Stair Calculator — Risers, Treads & Stringer

Quick answer: a 9 ft floor-to-floor height (108 in) needs 14 risers at about 7.71 in each — 13 treads, a 130 in run, and a ~169 in stringer at a 10 in tread.

Stairs are sized from the floor-to-floor height: divide the total rise into equal risers, keep every riser identical (uneven steps are a trip hazard), then lay out treads and cut stringers. This calculator does the division properly — rounding to a whole number of risers so the actual riser lands under the 7¾ in IRC maximum — and gives you the stringer length and angle for the lumber yard.

Code checks are shown live: riser height (4–7¾ in), tread depth (10 in min) and the comfort rule (2 × riser + tread ≈ 24–25½ in).

How to use it

Measure the total rise — floor to floor, finished surfaces, in inches (9 ft = 108 in).
Set a target riser height (7–7.5 in is comfortable) or override the riser count once you've picked it.
Enter your tread depth (10 in is the code minimum and a comfortable standard).
Read the result: risers, exact riser height, treads, total run, stringer length and angle — plus pass/fail on the code checks.

How it works

Risers = round(total rise ÷ target riser), adjusted so the actual riser ≤ 7¾ in. Actual riser = total rise ÷ risers. Treads = risers − 1. Run = treads × tread depth. Stringer = √(rise² + run²). Angle = atan(rise ÷ run).

IRC 2021 residential limits: riser 4–7¾ in (R311.7.5.1), tread ≥ 10 in (R311.7.5.2), headroom ≥ 6 ft 8 in.
Standard (attached-top) mount: treads = risers − 1.
Stringer length is the sloped span — add a few inches of lumber for the top and bottom cuts.
Local codes can be stricter than the IRC — verify with your building department.

Worked example

Total rise 108 in (9 ft), target riser 7.5 in: 108 ÷ 7.5 = 14.4 → 14 risers. Actual riser = 108 ÷ 14 = 7.71 in ✓ (under 7¾). Treads = 13 × 10 in = 130 in run. Stringer = √(108² + 130²) ≈ 169 in (14.1 ft) — buy 16 ft stock. Comfort: 2 × 7.71 + 10 = 25.4 in ✓.

Quick reference

Stair code quick reference (IRC 2021, residential)

Item	Requirement
Maximum riser height	7¾ in (7.75 in)
Minimum riser height	4 in
Minimum tread depth	10 in
Stair width	≥ 36 in
Headroom	≥ 6 ft 8 in
Handrail height	34–38 in

Comfort rule (2 × riser + tread)

Riser	Tread	2R + T	Verdict
7 in	11 in	25 in	Comfortable
7.5 in	10 in	25 in	Comfortable
7.75 in	10 in	25.5 in	At the limit
8 in	10 in	26 in	Too steep (code fail)

FAQ

What is the maximum riser height for stairs?

7¾ in (7.75 in) for residential stairs under the IRC 2021, with a 4 in minimum. All risers in a flight must be equal — variations over ⅜ in are a trip hazard and a code violation.

How do I calculate the number of stairs?

Divide the total rise by a target riser (7–7.5 in is comfortable) and round to the nearest whole number. For a 108 in rise: 108 ÷ 7.5 = 14.4 → 14 risers at 7.71 in each.

Why are there fewer treads than risers?

On a standard stair the top step is the upper floor itself — it has no separate tread. So a 14-riser stair has 13 treads. A free-standing stair with landings at both ends is the exception.

How long is the stair stringer?

The stringer is the hypotenuse: √(rise² + run²). For a 108 in rise and 130 in run that's ~169 in — buy 16 ft lumber and account for the birdsmouth cuts at top and bottom.

What headroom do stairs need?

At least 6 ft 8 in (80 in) measured plumb from the tread nosing to the ceiling above. Check it before cutting — a headroom fail is the most expensive stair mistake to fix.

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Sources & verification

Data verified: October 11, 2026.

IRC 2021 (International Residential Code) R311.7 — stairway limits.
Stringer geometry: right-triangle hypotenuse; standard carpentry practice.

Estimates are for planning only — always confirm quantities with your supplier and check local building codes before you buy or build.