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Stair calculator (risers & treads)
Stair layout from total rise: how many risers, the equal riser height, and a comfortable tread run.
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How it’s computed
Risers = ceil(total rise ÷ target riser) — this keeps every riser AT OR UNDER your target (default 7.5"). Riser height = total rise ÷ riser count, all equal. Tread run comes from the classic 2R + T = 25 comfort rule. ADVISORY: riser and tread limits are code items — commonly a 7-3/4" max riser under the IRC, but jurisdictions amend — verify against your local code before you cut stringers.
Decimal-exact and deterministic: identical inputs always return identical results. This is the same math Ask Donnie’s estimating engine runs — not an approximation, and never a language model doing arithmetic.
Common questions
Why must the risers be equal?
Codes tightly limit riser-height variation within a flight because uneven steps trip people. Laying out from total rise ÷ count guarantees equal risers by construction.
What's the 2R + T = 25 rule?
A long-standing comfort proportion: twice the riser plus the tread ≈ 25". A 7.5" riser pairs with a 10" tread. Your code's minimum tread still governs — check it.
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