Winder stair calculator

A landing takes up run without adding a riser; winders do the opposite, they add risers through the turn without a flat platform. A 3-winder quarter turn typically uses 3 risers, each turning 30 degrees, to complete a 90-degree change of direction.

For a 108 in rise at a 7.25 in target riser, total risers work out to 15 at 7.2 in each, exactly like a straight or landing stair with the same rise. Of those 15 risers, 3 become winders at the turn, and the remaining 12 risers are straight treads split between the flights before and after the turn.

Winder treads aren't a consistent depth; they're wide on the outside of the turn and taper to almost nothing on the inside. Because of that, tread depth is measured along a walk line, an arc roughly 12 in in from the narrow edge, commonly cited as the path most people actually walk on a turn.

For a 36 in wide stair, the walk line sits at a radius of 36 − 12 = 24 in from the inside pivot point. Each winder in a 3-winder quarter turn spans 30 degrees, so the arc length along the walk line per winder is 2π × 24 × (30 ÷ 360) ≈ 12.6 in, comfortably above the commonly cited 10 in minimum tread depth for that location.

Even with a generous walk-line depth, the inside point of each winder tapers down to a narrow tip. Commonly cited reference values put a 6 in minimum tread depth at that narrowest point, measured at the inside edge, so nobody's foot lands entirely on an unsupported sliver of tread.

This narrow-end minimum is one reason winders aren't stacked indefinitely; past 3 or 4 winders in a quarter turn, the inside edges get so tight that they start failing this minimum even when the walk line still looks fine.

Each of the 3 winders turns the stair 30 degrees (90 ÷ 3), pivoting around the inside corner post. On paper, that means drawing three 30-degree wedges radiating from the pivot point, each one riser higher than the last, fanned across the 90-degree turn.

The outside edge of each winder can be as wide as the straight treads before and after it, typically matching the stair's normal tread depth on the wide side. The framing under winders is more involved than a straight stringer, usually built with a center post or short header framing to support the tapering tread shape.

1. Determine total risers and riser height for the full rise.
2. Decide how many winders will complete the turn (commonly 2 to 4).
3. Divide 90 degrees by the winder count to get the angle per winder.
4. Set walk-line radius as stair width minus 12 in.
5. Check walk-line arc depth per winder against the commonly cited 10 in minimum.
6. Check the narrow inside edge against the commonly cited 6 in minimum.

Winders save floor space compared with a landing, but they're less forgiving underfoot, especially on the narrow side, and many people find them harder to descend confidently than a landing. Some jurisdictions restrict or add extra requirements for winders on required egress stairs, so check locally before designing around them.

If the stairwell has room for a full landing, most builders default to the L-shaped stair calculator or stair calculator with landing instead, reserving winders for tight retrofits, spiral-adjacent layouts, or attic and basement access where a full landing genuinely won't fit.