Stair Stringer Calculator

Calculate stair stringer dimensions for any deck, porch, or interior staircase. Enter the total rise height and desired tread depth to get riser height, stringer length, angle, and step count — all meeting standard building code guidelines. Free, no signup required.

Stair Stringer Layout Calculator

Stair Building Code Requirements

Most building codes in the United States (IRC Section R311.7) require stairs to meet the following standards:

  • Maximum riser height: 7-3/4 inches (7.75")
  • Minimum riser height: 4 inches
  • Minimum tread depth: 10 inches
  • Maximum variation between risers: 3/8 inch
  • Minimum stairway width: 36 inches

The classic rule of thumb is: 2 × riser + tread = 24 to 25 inches. This ensures comfortable and safe stair proportions. A 7-inch riser with a 10-inch tread gives 24 inches — within the ideal range.

Stair Stringer Formula

Number of Steps = ROUND(Total Rise / Target Riser Height) Actual Riser = Total Rise / Number of Steps Total Run = Number of Treads × Tread Depth (Note: Number of Treads = Number of Risers) Stringer Length = √(Total Rise² + Total Run²) Angle = arctan(Total Rise / Total Run) Example: 48" total rise, 10" treads Steps = ROUND(48 / 7.5) = 6 steps Riser = 48 / 6 = 8.0" (exceeds 7.75 code max — try 7 steps) Steps = 7 Riser = 48 / 7 = 6.86" ✓ Total Run = 7 × 10 = 70" Stringer = √(48² + 70²) = √(2304 + 4900) = √7204 ≈ 84.9"

Lumber Requirements

Stringer CountStair WidthApplication
2 stringersUp to 36"Narrow deck stairs
3 stringers36" – 48"Standard deck/porch stairs
4 stringers48" – 60"Wide stairs
5+ stringers60"+ Commercial or grand stairs

Frequently Asked Questions

What is the maximum riser height per building code? +
Per IRC (International Residential Code) Section R311.7.5.1, the maximum riser height is 7-3/4 inches (7.75"). The minimum is 4 inches. All risers in a stairway must be within 3/8 inch of each other. Local codes may be stricter — always verify with your local building department.
What size lumber do I need for stair stringers? +
The most common lumber for stair stringers is 2×12 dimensional lumber (actual size 1.5" × 11.25"). This gives enough material to cut the notches for risers and treads while maintaining the required minimum net section of 3.5 inches at the critical cross-section. For heavy-duty commercial stairs or long runs, engineered lumber may be used.
How do I calculate the number of stairs needed? +
Divide the total rise (in inches) by the desired riser height (typically 6.5"–7.75"). Round to the nearest whole number to get the number of risers (which equals the number of steps). Then recalculate the actual riser height by dividing the total rise by that number. The number of treads equals the number of risers for a free-standing stair.

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