Stair Calculator
Lay out a straight flight from the total rise: equal riser height, riser and tread counts, total run, and stringer length — checked live against the governing code's riser and tread limits (IBC commercial by default, with IRC residential and OSHA industrial selectable) and the 2R + T comfort rule, then printed as a cut sheet. The three codes carry different limits and must never be mixed. Free, no login.
Stairs
Layout
IBC 1011.5 (commercial). Also check max flight height, landings, nosing & handrails.
Stair Calculator — Riser & Tread Cut Sheet
Prepared with NORDIX — nordixhq.com/tools/stair-calculator
| Total rise: 108 in | Code: IBC | Code check: CHECK FAILED |
| Item | Qty | Unit | Notes |
|---|---|---|---|
| Risers | 15 | @ 7.2" | limit ≤ 7" (OVER) |
| Treads | 14 | @ 10.5" | limit ≥ 11" (UNDER) |
| Total run | 147 | in | 2R + T comfort rule met |
| Stringer length | 182.41 | in | point-to-point; add for cuts & waste |
IBC 1011.5 (commercial). Riser = total rise ÷ risers (equal risers); run = treads × tread depth. Verify max flight height, landings, nosing, headroom, and handrails against the adopted code before cutting stringers.
How this calculator works
Stair layout is division that has to come out even — every riser in a flight must be equal (the IRC and IBC both limit riser variation within a flight to 3/8"):
risers = round(rise ÷ target) · riser = rise ÷ risers · run = (risers − 1) × tread
Treads are always one fewer than risers (the upper floor is the last walking surface), the stringer is the diagonal √(rise² + run²), and the layout is then checked two ways. First the code check: maximum riser and minimum tread per the code you select — IBC 1011.5, IRC R311.7, or OSHA 1910.25 — with the default set to IBC because commercial limits are the strictest of the three. Second the comfort rule: 2R + T ≈ 24–25", a rule of thumb (not code) that flags stairs that pass code but walk badly. Measure total rise plumb, finished floor to finished floor — finish-thickness errors concentrate in the first or last riser, exactly where the 3/8" variation limit catches them.
Worked example
A residential flight, 108" total rise, 7" target riser, 10.5" treads, checked against the IRC:
- Risers: round(108 ÷ 7) = 15 risers at 108 ÷ 15 = 7.2" each — passes the IRC 7-3/4" maximum, and the 10.5" tread passes the 10" minimum.
- Run and stringer: 14 treads × 10.5" = 147"; stringer = √(108² + 147²) = 182.41" (~15 ft 3") — buy longer stock for the end cuts.
- Comfort: 2 × 7.2 + 10.5 = 24.9 — inside the 24–25" window.
- Switch the code to IBC and the same layout fails both limits (7.2" > 7" riser, 10.5" < 11" tread) — a commercial flight of the same rise needs 16 risers at 6.75" with 11" treads, stretching the run to 165".
Enter the same flight above (108" rise, 7" target, 10.5" tread, code IRC) to reproduce it, then print the cut sheet.
Reference: riser and tread limits by governing code
The verified limits behind the pass/fail check. Which code governs is set by the building’s use — and jurisdictions amend these values, so confirm against the adopted edition. Default here: IBC (commercial).
| Governing code | Max riser | Min tread | Citation |
|---|---|---|---|
| IBC (commercial) | 7" | 11" | IBC 1011.5 (commercial) |
| IRC (residential) | 7¾" | 10" | IRC R311.7 (residential) |
| OSHA (industrial fixed stairs) | 9½" | 9½" | OSHA 1910.25 (industrial fixed stairs) |
Frequently asked questions
What is the maximum riser height and minimum tread depth?
It depends on which code governs the stair — the numbers are different and must not be mixed. Residential stairs under IRC R311.7: maximum riser 7-3/4 in., minimum tread 10 in. Commercial stairs under IBC 1011.5: maximum riser 7 in., minimum tread 11 in. Industrial fixed stairs under OSHA 1910.25: 9-1/2 in. maximum riser and 9-1/2 in. minimum tread. The calculator checks your layout against whichever code you select; some states and cities amend these limits, so verify against the code adopted in your jurisdiction.
How do I calculate the number of risers from total rise?
Divide the total rise by a target riser height (around 7 in.), round to the nearest whole number, then divide the total rise by that count to get the exact riser height — every riser must be equal. A 108 in. rise at a 7 in. target gives 15 risers of 7.2 in. each, which is exactly what this calculator does. The IRC and IBC both limit the variation between the tallest and shortest riser in a flight to 3/8 in., so the division has to come out even in the field, not just on paper.
Why are there fewer treads than risers?
The top riser lands you on the upper floor, which acts as the final walking surface, so a flight always has one fewer tread than risers — 15 risers means 14 treads. Total run is treads times tread depth, which is why adding a riser to soften the climb also stretches the stair by a full tread. If the stair ends at a landing instead of the floor, the landing plays the same role.
What is the 2R + T rule for stairs?
A comfort rule of thumb, not a code requirement: twice the riser height plus the tread depth should land around 24–25 in., which matches a natural stride. A 7 in. riser with an 11 in. tread hits 25 exactly; a 7-3/4 in. riser with a 10 in. tread comes to 25.5, right at the edge of the window, and walks noticeably steeper. The calculator flags layouts outside the window — a stair can pass code and still walk badly.
How do I measure total rise for a stair?
Plumb, from finished floor below to finished floor above — not subfloor to subfloor, unless you adjust for the finish thicknesses. If the two floors will get different finishes (say tile below and hardwood above), account for the difference now, because any error ends up concentrated in the first or last riser. That is exactly where the 3/8 in. riser-variation limit gets stairs flagged at inspection.
How long a board do I need for a stair stringer?
The calculator reports the stringer length as the diagonal of the total rise and total run — √(rise² + run²) — which for a 108 in. rise over a 147 in. run is about 15 ft 3 in. Buy stock longer than that number to cover the plumb and level cuts at each end and any tail at the bottom. Cut stringers are typically laid out on 2x12 so enough solid wood remains below the notches; the commonly published guideline (AWC DCA6) is to keep at least 5 in. of uncut depth (the throat).
Method: equal-riser division (risers = rise ÷ target, rounded; riser = rise ÷ risers), run = treads × tread depth, stringer as the rise/run diagonal; limits per IBC 1011.5 (max riser 7", min tread 11"), IRC R311.7 (7-3/4"/10"), and OSHA 1910.25 (9-1/2"/ 9-1/2" for industrial fixed stairs). The 2R + T ≈ 24–25" check is a comfort rule of thumb, not a code requirement. Riser and tread are only two of the requirements on a stair: maximum flight rise between landings, landing dimensions, nosing, headroom, stair width, guards, and handrails all apply and are not checked here — and state and local amendments modify the model-code values, so the adopted code and the AHJ govern. Stringer stock: keep enough uncut depth below the notches (AWC DCA6 suggests a 5" minimum throat on cut 2×12 stringers).
Pair with the Right-Angle (3-4-5) Calculator for the stringer diagonal and layout squaring, the Rafter & Roof Pitch Calculator for the other sloped-member layout, or the Decking Calculator for the deck the stairs land on.