Stair Stringer Calculator — with Code Check
Enter your total rise (floor-to-floor height) and get the full stringer layout: number of risers and treads, exact riser height, total run, stringer length, and stair angle — with IRC residential code flags and a to-scale side-view diagram. Works in inches or centimeters.
Private by design. All stair math and the to-scale diagram run entirely in your browser — nothing is uploaded to a server.
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IRC residential stair limits (R311.7)
The International Residential Code sets hard limits on stair geometry because riser/tread proportions are directly tied to trip-and-fall rates. Most US jurisdictions adopt the IRC with local amendments; commercial stairs (IBC) are stricter. These are the numbers the pass/warn badges check.
| Dimension | IRC residential limit | Why it matters |
|---|---|---|
| Riser height | 7.75" maximum (and ≥ 4" typical minimum) | Taller risers are the single biggest stair-fall factor; short risers cause toe-catch |
| Riser uniformity | Largest and smallest riser within 3/8" | A single odd step breaks stride rhythm — this calculator divides rise evenly, so uniformity is automatic |
| Tread depth | 10" minimum (nosing can add ~1") | Shallow treads leave no room for the foot on descent |
| Stair angle | ≈ 37.5° comfortable maximum | Steeper pitches feel ladder-like; 30–35° is the comfort sweet spot |
| Headroom | 6'8" minimum vertical clearance | Measured plumb from the nosing line — check this separately for basement stairs |
- A classic carpenter rule of thumb: riser + tread ≈ 17–18 inches, or 2 × riser + tread ≈ 24–25 inches. The 7.5" / 10" defaults land right in that comfort zone.
- This calculator handles the stringer math; it does not model landings, winders, nosing projection, or handrail requirements — those have their own IRC sections.
How to calculate stairs from total rise
- Measure total rise. Measure the vertical distance from the lower finished floor to the upper finished floor, in inches or centimeters.
- Set your targets. Keep the 7.5" target riser and 10" tread defaults, or adjust. Optionally cap the total run if the staircase must fit a limited footprint.
- Read the layout. The tool rounds the riser count up (so actual risers never exceed your target), then shows exact riser height, treads, total run, stringer length, and angle.
- Check the code flags. Green badges mean the layout passes IRC residential limits; warnings flag risers, treads, or angles outside code — adjust and recheck before cutting.
Frequently Asked Questions
- Is anything uploaded?
- No. All stair math and the diagram render entirely in your browser.
- Why did I get 14 risers when 100 ÷ 7.5 is 13.3?
- The riser count is rounded up so the actual riser height never exceeds your target: 100" ÷ 14 = 7.14" per riser. Rounding down would give 7.69" risers — taller than you asked for.
- Why is there one fewer tread than risers?
- The top riser lands on the upper floor itself, which acts as the final "tread". A 14-riser stair has 13 treads.
- What stringer board length should I buy?
- The stringer length shown is the diagonal √(rise² + run²) — buy the next standard board length up, and add extra if your layout needs a top or bottom plumb cut beyond the step profile.
- What are the IRC limits this tool checks?
- IRC R311.7 residential: maximum riser 7.75", minimum tread depth 10", and the common ≤37.5° comfort/pitch guideline. Local amendments vary — always confirm with your building department.
- Does it handle metric?
- Yes. Toggle to centimeters and all inputs and outputs convert; the code check still evaluates against the IRC inch limits behind the scenes.
Use Cases
- Lay out deck stair stringers from a measured total rise
- Check a planned basement stair against IRC riser/tread limits
- Fit a staircase into a limited total-run footprint
- Convert a metric plan to inch-based stringer cuts
- Estimate stringer board length before a lumber run