About this tool
Stair Calculator is a free, in-browser tool that turns a floor-to-floor rise into a buildable staircase: the number of steps, the exact riser height, the tread count, total horizontal run and stringer length. Everything is computed locally as you type — nothing is uploaded.
The number of risers is the total rise divided by your target riser height, rounded to the nearest whole step (at least one). Riser height is then total rise ÷ number of risers, so every step is identical, and the number of treads is one less than the number of risers. Total run is tread depth × number of treads, and the stringer length is the diagonal √(total rise² + total run²). A verdict cell checks the result against common building-code limits: risers between 6 and 7.75 inches, treads at least 10 inches, and the comfort rule 2 × riser + tread landing between 24 and 25 inches.
Use it to lay out a deck, basement or interior stair: adjust the target riser or tread until the verdict passes and every riser comes out equal, which is what inspectors and comfortable stairs both require.
Frequently asked questions
Why does the actual riser height differ from my target?
The total rise rarely divides evenly by the target, so the tool picks the whole number of risers closest to your target and recomputes an exact equal riser height. Building codes require all risers to be equal, so this even division is what you build to.
How is the stringer length found?
It is the straight-line diagonal of the stair triangle: the square root of (total rise squared + total run squared). This is the minimum board length for the stringer before cutting the notches; add a little for the top and bottom connections.
What is the code-comfort check testing?
It flags the stair as a pass only when the riser is 6 to 7.75 inches, the tread is at least 10 inches, and 2 × riser + tread falls between 24 and 25 inches — a longstanding stair-comfort rule. Otherwise it says adjust, and you can change the target riser or tread depth.
Should I still check local code?
Yes. Limits vary by jurisdiction and stair type. This tool uses widely accepted residential values as a guide; confirm the exact riser, tread and headroom rules with your local building code. All math runs in your browser.
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