Deck Stair Calculator
Calculates code-compliant deck stairs under IRC Section R311.7 (max 7-3/4" rise, min 10" run), including number of notched 2x12 stringers (spaced 12" to 16" OC), 5/4x6 treads, and bottom concrete landing pad.
Formula
Steps = Round(Total Rise ÷ 7.5); Unit Rise = Total Rise ÷ Steps; Stringers = Ceil(Width ÷ 1.33) + 1
1. Number of Risers (Steps) = Round(Total Rise in ÷ 7.5 in) 2. Exact Unit Rise = Total Rise in ÷ Number of Risers 3. Number of Treads = Number of Risers - 1 4. Total Stair Run = Number of Treads × 10.5 in 5. Stringer Count = Ceil((Stair Width ft × 12) ÷ 12 in (composite) or 16 in (wood)) + 1
Inputs
- Total Vertical Height (Deck surface to ground in inches)
- Stair Tread Width
- Stringer Material
Worked example
Building code-compliant stairs for a deck 45 inches above ground level.
Outcome: 45" ÷ 7.5" = 6 risers (each exactly 7.50" high). 5 tread steps (each 10.5" deep). Total horizontal stair run is 52.5 inches (4.38 ft). Requires 4 cut 2x12 stringers (spaced 16" OC) and 10 five-quarter deck boards for treads.
Why can stair stringers not be spaced more than 12" to 16" apart?
Because stair treads take high dynamic impact when people step down. Spacing stringers at 12" (for composite) or 16" (for 5/4 wood) prevents treads from snapping or bowing dangerously.