Stair Rise & Run Calculator
A 109-inch total rise solves to 16 risers at 6.813 inches each, with 15 treads and a total run of 13.7 feet. That passes the IRC 2021 limits of 7-3/4″ maximum riser and 10″ minimum tread. This calculator checks every dimension against the code as you type and flags what fails. Code figures from IRC 2021 §R311.7, verified 2026-08-04.
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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.
How is stair rise and run calculated?
Total rise is fixed by the building — you cannot change the distance between two floors. Everything else follows from choosing how many risers to divide it into, and that single choice determines whether the stair is legal and whether it is comfortable.
The risers − 1 line trips people up constantly. If you cut
a stringer with a tread for every riser, the top tread sits one tread
thickness above the upper floor. The upper floor is the last
tread.
A worked example: a standard 9-foot floor-to-floor
A 109 inch total rise — typical for a 9 ft ceiling with floor structure above — with an 11 inch tread and a 36 inch wide stair.
Step by step
- Riser count
- 109 ÷ 7 = 15.57 → round to 16 risers
- Actual riser height
- 109 ÷ 16 = 6.8125″ — under the 7¾″ limit
- Treads
- 16 − 1 = 15 treads
- Total run
- 15 × 11″ = 165″ = 13.75 ft
- Stair angle
- 33.4°
- Stringer length
- √(109² + 165²) = 16.48 ft
- 2R + T = 24"–25"
- 24.63 in — within range
- R + T = 17"–18"
- 17.81 in — within range
- R × T = 72–75 in²
- 74.94 in² — within range
Notice the trade. Fifteen risers would give 7.267" — also legal, also comfortable, and it would save a tread's worth of floor space, about 11 inches. Sixteen risers at 6.813" is gentler to climb but consumes 13.7 feet of floor. On a tight landing that difference decides the stair. Use the riser-count override to compare them directly.
Code limits at a glance
| Dimension | IRC 2021 (dwellings) | IBC 2021 (commercial) |
|---|---|---|
| Maximum riser | 7-3/4″ | 7″ |
| Minimum tread | 10″ | 11″ |
| Riser variation in a flight | 3/8″ | 3/8″ |
| Minimum width | 36″ above the handrail | 44″ for most occupancies |
| Minimum headroom | 6 ft 8 in | 6 ft 8 in |
| Nosing | 3/4″–1-1/4″ where tread < 11″ | Required, same range |
| Handrail height | 34″–38″ from the nosing line | 34″–38″ |
| Guard opening | No opening passes a 4″ sphere | 4″ sphere |
From IRC 2021 §R311.7 and IBC 2021 §1011. Local amendments override both, and adoption lags — several jurisdictions still enforce the 2018 or 2015 edition. Confirm with your building department before cutting.
Who this calculator is for
- Carpenters laying out stringers, who need the exact riser height to four decimal places before marking a framing square.
- Deck builders — deck stairs are the same arithmetic and the same code section, and they get inspected. Pair it with the deck calculator.
- Renovators checking whether an existing stair is compliant before a remodel triggers an upgrade requirement.
- Anyone fitting a stair into a tight space — the riser-count override lets you compare a steeper flight against a gentler one on floor space consumed.
What this calculator does not account for
- Winders and spiral stairs. Both have their own code sections with different tread measurement rules. Nothing here applies to them.
- Landings. A flight over 12 feet of vertical rise needs an intermediate landing, and landings have their own minimum dimensions. Not modelled.
- Structural design. Stringer sizing, bearing and attachment are not calculated. The 3-1/2" minimum uncut throat behind each notch is a rule of thumb, not an engineered figure.
- The nosing overlap on tread depth. Tread depth is measured nosing to nosing, not the physical width of the tread board. Enter the nosing-to-nosing figure.
- Handrail extensions and returns. Required at both ends and their length is not in the handrail quantity beyond a flat 15% allowance.
- Local amendments. The two code sets here are the model codes. Your jurisdiction adopts a version and may amend it.
Common floor-to-floor heights
Click a column to sort — for example by riser height, to see which rises land closest to the ideal 7 inches.
| Total rise | Risers | Riser height | Run at 10″ tread | Run at 11″ tread |
|---|---|---|---|---|
| 24″ (2.0 ft) | 3 | 8.000″ | 1.7 ft | 1.8 ft |
| 36″ (3.0 ft) | 5 | 7.200″ | 3.3 ft | 3.7 ft |
| 48″ (4.0 ft) | 7 | 6.857″ | 5.0 ft | 5.5 ft |
| 60″ (5.0 ft) | 9 | 6.667″ | 6.7 ft | 7.3 ft |
| 84″ (7.0 ft) | 12 | 7.000″ | 9.2 ft | 10.1 ft |
| 96″ (8.0 ft) | 14 | 6.857″ | 10.8 ft | 11.9 ft |
| 97″ (8.1 ft) | 14 | 6.929″ | 10.8 ft | 11.9 ft |
| 104″ (8.7 ft) | 15 | 6.933″ | 11.7 ft | 12.8 ft |
| 108″ (9.0 ft) | 15 | 7.200″ | 11.7 ft | 12.8 ft |
| 109″ (9.1 ft) | 16 | 6.813″ | 12.5 ft | 13.8 ft |
| 112″ (9.3 ft) | 16 | 7.000″ | 12.5 ft | 13.8 ft |
| 120″ (10.0 ft) | 17 | 7.059″ | 13.3 ft | 14.7 ft |
| 128″ (10.7 ft) | 18 | 7.111″ | 14.2 ft | 15.6 ft |
All riser heights above are within the IRC 7-3/4″ limit. Note how much floor space the tread depth costs: going from 10″ to 11″ treads adds over a foot of run on a typical flight.
