Stair Stringer Calculator
Riser height, tread depth, total run, and stringer length for any total rise — equal risers, IRC-compliant.
Riser height (each)
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—
Risers
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Treads
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Total run
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— ft
Stringer length
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— ft
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How stair layout works
The starting point is the total rise — the vertical distance from finished floor (or ground) to the top of the upper landing. Divide it into equal risers no taller than code allows, and the geometry falls out from there.
Risers = ceil(Total_rise / Max_riser_height)
Actual riser = Total_rise / Risers
Treads = Risers − 1
Total run = Treads × Tread_depth
Stringer length = √(rise² + run²)
The "−1" on treads is because the upper landing acts as the last tread — you don't build a tread that's level with the floor you're stepping onto.
Code limits — IRC R311.7
- Riser height: 4" min, 7-3/4" max
- Tread depth: 10" min (measured nosing to nosing)
- Nosing projection: 3/4"–1-1/4" if treads are less than 11" deep
- Variation between risers (or treads): 3/8" maximum in any flight
- Headroom: 6 ft 8 in minimum, measured plumb from tread nosing to ceiling
- Width: 36" minimum (residential)
Stringer rules of thumb
- Use 2x12 stringers — anything smaller leaves less than 5" of solid wood after notching.
- Three stringers minimum at any width 36" or more. Four stringers if width exceeds 48".
- Maximum unsupported span: ~6 ft for 2x12 stringers carrying typical residential loads.
- Add 6–12" to the calculated stringer length for the top hanger and bottom kickplate cuts.
Worked example
Deck is 42" above grade. Max riser 7-3/4". Desired tread 10-1/2".
- Risers needed: ceil(42 ÷ 7.75) = ceil(5.42) = 6 risers
- Actual riser height: 42 ÷ 6 = 7.00"
- Treads: 6 − 1 = 5 treads
- Total run: 5 × 10.5 = 52.5" (4 ft 4-1/2")
- Stringer length: √(42² + 52.5²) = √(1764 + 2756) = 67.2" (5 ft 7-1/4"). Use a 12 ft 2x12 — that gives plenty of stock for cuts and a 1 ft offcut.
Frequently asked questions
Max riser height?
7-3/4" residential (IRC), 7" commercial (IBC). Variation in a flight ≤ 3/8".
Min tread depth?
10" nosing-to-nosing. Nosing projects 3/4"–1-1/4" if used.
Number of risers?
Divide total rise by your max riser, round up. That gives equal risers under the code limit.
Stringer length?
Pythagorean — √(rise² + run²). Add 6–12" for top and bottom cuts before buying stock.
How many stringers?
Three 2x12s for a 36"+ wide stair, four at 48" or wider.