How to Measure for Bay Window Curtains: The 6-Step Method for Angled Windows

Most people measure the window frame. On a bay window, that is the wrong measurement. The track path around the two corners runs 20 to 40 percent longer than the frame width you see head-on, and every ready-made panel on the market is sized for the frame. That single confusion is why 4 buyers out of 5 end up with panels that stop 8 inches short of the flanking segments, or a straight rod screwed across a bay that leaves the corners exposed to morning sun.

Bay windows break the flat-window formula on three counts. The track has to follow three segments (left flank, front, right flank), the two corners sit at 22.5, 30, or 45 degrees to the front plane, and the stack-back cannot cross the bend, so each corner eats its own fabric. If you want to zoom out to the entire pillar of bay window treatments, the bay window curtain hub covers every mounting variation, header style and fabric weight.

Anatomy of a Bay Window (Vocabulary Before Numbers)

Every measurement error on a bay traces back to the same shortcut: treating the front segment as the whole window. It is not. The bay is three windows fused into one recess, and the two corner walls carry as much drape as the front does. Naming the parts once removes the “which number am I quoting” confusion that costs the second order.

Bays come in three common geometries: shallow bows at 22.5 degrees per corner, standard bays at 30 degrees, and deep angled bays at 45 degrees. The angle is the single number that decides whether a bendable track absorbs the geometry or three separate rods are the only path, and it is the number most homeowners never actually measure.

The 6-Step Method at a Glance

The sequence matters. Mount decision first, because it caps every hardware option that follows. Segment widths and corner angles next, because those are the two data points a bendable track fabricator needs. Drop, stack-back and order flow from there. Skip mount and every subsequent number is provisional.

Step 1: Choose Ceiling Mount or Wall Mount Before You Touch a Tape

Why ceiling wins on bays 8 times out of 10

Sounds like a preference call. It is not. A 250 gsm blackout drape running across a 120-inch bay track puts 7 to 9 pounds of fabric on the hardware. Wall brackets rated for lighter loads start to sag on 10-foot spans, and the two corners force elbow brackets that create a header gap of 1 to 2 inches at each bend.

Ceiling joists absorb the load, and a single bendable rail like the Velin removes the elbow bracket problem entirely. The drape line stays continuous through both corners instead of jumping at each junction. That continuity is what makes the bay read as one window rather than three windows glued together.

When wall mount can still work

Two scenarios keep wall mount viable. First, a lightweight sheer under 150 gsm on a shallow bow at 22.5 degrees, where the load is low enough that elbow brackets stay clean. Second, when the ceiling has a bulkhead, crown molding, or coffered detail directly above the bay that would compete visually with a ceiling track.

Beyond those, ceiling mount is the correct call. If it were my client’s living room with a 108-inch angled bay, I would never spec three rods. The elbow bracket gap alone reads as an install shortcut every time you draw the drapes.

Step 2: Measure Each Segment Width and Add the Two Bend Allowances

The three segment measurements

Three measurements, not one. Measure the front segment head-on, from left corner to right corner along the parallel-to-the-wall face. Then measure each flanking segment along its actual angled wall face. Do not project the flanks onto the front plane, and do not use the flat-projected bay width as your track path input.

The distinction matters. A bay that reads 90 inches wide head-on because you measured the recess opening across the front plane hides two angled walls behind it. If those walls each run 30 inches along their own face at 30 degrees, the true track path is 30 plus 60 plus 30 plus 4 inches of bend allowance, which lands at 124 inches. The Two Pages custom curtain measurement guide spells out the same rule with photographs of the correct measurement path.

The bend allowance the fabricator needs

Every corner needs 2 inches of extra track length for the bend radius. The bendable Velin cannot fold at a sharp 90-degree corner (nothing can without kinking), so the fabricator absorbs each bend with a gentle 2-inch radius that consumes a bit of track length in exchange for a clean curve.

Add 2 inches per corner to the sum of your three segment widths, and that is your total order length. Round up to the nearest half inch. Two Pages configurators accept half-inch increments up to 216 inches total.

Step 3: Capture Each Corner Angle to the Half Degree

22.5°, 30°, 45° — get it exact

Sounds obvious. It is not. Standard residential bays fall in three buckets and homeowners guess wrong 3 times out of 5. A phone protractor app placed flat against the corner wall delivers 1-degree precision in under a minute, and the number goes on the order sheet next to the segment widths.

The angle is what the Velin fabricator bends the track to before shipping. Guessing 30 when the corner is actually 45 puts the track 3 inches off the ceiling header at each corner, the drape misses the glass, and the whole install reads as amateur. The Two Pages configurator asks for exact values in half-degree increments.

