Stair Design & Stringer Calculator (IRC R311.7)
Calculate stair riser height, tread run, stringer length, 2x12 lumber cuts, and throat depth conforming to IRC R311.7.
Stair Design & Stringer Calculator
Calculate exact riser height, tread run, incline pitch, cut dimensions, and material takeoff for IRC-compliant stairs.
Floor-to-floor vertical drop
IRC Max: 7.75" | Ideal: 7.25"–7.5"
IRC Min: 10.0" | Standard: 10.5"–11"
IRC Min: 36" clear width
Critical for bottom stringer cutback.
IRC requires 0.75"–1.25" if run < 11".
Checked against 80" (6'8") min IRC headroom.
14 Risers total
13 Treads total
11.38 ft
Optimal Comfort
Order 4 boards
≥ 3.5" (Code Pass)
Carpenter's Layout Rule: Bottom Stringer Foot Deduction
Before notching your 2x12 stringers, you must cut off 6.714" from the bottom foot of the stringer (Unit Rise 7.714" minus Tread Thickness 1"). This compensates for the added height of the first tread board, ensuring all finished step rises measure exactly 7.714" from finished subfloor to top landing!
Interactive Elevation & Stringer Geometry
Visual cut profile showing treads, risers, uncut throat depth (5.03"), and total run (136.5").
Cut off bottom of stringer so 1st finished riser equals 7.714" with 1" tread.
Top tread sits one full unit rise below upper finished floor level.
Falls in acceptable human walking stride zone (Standard: 24"–25").
About This Calculator & Field Methodology
Professional residential stair layout and stringer calculation engine conforming strictly to International Residential Code (IRC 2021/2024 Section R311.7). Computes exact riser counts (max 7-3/4"), tread runs (min 10"), incline pitch angles (30°–37°), 2x12 stringer cut dimensions, uncut throat depth (≥ 3.5"), bottom foot cut deductions, and minimum 80" clear headroom clearance. Generates itemized material takeoff lists and contractor proposal PDFs.
Standard formulas derived from international trade engineering standards (NEC Table 310.16, ACCA Manual J 8th Ed, ACI Concrete Mix Manual). Actual physical installations may require local authority having jurisdiction (AHJ) approval.
ToolHub Trade Engineering Team
Lead Framing Carpenter & Structural Code Consultant · Master Carpenter & ICC Residential Building Inspector Certified
Strict compliance with NEC, ACCA, and ACI code standards.
Frequently Asked Questions
Q:What is the maximum riser height and minimum tread run allowed by IRC code?
Under IRC Section R311.7.5.1 and R311.7.5.2, the maximum unit riser height is 7-3/4 inches (197 mm) and the minimum unit tread run is 10 inches (254 mm). Additionally, the greatest riser height within any flight of stairs cannot exceed the smallest by more than 3/8 inch (9.5 mm) to prevent tripping hazards.
Q:Why must you cut the tread thickness off the bottom of the stringer?
When laying out a stair stringer, the carpenter must deduct the thickness of one tread board (typically 1.0" or 1.5") from the bottom foot of the stringer. Because the first tread will sit on top of the first stringer notch, failing to cut this thickness off will make the first step too tall by the tread thickness, and make the top landing step too short, violating building code.
Q:What is the minimum throat depth required for a 2x12 cut stringer?
The uncut 'throat' (waist) of a 2x12 stringer—the remaining uncut wood perpendicular to the bottom edge behind each notch—must be at least 3.5 inches (89 mm) deep per IRC and structural engineering standards. A throat smaller than 3.5 inches compromises the load-bearing capacity of the stringer and can sag or snap under dynamic live loads.
Q:What is Blondel's Rule for comfortable stair walking stride?
Blondel's formula states that (2 × Rise) + Run should equal between 24 inches and 25 inches (61 cm to 64 cm). This reflects the natural human walking gait: when steps are steeper, treads should be shorter; when steps are shallower, treads should be deeper for effortless climbing.
Q:How much clear headroom is legally required above a staircase?
IRC Section R311.7.2 mandates a continuous clear headroom of not less than 6 feet 8 inches (80 inches / 2032 mm) measured vertically from the sloped plane adjoining the tread nosing to the ceiling or ceiling opening header.
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