Alternating tread stair calculator

Instead of full-width treads that both feet share, an alternating tread stair uses narrow treads that alternate sides, cut away in a zigzag pattern down the center. You climb it more like a ladder, right foot then left foot on alternating steps, which is what lets the pitch run so much steeper than a normal staircase.

Because each tread only needs to support one foot at a time, the effective tread depth per footstep can be shallower than what a conventional stair would require, while the overall angle stays walkable rather than a true vertical ladder.

Alternating tread stairs are commonly built in the 50 to 70 degree range, noticeably steeper than the roughly 30 to 37 degree range typical of conventional interior stairs. Riser heights commonly run 8 to 9.5 in, taller than a standard residential stair, which is part of what compresses the flight's footprint.

Tread depth on an alternating tread stair is usually measured per side rather than as one continuous run, commonly around 6 to 8 in per alternating step, since each foot only needs enough surface for a single footfall, not a full stride.

The core arithmetic is the same as any stair: divide total rise by a target riser height, round to a whole number of risers, and recalculate the exact riser height. From there, total run is the tread depth multiplied by the number of tread positions, and the diagonal length is still √(rise² plus run²).

The difference is scale: because the tread depth is short and the riser is tall, the resulting angle is steep and the horizontal run is compressed, which is the entire point of choosing this stair type over a conventional flight.

1. Measure total rise from the lower floor to the upper opening
2. Divide by a target riser height (commonly 8 to 9.5 in) and round to a whole number of risers
3. Recalculate the exact riser height by dividing total rise by that riser count
4. Choose an effective tread depth (commonly 6 to 8 in) for the alternating pattern
5. Multiply tread depth by the tread count to get overall run
6. Check the resulting angle with arctan(riser ÷ tread); expect 50 to 70 degrees

Alternating tread devices are typically used for access to spaces like lofts, mezzanines, equipment platforms, or attic storage rather than as a primary means of egress for living space, and many codes restrict where they can be used for exactly that reason.

Whether an alternating tread stair is permitted for a given space in your project depends heavily on local code and occupancy classification; this varies by jurisdiction, so check with your local building department before committing to this stair type for a code-required exit or a primary living-space access route.

Because the pitch is so steep, handrails on both sides are strongly recommended even where not strictly required, since the climbing motion on an alternating tread stair is closer to climbing a ladder than walking a conventional flight.

Headroom checks matter more here too: a steep 60-plus degree stair reaches a given rise in a much shorter horizontal distance, so overhead framing, ceiling openings, or floor hatches need to be sized for the compressed footprint, not just the total rise.

If you find a 50-plus degree pitch is too steep for the space's intended use, compare it against a standard stair layout using the stair rise and run calculator, or a compact but code-friendlier option using the winder stair calculator.

Need to double-check headroom clearance for the steep opening above the stair? The stair headroom calculator on this site walks through that measurement separately.