Stair Calculator

Enter your floor-to-floor height and this calculator lays out the whole flight — equal riser height, tread depth, total run, stringer length, the walking angle and the stairwell opening you need for headroom. Every result is checked live against IRC and IBC limits, so you see a green flag or a red one before anything gets cut.
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How the Stair Math Works

A stair is one constraint with four consequences. The constraint is the floor-to-floor height — the vertical distance from finished lower floor to finished upper floor. Everything else follows:

Risers. Divide the height by a comfortable target of about 7 inches and round to a whole number. That is the riser count. Divide the height by that count and you have the exact, equal riser height — equal being the point, since code caps the variation between risers at 3⁄8 inch.

Treads. There is always one fewer tread than riser, because the top riser lands on the finished upper floor. Total run = treads × tread depth. The tread depth you choose trades directly against footprint: a deeper, more comfortable tread lengthens the run.

Stringer. The stringer is the hypotenuse of the rise-and-run triangle: √(rise² + run²). The stair angle is arctan(rise ÷ run) — keep it between 30° and 37° and the flight will feel right underfoot.

Headroom. The quiet deal-breaker. You need 6′8″ of clear height above the nosing line, which sets a minimum length for the floor opening above the stair. This calculator returns that opening length for your floor assembly thickness — the number that decides whether a straight run fits or the stair has to turn.

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Code Limits at a Glance

Values are the widely adopted model-code minimums (2021 IRC / IBC). Local amendments, occupancy, and stair type (spiral, winder, alternating tread) can differ — confirm with your authority having jurisdiction before construction.

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Frequently Asked Questions

‍What is the maximum riser height?
7¾″ under the IRC (residential), 7″ under the IBC (commercial). Risers must also be uniform to within ⅜″. This tool divides your height into equal risers and flags any over the limit.

‍What is the minimum tread depth?
10″ under the IRC, 11″ under the IBC (nosing to nosing). Below 11″, add a ¾″–1¼″ nosing so the walking surface is deeper.

‍How many stairs for a given floor height?
Divide the floor-to-floor height by ~7″ and round to the nearest whole number for the riser count; divide the height by that count for the exact riser height. Treads = risers − 1.

‍What is the minimum headroom?
6′8″ (80″) clear above the nosing line, in both codes. The calculator returns the stairwell opening length that keeps you above it for your floor thickness.

‍What is a comfortable stair angle?
30°–37°. Check the proportioning rules too: 2R + T near 24–25″ and R + T near 17–18″ both signal a comfortable flight.