How to lay out a stair stringer
- Measure the total rise from the finished surface at the bottom to the finished surface at the top.
- Number of risers = total rise ÷ 7¾, rounded up. Exact riser height = total rise ÷ number of risers.
- Treads = risers − 1 (the top floor or deck is the last step). Total run = treads × tread depth.
- Mark the riser and tread with a framing square and stair gauges, repeating down the board.
- Cut the bottom riser shorter by one tread thickness so every step ends up the same height.
risers = ⌈total rise ÷ 7.75⌉ · riser = total rise ÷ risers · run = (risers − 1) × tread
Stair code basics (IRC residential)
- Riser height: 7¾" maximum, and no two risers in a flight may differ by more than ⅜".
- Tread depth: 10" minimum measured nosing to nosing; nosings of ¾"–1¼" are required on solid risers with treads under 11".
- Headroom: 6' 8" minimum; stair width: 36" minimum.
- Handrail at 34"–38" when there are four or more risers.
Local codes and decks (see AWC DCA 6) can differ — check with your building department before you build.
Stair stringer chart
With a 7¾" maximum riser, 10½" treads and 2×12 stringers:
| Total rise | Risers | Riser height | Treads | Total run | Stringer |
|---|---|---|---|---|---|
| 24" | 4 | 6" | 3 | 2' 7 1/2" | 3' 3 1/4" (8 ft board) |
| 30" | 4 | 7 1/2" | 3 | 2' 7 1/2" | 3' 7 1/16" (8 ft board) |
| 36" | 5 | 7 3/16" | 4 | 3' 6" | 4' 7" (8 ft board) |
| 48" | 7 | 6 7/8" | 6 | 5' 3" | 6' 7" (8 ft board) |
| 60" | 8 | 7 1/2" | 7 | 6' 1 1/2" | 7' 10 11/16" (8 ft board) |
| 72" | 10 | 7 3/16" | 9 | 7' 10 1/2" | 9' 10 5/8" (10 ft board) |
| 96" | 13 | 7 3/8" | 12 | 10' 6" | 13' 2 5/16" (14 ft board) |
| 108" | 14 | 7 11/16" | 13 | 11' 4 1/2" | 14' 5 15/16" (16 ft board) |
| 120" | 16 | 7 1/2" | 15 | 13' 1 1/2" | 16' 5 15/16" (18 ft board) |
FAQ
How many stringers do I need?
Space them about 16" apart for 5/4 wood deck boards and 2× treads; many composite treads need 12" or less — check the maker. That means 3 stringers for a 36" stair and 4 for a 48" stair.
What size lumber for stair stringers?
2×12 for cut (notched) stringers. After notching you must keep at least 5" of solid wood (the "throat"); the calculator checks this for you.
What is the ideal stair angle?
Between 30° and 37°. The classic comfort rule is 2 × riser + tread ≈ 24–25 inches.