Stair Calculator
Given the total rise (floor-to-floor height), how many steps, how tall each riser, how deep each tread, and what stringer length you need.
Layout
Number of steps
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Riser height (each)
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Tread depth
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Total horizontal run
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Stringer length
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Angle
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IRC compliance
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The IRC limits
International Residential Code limits residential stairs to: maximum 7.75 in riser, minimum 10 in tread (technically: 11 in with no nosing, 10 in with a ¾ in nosing). Maximum variation between any two risers in the same flight: 0.375 in.
The comfort rule of thumb
Two empirical rules architects use: riser + tread ≈ 17–18 in, OR 2 × riser + tread ≈ 24–25 in. A 7 in riser + 11 in tread = 18, which satisfies both. Anything steeper feels like a ladder; anything shallower wastes floor space.
Stringer length
The stringer is the angled board that supports the treads — its length is the hypotenuse of a right triangle whose vertical side is the total rise and horizontal side is the total run. Add a few inches for the top and bottom cuts.