Asymmetric bays and settled foundations

Older homes almost never deliver a symmetric bay. Foundations settle, framing shifts, and the two corners can differ by 2 to 5 degrees. Measure both independently and note them left / right on the order sheet.

The bendable track is bent to your two values, not to a symmetric assumption. A bay that reads 30 degrees on the left and 32 degrees on the right ships with those exact bends. Filing a single “30 degrees” value on the order form when the reality is asymmetric produces a track that fights the ceiling line at one corner.

Bay type	Corner angle	Stack-back per corner	Typical flat-projected width
Shallow bow	22.5°	5 to 7 inches	60 to 84″W
Standard bay	30°	6 to 8 inches	72 to 108″W
Deep angled bay	45°	8 to 10 inches	96 to 120″W
Wide bow	60°	9 to 11 inches	108 to 144″W
Box bay	90°	N/A (three rods)	96 to 120″W
Custom	Measure to 0.5°	Formula-based	Any

Step 4: Apply Fullness and Corner Stack-Back

The corner stack formula bays force

Picture the moment you draw the curtains at 8am and the fabric jumps the corner in one continuous fold. That is what a bendable track delivers. But there is a catch: the fabric cannot cross the corner bend when stacking open, so each corner absorbs its own stack-back independently.

The formula runs by header style. Corner stack per side = panel width × 0.20 for pinch pleat, × 0.16 for grommet, × 0.12 for ripple fold. The two corner stacks stay on the flanking segments; they do not slide onto the front. That means the front glass exposure when fully open equals the full front segment width, but each flank exposes only its width minus the corner stack.

Converting corner stack into total track length

Worked example on a 124-inch track path with 2x fullness. Total panel width = 248 inches. Split as four panels (one per flank, two center-split on the front) at 62 inches wide each. Pinch pleat corner stack = 12 inches per corner.

When the drapes are fully open, the front segment exposes 60 inches of glass (both center-split panels stack onto the corners). Each flank exposes 30 minus 12 = 18 inches of usable glass. Skip the corner stack calculation and the flanks stay 40 percent covered even when the drapes are drawn.

Step 5: Set the Drop and Decide Panel Split

Drop options and the window seat problem

Drop measures from the track roller position to the hem, not from the ceiling to the floor. On ceiling mount, drop = floor-to-ceiling minus 3 inches for the roller offset. On wall mount, drop = rod-to-floor.

The bay-specific constraint sits below the window. If a window seat, radiator, or heat register occupies the wall under the flanking segments, the two flanks finish at sill or apron kiss while the front finishes at floor kiss, and the whole bay looks broken. Match all three segments to the same hem finish or accept that the window seat wins the drop decision.

Four-panel configuration for bays

The default panel split for a 3-sided bay is four panels: one per flanking segment plus a two-panel center split on the front. The configuration gives a symmetric look and lets the front panels open without disturbing the two flanks.

Three-panel single-track setups (one per segment, joined at the corners) are possible on shallow bows past 22.5 degrees but not recommended past 30 degrees, because the fabric bunches awkwardly at the bend when the drapes are drawn open. If it were my client, I would default to four panels every time past 22.5 degrees. Full stop.

Step 6: Order Custom (Ready-Made Almost Never Fits a Bay)

Single bendable track vs three rods

Real talk on ready-made drapes over 96-inch bays: they miss the floor by 4 to 6 inches on ceiling-mount installs, they cannot follow the two corners on a single rod, and they force a compromise on every single number captured in the last five steps. Custom is not a luxury on a bay window, it is the only way to hit the geometry.

The two paths forward are a single bendable track like the Velin, or three separate rods joined by elbow brackets. The bendable track wins on drape continuity and header aesthetics; the three-rod setup wins on upfront cost and stays viable on very deep bays past 45 degrees with structural columns that would fight a continuous rail. The Velin bendable track product page lists every configurator field the fabricator needs. The Velin configurator accepts three segment widths, two angle values, drop, and header type in a single order form.

Standard Bay Sizes vs Custom (When Ready-Made Falls Short)

Residential bays cluster into 6 flat-projected widths, per the This Old House bay window treatments reference. The head-on width ranges from 60 to 144 inches, and the height from 48 to 84 inches, with box bays (90-degree corners) at the upper end. Ready-made drape SKUs are built around standard 50 to 84-inch flat panel widths and 63, 84, 95, 108-inch drops, which cover only the front segment on paper and almost none of the bay geometry in practice.

