What angles for crown molding on a 135° bay window corner?
For 52/38 crown (38° spring) in a 135° bay window corner, lying flat you set a miter of 14.3 degrees and a bevel of 17.6 degrees. Nested upside down against the fence it is a single miter of 22.5 degrees with the blade square.
How to work it out
- miter = atan(sin 38° ÷ tan(135° ÷ 2)) = atan(0.616 ÷ 2.414) = 14.3 degrees
- bevel = asin(cos 38° × cos(135° ÷ 2)) = asin(0.788 × 0.383) = 17.6 degrees
- Nested upside down: (180° − 135°) ÷ 2 = 22.5 degrees with the blade square
A standard 45° bay window turns the wall twice on each side, and every one of those turns is a 135° corner. Two of them are inside corners — where the angled side walls meet the window wall — and two are outside corners, where the bay opens off the main wall.
Adjust for your project
- Miter angle — crown nested upside down (bevel 0°)
- 22.5 degrees
- Left-hand piece (as you face the corner)
- Lying flat (blade tilted left 17.6°): top edge against the fence, table swung right 14.3°, keep the left end. Nested: swing right 22.5°, keep the right side.
- Right-hand piece (as you face the corner)
- Lying flat (blade tilted left 17.6°): bottom edge against the fence, table swung left 14.3°, keep the left end. Nested: swing left 22.5°, keep the left side.
- Inside corner: many carpenters cope it instead — run the first piece square into the corner, cut the second nested at the miter above, then back-cut along the profile line with a coping saw. The coped piece rides over small wall errors.
A 135° bay corner by spring angle
| Spring angle | Miter (table) angle — crown lying flat | Blade bevel — crown lying flat | Miter angle — crown nested upside down (bevel 0°) |
|---|---|---|---|
| 38 | 14.3 degrees | 17.6 degrees | 22.5 degrees |
| 45 | 16.3 degrees | 15.7 degrees | 22.5 degrees |
| 52 | 18.1 degrees | 13.6 degrees | 22.5 degrees |
What to know about crown in a 135° bay
The most common error is reaching for the 90° numbers. The saw detents at 31.6° and 33.9° are for a square corner only; a 135° corner needs about half the miter and half the bevel. The second error is mixing up the two methods: 22.5° is the nested miter, and cutting 22.5° with the crown lying flat gives a joint that will not close.
Bay corners are rarely exactly 135°. Factory bay units are usually close, but site-built bays and old houses drift by a degree or two, and with four corners the errors stack up across a short run. Read each corner with an angle finder at ceiling height and cut each joint to its own reading. The outside corners read 135° measured across the outside; from inside the room the finder shows 225°.
The pieces are short. The angled side walls of a bay are often only 12 to 20 in wide, so cut both ends of a side piece from a longer offcut before cutting it to length, and nail the side pieces last so you can adjust them. An octagon has the same 135° corners and the same 22.5° cut — see What miter angle for an octagon for that flat version of the problem.
If the rest of the room has square corners, the settings for those are on Crown molding miter angle for a 90° inside corner (38° crown).
Different size? Use the full Crown Molding Calculator.
What you’ll need
- Digital angle finderBudget price
Reads angles to a tenth of a degree — blade-to-fence, blade tilt and existing corners — so saw settings match the calculator.
See options on Amazon (opens in a new tab) - Compound miter sawPremium price
Fast, repeatable crosscuts, miters and bevels for trim, frames and cut lists. A sliding model handles wider boards.
See options on Amazon (opens in a new tab) - 18-gauge brad nailerMid-range price
Fixes trim, battens and moulding with small nails that are easy to fill. Cordless models skip the compressor and hose.
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As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
Are the inside and outside corners of a bay window cut at the same angle?
Yes, in a regular 45° bay all four are 135°, so the miter and bevel are the same. What changes is which edge goes against the fence and which end you keep — the outside corners are the mirror image of the inside ones.
What miter and bevel for 45/45 crown in a 135° bay corner?
Lying flat, a miter of 16.3 degrees and a bevel of 15.7 degrees. Nested, 22.5 degrees — the same as for 52/38 crown, because the nested cut depends only on the corner.
How do I cut crown for the outside corners of a 135° bay?
Same angles — a miter of 14.3 degrees and a bevel of 17.6 degrees lying flat, or 22.5 degrees nested — with the pieces mirrored. Left-hand piece: Lying flat (blade tilted left 17.6°): bottom edge against the fence, table swung left 14.3°, keep the right end. Nested: swing left 22.5°, keep the right side. Right-hand piece: Lying flat (blade tilted left 17.6°): top edge against the fence, table swung right 14.3°, keep the right end. Nested: swing right 22.5°, keep the left side.
What crown angles for a 30° bay window with 150° corners?
For 52/38 crown, lying flat: a miter of 9.4 degrees and a bevel of 11.8 degrees. Nested upside down: 15 degrees. The corners are shallower, so every setting is smaller than in a 45° bay.
Other common sizes
Sources
- DEWALT DWS715 12 in. compound miter saw instruction manual — Cutting Crown Molding (opens in a new tab) — 52°/38° crown profile, 31.6° miter and 33.9° bevel for a 90° corner, and the left/right cut charts for both methods.