Stair Stringer Calculator

Stair Stringer Calculation determines the rise, run, and number of steps needed for a safe and code-compliant staircase.

Key Terms:

• Total Rise: The vertical distance from the bottom landing to the top landing.
• Total Run: The horizontal distance the stairs will cover.
• Unit Rise: The height of each individual step (riser).
• Unit Run: The depth of each individual step (tread).
• Stringer: The diagonal board that supports the treads and risers.

Building Code Requirements (International Residential Code, or IRC):

IRC R311.7.3 is the row worth reading twice. A single flight cannot climb more than 151 inches between floors or landings, so a 14-foot floor-to-floor needs a landing regardless of how the risers work out. This calculator flags it.

The 17-18 Rule: A comfortable stair satisfies: Unit Rise + Unit Run = 17 to 18 inches (43–46 cm). For example: 7" rise + 11" run = 18", which is very comfortable.

Stringer Length Formula: Stringer Length = √(Total Rise² + Total Run²)

This is the Pythagorean theorem applied to the diagonal stringer board.

Number of Steps: Number of Risers = Total Rise / Desired Unit Rise (rounded to nearest whole number) Actual Unit Rise = Total Rise / Number of Risers Number of Treads = Number of Risers - 1 (the top landing counts as the final step)

Practical Example: Total rise: 108 inches (9 feet / 2.74 m). Desired unit rise: 7.5 inches. Number of risers: 108 / 7.5 = 14.4 → round to 14 risers. Actual unit rise: 108 / 14 = 7.71 inches, which is within code. Number of treads: 14 - 1 = 13. With 10.5" run: Total run = 13 × 10.5 = 136.5 inches (11.4 ft / 3.47 m). Stringer length: √(108² + 136.5²) = 174.1 inches (14.5 ft / 4.42 m).

Tips:

• All risers in a staircase must be the same height (max 3/8" / 9.5 mm variation allowed by code).
• Use a 2×12 board for stringers, it provides enough width for the notch cuts.
• Mark the stringer with a framing square and stair gauges for accuracy.
• Account for the thickness of the tread material when calculating the first riser height.
• Check local building codes, as requirements vary by jurisdiction.