The two key concepts
Crown molding sits at the wall-ceiling junction at an angle — the "spring angle." Standard US homes use 38° spring angle (sometimes 45° or 52° for vaulted ceilings). The molding profile itself runs vertically when installed, so when you lay it on the saw, you tilt the molding to mimic its installed position.
The two cuts are different on each saw axis:
- Miter (left-right rotation): The cut angle around the vertical axis of the molding. This matches the wall angle.
- Bevel (forward tilt): The cut angle around the horizontal axis perpendicular to the blade. This compensates for the spring angle.
Inside vs outside corners
An inside corner is where two walls meet forming an interior angle (like the corner of a room). The molding wraps inward. The miter cut goes one direction, and the bevel complements it.
An outside corner is where walls form an exterior angle (like a wall bump-out or chimney). The molding wraps around the outside. The miter and bevel angles are different from inside corners.
The most common beginner error: cutting inside-corner angles for outside corners, or vice versa. The molding looks almost right until you hold it up — and then there's a gap that no amount of caulk will hide.
The formulas
For a corner with wall angle A (90° for standard corners) and spring angle S:
Inside corner:
Miter = arctan(sin(A/2) × tan(S))
Bevel = arctan(cos(A/2) × tan(S))
Outside corner (subtract from 90°):
Miter = 90° − arctan(sin(A/2) × tan(S))
Bevel = 90° − arctan(cos(A/2) × tan(S))
Why most crown jobs go wrong
- Molding flipped wrong on the saw. Always lay it with the ceiling edge (top of the profile when installed) against the fence.
- Inside and outside angles confused. Test-fit before cutting. The joint will tell you which way it should close.
- Sprung walls. Older homes are never exactly 90°. Measure the corner with a protractor or angle-finder; adjust the corner angle.
- Coping not used for inside corners. Coping (scribing the profile) gives a tighter joint on inside corners than a miter. Use this calculator for the rough angle, then cope for the final fit.
The cope joint advantage
Even with the perfect miter, inside corners can open up slightly as the wood moves seasonally. A cope joint — where one piece is cut to match the profile of the other — accommodates that movement and stays tight year-round.
To cope: miter one piece at the inside corner (left piece usually), cut the second piece at a 45° outside angle to expose the profile, then use a coping saw to follow the profile, angling back slightly.
Frequently asked questions
What if my spring angle isn't 38°?
Measure it: hold the molding on the wall, mark the top and bottom edges, measure the angle from the wall. Modern vaulted ceilings often have 45° or 52° spring. Use the custom field.
Can I cut crown without a compound miter saw?
Yes, but it requires a setup jig. Most woodworkers either buy a 10" or 12" compound miter saw (~$200-500) or build a sled for their table saw. The sled is cheaper but slower per cut.
Why does my cut look correct but doesn't fit?
Almost always: the molding is flipped. The "ceiling" edge must be against the saw fence (the vertical face). The "wall" edge goes against the table.