Stair Stringer Calculator
Stringer count and board length for a set of deck or porch stairs.
Input sheet
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Stringers are the sawtooth backbones of a stair, and two numbers decide the lumber run: how long the diagonal is, and how many you need across the width. This calculator works both out from the total rise, tread depth, and stair width.
How it works
Risers = total rise ÷ 7.75 in max riser, rounded up. Run = (risers − 1) × tread. Stringer length is the diagonal √(rise² + run²); stringers sit 16 in apart across the stair width.
Riser count comes from code's 7¾ in maximum riser: total rise ÷ 7.75, rounded up, then the actual riser height is the rise divided evenly by that count. The horizontal run is one tread fewer than the riser count (the top 'tread' is the deck or floor itself).
The stringer's length is the diagonal of the rise/run triangle — √(rise² + run²) — and it must come out of a single 2×12, since sawtooth cuts leave only a narrow uncut spine. Stringers are spaced 16 in on center across the width: three for 36 in stairs, four for 48.
Risers = total rise ÷ 7.75 (rounded up); run = (risers − 1) × tread depth; stringer length = √(rise² + run²). Stringers = stair width ÷ 16 + 1 (rounded down + 1).
Worked examples
Deck stairs with 108 in total rise, 10 in treads, 36 in wide. → 3 stringers about 14.1 ft long — buy 16 ft 2×12s.
108 ÷ 7.75 → 14 risers at 7.71 in; run = 13 × 10 = 130 in; √(108² + 130²) ≈ 169 in ≈ 14.1 ft.
Low porch steps: 30 in rise, 10 in treads, 48 in wide. → 4 stringers about 3.5 ft long — one 2×12 yields them all.
30 ÷ 7.75 → 4 risers at 7.5 in; run = 3 × 10 = 30 in; √(30² + 30²) ≈ 42.4 in ≈ 3.5 ft.
Tips & gotchas
- Lay out with stair gauges on a framing square and cut the first stringer carefully — then use it as the template for the rest.
- The bottom riser must be cut shorter by one tread's thickness, or the first step ends up taller than the rest once treads go on.
- Use pressure-treated 2×12 outdoors and hang stringers with rated hangers or a ledger — toe-nails alone are how stairs pull away.
FAQ
How many stringers do stairs need?
One every 16 in of width: 36 in stairs take three, 48 in take four. Composite treads often require the tighter spacing regardless.
Why 7.75 inches for the riser?
That is the IRC maximum riser height. Dividing by it gives the fewest code-legal steps; the calculator then spreads the rise evenly so every step matches — code also caps riser variation at ⅜ in.
Can I use pre-cut stringers from the store?
Only if their riser height lands within ⅜ in of your exact rise ÷ steps. Pre-cuts assume standard rises; an inch of mismatch makes an uneven, code-failing step.
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