How do you calculate stringer length?

Carpentry & FramingIRC R311.7 Code Compliant

Stair Calculator (IRC Code Compliant)

Calculate exact riser count, step heights, tread depth, stringer layout, and total run length complying strictly with residential building codes.

Carpentry & Framing Field StandardsIRC R311.7 Code Compliant

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

Under International Residential Code (IRC Section R311.7), the maximum allowable stair riser height is 7-3/4 inches (197 mm) and the minimum tread depth is 10 inches (254 mm). For a standard 9-foot (108-inch) total rise, you need exactly 15 risers at 7.20 inches each, with 14 treads providing a total horizontal run of 12.25 feet.

Maximum riser height = 7-3/4" (IRC R311.7.5.1).
Minimum tread depth = 10" (IRC R311.7.5.2).
Minimum clear headroom = 6 feet 8 inches measured vertically.

Interactive Calculator

Adjust inputs for real-time results

Total Floor-to-Floor Rise Heightinches

inches

Finished lower floor to finished upper floor vertical distance.

Target Riser Heightinches

inches

Tread Depth (Run Per Step)inches

inches

Calculated OutputIRC R311.7 Compliant

Exact Riser Height

7.200"inches

Number of Risers (Steps Up): 15
Number of Treads: 14
Total Horizontal Run: 12.25 ft (147.0")
Minimum Stringer Lumber Length: 15.20 ft (16 ft board)
Stair Incline Angle: 36.3°

Note: IRC R311.7 limits maximum riser height to 7-3/4 inches and minimum tread depth to 10 inches. Riser height variance cannot exceed 3/8 inch.

IRC R311.7 Stair Code Summary

Uniformity is critical: the greatest riser height within any flight of stairs cannot exceed the smallest by more than 3/8 inch to prevent tripping.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

Use the Pythagorean theorem: Stringer Length = √(Total Rise² + Total Run²).