Related Codes, 2024 edition, §1011), which is looser than either model code. Confirm the adopted edition and the local amendment before you cut a stringer.
What I check on a job
Lay out from the exact riser. The riser count is your rise divided by the target you typed, rounded, not anything the code sets, so a count is only a candidate until the flag clears. Once a count passes, I check which riser figure I am about to use. In imperial the page prints the riser to the nearest sixteenth and the exact value beside it, and on a stringer the difference compounds. Take the 108-inch rise at 14 risers under IRC R311.7: the exact riser is 7.7143 inches, the fraction shown is 7-11/16. Step that fraction off thirteen times and the last riser is left at 8-1/16, which is over the 7-3/4 maximum and a full 3/8 inch taller than the rest, the whole uniformity allowance gone before any flooring goes down. I divide from the total, or mark off the exact figure.
Code values: IBC 1011.5 sets riser 7" max, tread 11" min. IRC R311.7 sets riser 7-3/4" max, tread 10" min. Riser uniformity within a flight is limited to 3/8" (IBC 1011.5.4 / IRC R311.7.5.1). Codes are amended by state and local jurisdiction, so confirm the adopted edition with your building department. This is a design aid, not a compliance certification.
Worked example
A 108-inch floor-to-floor rise in a commercial building: how many risers, and how tall?
- At the page's default target riser, 108 in divides into 15 risers of 7.2 in (7-3/16"), with 14 treads.
- IBC 1011.5.2 caps a riser at 7 in, so 7.2 in fails: the page flags it and says 16 risers of 6-3/4" are the least that pass.
- Stringer angle at an 11 in tread: atan(7.2 ÷ 11) = 33.2°; the page prints the angle for its own tread, 35.0° at the default.
15 risers at 7.2 in is the arithmetic and it does not pass — read the flag before the count. Set the target to 6.75 in and it becomes 16 risers, 15 treads, and a stair an inspector will sign.
14 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
What is the maximum riser height?
7 inches under IBC 1011.5.2 for most commercial stairs, 7¾ inches under IRC R311.7 for dwellings. The two codes are not the same, and the page checks the one you choose.
How many risers for a 108-inch rise?
15 risers give 7.2 inches each, which fails the IBC; 16 risers give 6¾ inches, which passes. There is always one fewer tread than risers.
Can an apartment building's common stair use 7-3/4 in. risers?
No. IBC 1011.5.2 Exception 3 (7-3/4" and 10") covers Group R-3, stairs within dwelling units in Group R-2, and Group U accessory to them. The common stair serving the building is a Group R-2 exit and keeps 7" and 11".
Why does finish flooring break riser uniformity?
Because the spread between the largest and smallest riser in a flight is capped at 3/8" (IBC 1011.5.4, IRC R311.7.5.1). Stringers cut subfloor to subfloor put 3/4" of hardwood on the top riser and take 3/4" off the bottom one: 1-1/2", four times the limit. Measure finish floor to finish floor.