Stair Calculator - Riser, Tread, Stringer & IRC Code Limits

Calculate stair riser height, tread depth, stringer length, number of steps, and stair incline angle according to International Residential Code (IRC) standards.

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⚙️ Configuration & Inputs
Finished lower floor to finished upper floor (e.g. 9 ft = 108")
IRC code standard: maximum 7.75 inches
IRC code standard: minimum 10.0 inches
Ceiling clearance (IRC min: 80 inches = 6'8")
📊 Results & Analysis
Step Count (Risers) 14 Risers (13 Treads) Exact Riser Height: 7.71 in
Total Horizontal Run 136.5 in (11 ft 4.5 in) Total floor space occupied
Stringer Board Length 174.1 in (14.5 ft) Recommended: Buy 16 ft 2x12 boards
Incline Stair Angle 36.3° Comfort range: 30° - 37°
Blondel's Rule of Comfort 25.92 in Optimal (Target: 24 - 26 in)
IRC Code Compliance PASSED (IRC 2024) Rise ≤ 7.75", Tread ≥ 10.0"
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International Residential Code (IRC) Stair Standards

Proper stair framing is strictly enforced by municipal building inspectors to prevent slip-and-fall injuries.

1. IRC Building Code Rules for Residential Stairs

  • Maximum Riser Height: 7.75 inches (197 mm).
  • Minimum Tread Depth: 10.0 inches (254 mm) with nosing, or 11.0 inches without nosing.
  • Uniformity Rule: The difference between the tallest and shortest riser in a flight cannot exceed 0.375 inches (3/8").
  • Minimum Headroom: 80 inches (6 ft 8 in) measured vertically from the sloped plane connecting tread leading edges.

2. Blondel's Comfort Formula

The standard architectural comfort ratio: 2 × Rise + Run = 24 to 26 inches (ideal is 25 inches).

Frequently Asked Questions

The final step up lands directly onto the upper finished floor surface, which serves as the final tread. Therefore, an assembly with 14 risers requires exactly 13 wood treads.
Building codes require solid 2x12 dimensional lumber (actual size 1.5" × 11.25") so that at least 3.5 inches of uncut structural 'throat' remains after notching the steps.
Under IRC Section R311.7.5.1, the greatest riser height within any flight of stairs cannot exceed the smallest by more than 3/8 inch (9.5 mm).