Bay Window Angle Calculator
Calculate left and right bay included angles from face widths and projection, then convert them into miter cuts, track bends, back-span checks, template marks, and corner clearance values.
Choose a close bay shape, then edit the measured side faces, center face, projection, return allowance, and tolerance for your exact window.
| Bay shape | Included angle | Turn from straight | Equal miter per piece |
|---|---|---|---|
| Square box bay | 90° | 90° | 45° |
| Traditional 45-degree side bay | 135° | 45° | 22.5° |
| Shallow bay or bow-like bay | 150° | 30° | 15° |
| Very shallow faceted bay | 160° | 20° | 10° |
| Measurement | How to take it | Used for | Common mistake |
|---|---|---|---|
| Side face length | Along the angled wall, sill, rod, or track route | Hypotenuse of each bay side triangle | Measuring straight across the room |
| Projection depth | Perpendicular from back opening to center face | Finds side offsets and included angles | Using sill depth when the trim is angled |
| Back opening span | Wall-to-wall at the rear of the bay | Checks calculated side offsets | Including casing only on one side |
| Track offset | Distance from glass or trim to hardware line | Template mark and clearance estimate | Ignoring bracket projection at corners |
| Target | Formula | Example | Use case |
|---|---|---|---|
| Side offset | square root of side squared minus depth squared | 25.5 in offset | Find rear opening span |
| Included angle | 180 minus arctangent of depth over offset | 135° | Bay corner angle |
| Turn angle | 180 minus included angle | 45° | Track or elbow bend |
| Equal miter | Turn angle divided by 2 | 22.5° | Trim, cornice, rod segment cuts |
| Layout | Angle priority | Length priority | Check before fixing |
|---|---|---|---|
| Flexible ceiling track | Turn angle and bend radius | Total route plus returns | Track bends without kinking |
| Segmented curtain rod | Elbow angle or half-miter setting | Each straight rod segment | Brackets do not block rings |
| Cornice or valance | Equal miter per corner | Outside face lengths | Dry-fit both corners together |
| Bay window seat template | Left and right skew difference | Back span and projection | Template touches all three faces |
Signal: left and right included angles are within the tolerance setting.
Use one miter setting for both corners and mirror the template marks.
Signal: side face lengths or corner angles differ noticeably.
Label each side before cutting, because one repeated angle will leave a visible gap.
Signal: projection is almost the same as each side face offset.
Expect near-90 degree corners and larger returns for fabric or blind clearance.
Signal: projection is small compared with the side face length.
Small angle errors show up over long track runs, so check the rear span carefully.
Make the depth line square. Projection must be perpendicular to the back opening, not diagonal from a corner. A square or laser line keeps the angle math honest.
Transfer the actual corners. For trim, rods, or a bay seat, cut a paper or cardboard triangle from the calculated back span and projection before making final hardware marks.
You get a curtain track that seems like it should work on paper. But it won’t conform to the shape of the room, it kinks sharply at the corners. It bends hard at the corners. It drags along the walls. That’s typically for one reason: no one bothered to measure their angles before they cut.
A bay window is a seemingly straightforward box extending outward into the rest of your home. Yet these are hardly ever perfectly rectangular. Builders will tweak things slightly and then walls settles. Most of the time the left and right sides is slightly different by degrees which start to add up when you attempt to mate two pieces of trim together cleanly.
How to Measure Your Bay Window Correctly
Once you enter your unique dimensions, the calculator do all the heavy lifting while avoiding any need to estimate visually, guess at coefficients or do the work yourself. Most people say, “Okay, I’ll just hang a picture,” by measuring straight line across the back opening and calling it good. Except that doesn’t tell you anything about geometry of the corners.
To determine the miter cuts you must figure out distance the window juts out from the line you used at the back wall. Call this depth. This is starting point for all other measurements. You must have your tape measure absolutely perpendicular to the rear opening. Otherwise every calculation that follows go off in some sideways direction.
It’s a little thing, but it matters a lot, since a two-degree error doubles as you get further from wall. And that’s where the reference tables on the page make things clear. They show that basic one hundred thirty-five degree bay means a twenty-two and a half degree miter cut. Many carpenters shoots for this as their sweet spot, but maybe your window is shallow or skewed, needing an entirely different approach.
So what does all this mean for your tools? That’s the trick. A hundred-and-fifty-degree included angle sounds like precision, but your miter saw doesn’t work in terms of included angles; it works in terms of turning away from square. That turns abstract geometrical world into something practical: where on the saw do I set my miter? It is the same deal with an elbow joint for a track system.
Geometry is even more important when you are dealing with flexible ceiling tracks. Plastic has memory, so forcing a too-tight bend risks cracking material or causing it to snap back under weight of heavy drapes. Can hardware take the geometry you are planning? Before you drive a hole in your drywall, you would of wanted to know.
Symmetry is another silent killer. On many older homes, the left and right halves of the bay aren’t symmetric. The calculator detects that skew; it also checks whether two corners line up (within some reasonable tolerance). If not: The miter cuts won’t be a perfect copy-paste from one side to the other, which results in exposed gaps, no amount of caulk can mask convincingly. Each corner has to be treated like a unique puzzle piece, especially with wooden cornices or seats. Wood movement and grain direction require precise joints. Use a paper template beforehand to get your head around it before committing to pricey lumber.
Precision always beats speed when it comes to trim work. If you take 10 minutes to double-check your length for each side face and projection depth now, you won’t have to spend hours cutting and resanding later. It’s just basic trigonometry math, but it’s no easy feat performing that mental arithmetic with a piece of molding in-hand.
Measure twice, cut once; get out the measuring tape, check your square, and follow the numbers. When curtain-hanging day arrives, your eyes will thank you for a perfectly-straight installation that doesn’t fight against the architectural elements.
