Compound Miter Crown Molding Calculator

Explanation

Crown molding sits at an angle between the wall and ceiling, so its profile is neither flat nor vertical. The spring angle is the angle between the back of the molding and the wall or ceiling. A 38 degree spring angle is common for traditional profiles, while 45 and 52 degree spring angles are also standard. The corner angle is the angle between the two walls you are joining. A typical interior corner is 90 degrees, but real rooms are often out of square and can be 88 to 94 degrees or more. This calculator solves the compound cut that blends those two geometric realities.

When crown is cut flat on a miter saw, the tool needs two settings: the miter (rotation in the horizontal plane) and the bevel (tilt of the blade). The miter controls how the piece swings around the corner, while the bevel compensates for the spring angle so the molding returns to the correct wall and ceiling surfaces after the cut. Because the molding is installed at a spring angle, the effective corner angle seen by the saw is not simply half of the room corner. Trigonometry projects the spring angle onto the miter and bevel planes, producing angles that are not intuitive but are repeatable.

The optional molding width is included for layout checks. While the width does not change the miter or bevel, it changes the length of the cut face across the molding. Knowing that length helps you mark the long point, verify the saw fence setup, and plan for complex returns. For precision building and finish carpentry, this extra check can save material and reduce cumulative error over long runs.

For inside corners, keep the crown upside down on the saw and cut the left and right pieces as a mirrored pair. For outside corners, the same angles apply but the miter direction flips. If your saw has crown stops or detents, verify that the spring angle setting matches your profile, since many crowns vary slightly from the listed angle. A quick test cut and dry fit will confirm that the calculated angles are aligned with the real molding geometry.

Formula

Let C be the wall corner angle and S be the spring angle (degrees). Convert to radians for trigonometry. The compound angles are:

miter = arctan( sin(S) / tan(C/2) )
bevel = arcsin( cos(S) x cos(C/2) )

Optional cut-face length for molding width W: face = W / cos(miter).

Example Calculation

Suppose your room corner is 90 degrees and your crown has a 38 degree spring angle. The calculator returns a miter angle of 31.62 degrees and a bevel angle of 33.86 degrees. If the molding width is 5.5 inches, the cut-face length is about 6.48 inches. Set your saw to those angles, make a test cut, and adjust only if your actual spring angle differs from the nominal profile.

FAQs

What is the difference between spring angle and corner angle?

The spring angle is the molding profile angle against the wall or ceiling. The corner angle is the angle between the walls you are joining.

Why are the miter and bevel angles different from half the corner?

Because the molding is installed at an angle, the saw has to project that angle into the miter and bevel planes.

Does this work for nested crown cuts?

This calculator is for flat-on-saw compound cuts. Nested cuts use different geometry and usually a single miter setting.

How do I handle an outside corner?

Use the same angle values but reverse the miter direction. The bevel remains the same for the matching piece.

Should I still test cut?

Yes. Real-world spring angles vary by manufacturer, and walls are rarely perfect.

How it works

This tool applies standard compound miter trigonometry and outputs saw settings that match your corner and spring angles. All calculations run client-side for privacy.

Disclaimer

Results are for planning and layout. Always follow tool safety practices and verify angles with test cuts before production work.