Stair Calculator
Calculate step count, riser, and stringer length for a straight staircase from its total rise.
How we calculate it
Steps = ceil(Total rise / Desired riser), Actual riser = Total rise / Steps, Stringer length = √(Total rise² + (Tread × (Steps − 1))²).
Understanding your estimate
Get one riser height wrong on a straight-run staircase and every other riser in that run inherits the mistake, because a stair only works when every step is the same height. This calculator solves for that one number instead of asking you to guess it, then checks the result against the two limits a residential stair actually answers to: a building code that sets a hard ceiling, and a comfort rule that is not code at all.
A 9 ft rise, worked out
Say you're framing a straight run with 108 in of total rise, a 7.5 in riser you'd like to hit, and 10 in treads.
- Steps: 108 ÷ 7.5 = 14.4, rounded up to 15.
- Actual riser: 108 ÷ 15 = 7.2 in. Fifteen is the smallest whole step count that keeps every riser at or under your 7.5 in target.
- Treads: 15 steps means 14 treads, not 15. The 15th step is the upper floor itself, not a new board.
- Total run: 10 × 14 = 140 in.
- Stringer length: √(108² + 140²) = √31,264 ≈ 176.8 in, the board length a stringer gets cut from.
- Check both limits: 7.2 in stays under the 7.75 in code cap, and 10 in meets the tread minimum, so no code warning. 2 × 7.2 + 10 = 24.4, inside the 24-26 in comfort band, so no comfort warning either.
Why the step count always rounds up
14.4 becomes 15, never 14. That's the same rule this catalog's Fence Calculator uses for post counts: when a formula could go either direction, round toward the outcome you can't undo once the material is cut. Rounding down here would let the actual riser climb above what you asked for, and on some rise-and-riser combinations, above the code limit too. Rounding up means actualRiserIn can only ever land at or below your desired riser, never above it, no matter how the numbers divide.
Why 15 steps means 14 treads, not 15
A straight run always has one more riser than it has treads, because the top riser lands you on the floor you were climbing to, not on a new physical board. A 3-step stair has 3 risers but only 2 real treads between them. This calculator counts risers as stepCount and uses stepCount − 1 for the run math, which is the one detail that trips up a formula copied without thinking about what a step actually is.
Why exceeding a limit never stops the calculation
The code check (riser over 7.75 in or tread under 10 in, per IRC R311.7.5.1 and R311.7.5.2) and the comfort check (2 × riser + tread outside 24-26 in) are two separate flags, and neither one blocks the result. Even if the riser you type in already breaks the code ceiling, the calculator still returns a full profile, amber warning attached, because the decision to fix it is yours, not a validation rule's. The two checks come from different places on purpose: the code limits are an IRC requirement, but the 24-26 in comfort band traces back to design convention, not law. Confirmed independently of the IRC itself, that comfort formula is the kind of guidance a stair builder follows because it feels right underfoot, not because an inspector checks for it.
What this calculator does not cover
It only handles a straight run: one flight, no landing partway up, no winders turning the stair around a corner, no spiral. It also doesn't pour the stair itself. If you're building a solid concrete staircase rather than treads and risers on a wood stringer, the volume math for that is its own problem, separate from this one and from the Concrete Calculator's slab and footing shapes.