Bay type	Flat width	Height range	Typical use case
Shallow bow	60″W	48″H	Kitchen breakfast nook
Medium bow	72″W	60″H	Dining alcove
Box bay	96″W	60″H	Living room seating alcove
Angled bay	108″W	72″H	Formal living room
Deep bay	120″W	72″H	Master bedroom bay
Wide bow	144″W	84″H	Great room, cathedral ceiling
Custom	Any	Any	Any

The 84-inch standard drop misses the floor by 4 to 6 inches on the smallest bay in the table and by 12 to 14 inches on the deep and wide variants. Ready-made panels sized to 50 or 84 inches wide cover the front segment on the smallest bows and leave the flanking segments naked on everything past 72 inches wide. Custom sizing to the half inch is the only path that hits the three-segment geometry, and it is the extension of the base flat-window method covered in the general curtain measurement guide.

6 Bay Measurement Mistakes That Force a Reorder

The reorder rate on bay window curtains sits above 30 percent industry-wide. The reason is not shipping damage or fabric defects, it is measurement. Six mistakes account for nearly all of them.

Now You Have the Path and the Angle: Order a Track Bent to Them

Coming back to the original question. With mount, three segment widths, two angle values, drop, and corner stack-back locked, the next click is the Velin configurator. Enter total path, both angle values, curtain height, and header type. The track ships pre-bent to your geometry in 5 to 7 business days, folded for delivery, unfolded on install.

FAQ

How do I measure a bay window for curtains?

Measure each of the three segments separately: the front segment head-on, then each flanking segment along its actual wall face without projecting it onto the front plane. Add 2 inches of bend allowance per corner. The total is your track path, which runs 20 to 40 percent longer than the frame width you see head-on. On a 90-inch flat-projected bay, expect a track path of 118 to 128 inches.

How wide should curtains be for a bay window?

Panel width must total at least 2 times the track path for proper folds, not 2 times the frame width. A 124-inch track path calls for 248 inches of total panel width, typically split into four panels: one per flanking segment plus a two-panel center split on the front. Corner stack-back adds another 6 to 10 inches per corner that cannot cross the bend.

How long should curtains be for a bay window?

On ceiling mount, drop equals floor-to-ceiling minus 3 inches for the roller offset. On wall mount, drop equals rod-to-floor. The bay-specific constraint sits below the window: if a window seat, radiator or heat register occupies the wall under the flanking segments, the two flanks finish at sill or apron kiss while the front finishes at floor kiss, and the whole bay looks unbalanced. Match all three segments to the same hem finish or accept the window seat wins the drop decision.

Do you need a special curtain rod for a bay window?

Yes on every bay past 22.5 degrees. A straight rod cannot follow the two corners, so the choices are a single bendable ceiling track like the Velin (aluminium, custom-bent to your two angle values, up to 216 inches long, 44 lbs load) or three separate rods joined by elbow brackets. The single track wins on drape continuity and header aesthetics, the three-rod setup wins on upfront cost and stays viable on very deep bays past 45 degrees with structural columns.

How do I measure the angle of a bay window?

Standard bays fall in three buckets: 22.5°, 30°, 45°. A phone protractor app delivers 1-degree precision on the wall angle at each corner. Measure both corners independently, because older homes with settled foundations produce asymmetric bays with 2 to 5 degrees of difference between left and right. Note them separately on the order sheet, the bendable track fabricator bends to your two values, not to a symmetric assumption.

What size curtains do I need for a 3-sided bay window?

For a standard 90-inch flat-projected 3-sided bay with 30-degree corners and 88-inch ceiling mount: total track path 118 to 128 inches, panel width 236 to 256 inches at 2x fullness, split as four panels of 60 to 65 inches wide each. Drop measures 85 inches on ceiling mount with floor kiss. Order custom, no ready-made SKU carries these numbers.

Can you use ready-made curtains on a bay window?

Only if the front segment is under 72 inches wide and the two flanking segments sit above a window seat that hides the drop mismatch, otherwise no. Ready-made panels come in 84, 95, 108-inch drops and 50 to 84-inch widths, but bay windows demand three segment widths, two corner angles, and a track path that no standard SKU carries. Custom sizing removes every compromise.

How much fabric do I need for bay window curtains?

Total fabric width equals track path × fullness ratio. For 2x fullness on a 124-inch track path, that is 248 inches of panel width. Add 4 to 6 inches per corner for the corner stack-back allowance the fabric needs to bunch when open, since the folds cannot cross the bend. Total order width lands at 268 to 272 inches for a standard 3-sided bay, typically split across four panels.