Common questions
What is the standard stair rise and run?
A riser of about 7 inches and a tread of 10 to 11 inches is the standard comfortable proportion, and it is what this calculator targets by default. The IRC caps the riser at 7-3/4″ and requires at least 10″ of tread — the standard 7×11 sits comfortably inside both limits with room to spare.
How many steps for an 8-foot rise?
An 8 ft (96") total rise needs 14 risers at 6.857" each, giving 13 treads and a total run of 10.83 ft at a 10" tread. Enter your own rise above for an exact count — the riser height changes with every inch of total rise.
How do I calculate stair rise and run?
Divide the total rise — finished floor to finished floor — by your ideal riser height, usually 7 inches, and round to the nearest whole number. That gives the riser count. Divide the total rise by that count and you have the exact riser height.
A 109" total rise ÷ 7" = 15.57, which rounds to 16 risers at 6.813" each. Treads are always one fewer than risers, so 15 treads at 11" gives a total run of 165" — 13.7 feet of floor space.
What is the maximum riser height allowed by code?
7-3/4 inches under the IRC for one- and two-family dwellings (§R311.7.5.1), with a minimum tread depth of 10 inches. Commercial and most multifamily stairs fall under the IBC, which is stricter: 7 inch maximum riser and 11 inch minimum tread.
Local amendments override both, and several jurisdictions still enforce the 2018 or 2015 edition. Check with your building department before cutting stringers — a stair that fails inspection gets rebuilt, not adjusted.
Why do risers have to be so consistent?
Because people climb stairs by muscle memory, not by looking. After two or three steps your body has learned the rhythm, and a riser that differs by more than about 3/8" breaks it — which is how people fall. The IRC caps the variation between the largest and smallest riser in a flight at 3/8 inch.
Dividing the rise evenly satisfies this automatically. The problem appears on site: the bottom riser has to be cut shorter by the thickness of the tread material, and forgetting that is the classic way to end up with one non-compliant riser at the bottom of an otherwise perfect flight.
What are the 2R+T and other stair comfort rules?
Three old proportion rules that check whether a stair will feel right to walk, as opposed to merely being legal:
- 2R + T = 24″–25″ — the most reliable, derived from an average stride of about 25 inches on the flat.
- R + T = 17″–18″ — a quick sanity check that fails on very shallow stairs.
- R × T = 72–75 in² — keeps the stair from feeling either steep or sprawling.
This calculator checks all three live. A stair can be fully code-compliant and still fail all three, and it will feel wrong every time you use it.
Do I measure total rise from the subfloor or the finished floor?
Finished floor to finished floor, and this is the single most common measuring error in stair building.
If the upper floor still has 3/4" hardwood to go down and you measured to the subfloor, every riser in the flight comes out 3/4" ÷ (number of risers) too short — except the top one, which is out by the full 3/4". That fails the 3/8" variation rule on its own. Add the thickness of any flooring not yet installed at both levels before you calculate.
How long a stringer do I need?
The straight-line hypotenuse: √(total rise² + total run²). For the example on this page that is √(109² + 165²) = 16.5 feet, so you would buy 2×12 stock at least 16 ft long.
Buy longer than the calculation. The hypotenuse does not include the cuts at either end, the bearing at the bottom, or the attachment detail at the top. And critically, at least 3-1/2 inches of uncut stringer must remain behind every notch — cut deeper than that and the stringer loses its strength.
How do I lay out a stair stringer?
Set a framing square to the riser height on the tongue and the tread depth on the body, walk it down a 2×12, and mark each step in turn — stair gauges clamped to those two measurements make every mark identical without re-measuring. Cut the bottom of the stringer short by one riser thickness (it sits on the floor, not floating), and leave at least 3-1/2" of uncut wood behind every notch.
How much headroom does code require over stairs?
6 feet 8 inches minimum under the IRC, measured vertically from the nosing line of each tread to the ceiling or any obstruction above — ducts, beams, and the underside of the floor framing above all count. This calculator checks your entered headroom against that limit live.
Sources for the figures used
Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.
- International Residential Code 2021, §R311.7 — Stairways — Riser, tread, width, headroom, nosing, handrail and guard requirements for dwellings (checked 2026-08-04)
- International Building Code 2021, §1011 — Stairways — Commercial and multifamily limits: 7" riser, 11" tread, 44" width (checked 2026-08-04)
- IRC 2021 §R311.7.5.1 — riser height and variation — Maximum 7-3/4" riser, maximum 3/8" variation within a flight (checked 2026-08-04)
- Stair proportion rules — The 2R+T, R+T and R×T rules are long-standing carpentry conventions, not code requirements; they are checked here as a comfort test (checked 2026-08-04)