Stair Stringer Calculator — Risers, Run & IRC Check
Who this is for: For deck builders and DIY framers cutting stringers from 2×12s — how many risers the rise takes, where the stringer lands, and whether the riser passes the 7¾-inch code check.
Enter total rise and tread depth — get riser count, actual riser height, total run, stringer length, and an IRC code check on the riser.
Quick answer: Risers = ceil(rise ÷ 7.75). A 42-inch rise takes 6 risers at 7.0 inches (IRC max 7¾), 50 inches of run at 10-inch treads, and a 65.3-inch stringer — order 2×12 stock a half-foot longer.
Grade to deck top, or floor to floor
IRC minimum 10 in
| Riser cap | 7¾ in (IRC R311.7.5) |
|---|---|
| Tread min | 10 in residential |
| Stringer | √(rise² + run²), from 2×12 stock |
| Run formula | (risers − 1) × tread depth |
How does the stair calculator lay out a stringer?
Stair geometry is one triangle: total rise (floor to floor, or grade to deck) is fixed, and the code caps each riser at 7¾ inches (IRC R311.7.5). Divide rise by 7¾ and round up — that's your riser count; the actual riser height is rise ÷ count, and it must not exceed the cap (the calculator flags it if it does). The run side: (risers − 1) treads, because the top riser lands on the finished upper level, each tread at least 10 inches deep for residential. Stringer length is the hypotenuse: √(rise² + run²), ordered as 2×12 stock long enough to keep the throat solid. Example: a 42-inch deck rise takes 6 risers at exactly 7 inches, 50 inches of run at 10-inch treads, and about a 65-inch stringer.
Common uses
- Laying out deck stringers before cutting 2×12s
- Checking a basement or porch stair against IRC riser limits
- Working out how much floor length a stair will eat
- Verifying equal risers when the rise is awkward