Measuring your turn before you calculate
A winder turn is defined by the angle it covers, the width of the stair, and how many treads split that angle between them — get those three right first.
Confirm your total turn angle
Most residential turns are a 90° corner (an L-shaped stair) or a 180° reversal (a U-shaped stair). If your layout turns a different amount, use Custom and enter the exact degrees.
Decide how many winder treads share the turn
Start low — 2 treads for a 90° turn, 4-5 for a 180° turn — and let the walk line depth chip tell you if that's too many. Adding treads narrows each one; it doesn't add capacity.
Measure clear stair width
Measure from the pivot side to the outside wall, the same clear width your straight flights use. This sets both the outer tread depth and how far out the walk line sits.
Read the compliance chips
Green means the walk line depth and riser height sit inside IRC residential limits. The narrow-edge chip is shown as a neutral flag rather than pass/fail — always confirm that measurement point with your local building department.
The trig behind every winder result
A winder tread is a pie slice around a pivot point. Every result on this page comes from the same angle and three radii — pivot, walk line, and outer wall.
Angle per Tread
The total turn split evenly across however many winders you're using. Every winder in a run should carry the same angle unless your layout has a specific reason not to.
Walk Line Radius
The walk line sits a fixed distance (12" by default) out from the narrow edge — not from the stair's centerline — so this radius barely changes even as stair width changes.
Tread Depth at a Radius
The straight-line chord distance between the two nosing lines of one tread, measured at radius R. Applied at the walk line, outer wall, and pivot radii to get the three depth results.
Outer & Pivot Radii
Outer radius is the pivot offset plus the full clear stair width; pivot radius is just the offset itself. Both feed the same chord formula above.
Walking through a 90° turn, 36" wide
Same formulas, filled in step by step, for the default this calculator loads with — a 90° corner, 36" clear width, 12" walk line offset, and a 1.5" pivot offset.
Try 3 treads first
90° ÷ 3 = 30° per tread. Walk line radius is 1.5 + 12 = 13.5". Walk line depth: 2 × 13.5 × sin(15°) ≈ 6.99" — well under the 10" minimum. Three treads splits the turn too thin.
Drop to 2 treads
90° ÷ 2 = 45° per tread. Same 13.5" walk line radius: 2 × 13.5 × sin(22.5°) ≈ 10.33" — this clears the 10" minimum, which is why 2 winders is the default for a standard 90° corner at typical widths.
Check the outer edge
Outer radius is 1.5 + 36 = 37.5". Outer depth: 2 × 37.5 × sin(22.5°) ≈ 28.7" — plenty of depth on the wide side; the walk line, not the outer edge, is always the binding constraint.
Check the narrow edge
Pivot radius is only 1.5": 2 × 1.5 × sin(22.5°) ≈ 1.15". This is the geometric reality of a pie-shaped tread — it tapers to almost nothing at the true pivot, which is exactly why the narrow-edge chip below is flagged neutral rather than a hard pass/fail.
This is the single most useful thing this calculator shows: adding a third winder to make each step feel gentler usually backfires, because it shrinks the one measurement — walk line depth — that code actually checks.
Typical winder counts by turn angle
These are common starting points at a 12" walk line offset — your actual stair width still determines whether each one clears the depth minimum.
| Turn | Typical Winders | Angle Each | Notes |
|---|---|---|---|
| 90° (L-shaped) | 2 | 45° | Most common residential quarter-turn layout. |
| 90° (L-shaped) | 3 | 30° | Gentler per-step angle, but often fails walk-line depth at a 12" offset — see worked example. |
| 180° (U-shaped) | 4 | 45° | Pairs two 90° turns' worth of winders with no straight run between them. |
| 180° (U-shaped) | 5–6 | 30–36° | Common where a landing isn't available; check walk-line depth carefully at this count. |
IRC winder-tread rules, quick reference
This calculator checks your result against the walk line and riser limits below. Full residential code also covers headroom and handrails — see the linked calculators for those.
| Element | Residential (IRC) | Notes |
|---|---|---|
| Walk line offset | 12" | Measured from the side where winders are narrower. [VERIFY] |
| Min tread depth at walk line | 10" | Same minimum as a straight-flight tread. |
| Min tread depth, any point in clear width | 6" | Measurement point varies by jurisdiction — see note below. [VERIFY] |
| Max riser height | 7.75" | Must stay uniform with the straight-flight risers, within 3/8". |
Reference only — always confirm against your local jurisdiction's adopted code before building. Winder-specific provisions (commonly found under an IRC section covering "winders") vary in section numbering between code editions and local amendments.
Pivot points, walk lines & tread numbering
A winder turn can pass the walk line depth check and still cause real problems on site — this section covers the three things that trip up an otherwise-correct layout.
Why the pivot offset matters
A true zero-radius pivot produces a tread that tapers to a mathematical point — impossible to frame or walk on. A small offset (a built-up stringer or center post, commonly an inch or two) gives the narrow end an actual physical width, at the cost of shrinking every other radius slightly.
Why walk line depth, not outer depth, is the binding limit
Because tread depth grows with radius at a fixed angle, the outer edge of a winder is always the deepest point and the walk line is always tighter than it. Code checks the walk line specifically because that's roughly where a person's foot actually lands.
Numbering winders in a turn
Each winder tread is still one full riser height, uniform with the straight flights on either side. Don't add or drop a riser to make the turn "work" — resolve a bad walk-line depth by changing tread count or width instead, not riser height.
Landing vs. winder at the same turn
Every winder turn can instead be built as a flat landing with no risers inside it — simpler to frame and code-compliant by definition, at the cost of a larger footprint. The L-Shaped and U-Shaped calculators below toggle between the two approaches directly.
These are planning-stage estimates, not a stamped structural design. Winder framing carries load differently than a straight stringer — have a carpenter or engineer confirm the support layout before cutting.
Common mistakes when planning a winder turn
Most winder problems trace back to one of these five planning errors, not to bad carpentry.
Adding treads to soften the turn
More winders means a smaller angle each, which sounds gentler but shrinks walk line depth — the worked example above shows this flipping a passing turn into a failing one.
Measuring the walk line from the wrong edge
The 12" offset is measured from the narrow side of the winders, not from the stair's centerline or the wide outer wall — get the reference edge backwards and every depth number is wrong.
Assuming a true zero-radius pivot
A mathematical point can't be framed. Every real winder turn needs some pivot offset — a post, a built-up stringer, or a small landing nose — decided before cutting, not improvised on site.
Changing riser height inside the turn
Winder risers must stay uniform with the straight-flight risers on either side, same as any other step in the run. Solve a bad turn with tread count or width, never by shortening the winder risers.
Checking only the outer edge
The outer edge of a winder always has the most depth and will always look fine. Code — and safe footing — care about the walk line and narrow edge, both of which are tighter than the number that's easiest to eyeball.
When to use a winder staircase calculator
Winder turns show up anywhere a straight stair needs to change direction without the floor space for a full landing.
Tight Remodels
Turning a stair inside an existing stairwell that's too short for a landing.
New Construction Layout
Setting winder geometry before framing, alongside the L- or U-shaped flight plan.
Code Compliance & Permits
Confirming walk line depth before a plan check, especially on a tight-footprint turn.
Custom Stringer Layout
Working out individual tread angles before cutting winder stringers or templates.
Basement & Attic Access
Fitting a turned stair into a confined opening where a landing won't fit.
Design Comparison
Comparing a winder turn against a landing-based L- or U-shaped layout for the same footprint.