Stair Stringer Calculator
Calculate stair stringer dimensions including rise, run, number of steps, and stringer length for building code compliance.
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 (IRC):
| Dimension | Minimum | Maximum | Ideal |
|---|---|---|---|
| Unit Rise | — | 7.75 in (197 mm) | 7.0–7.5 in (178–190 mm) |
| Unit Run | 10 in (254 mm) | — | 10–11 in (254–279 mm) |
| Headroom | 80 in (2032 mm) | — | — |
| Width | 36 in (914 mm) | — | — |
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" — 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 — 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.