Compound Miter Angle Calculator

Crown molding angles explained

Measure the spring angle from the wall

Spring angle here is the angle between the installed crown and the vertical wall. A 38° spring profile is therefore 52° from the ceiling. These settings are for crown laid flat, not nested at its installed angle against the fence.

Which corner angle should I enter?

Enter the smaller included angle between the wall lines, greater than 0° and less than 180°. For a normal square inside or outside corner, enter 90°. If you measured a reflex angle such as 267°, enter 360 − 267 = 93°. Inside/outside changes the orientation of the cuts, not their magnitudes under this convention.

The corrected formulas

With spring angle S and included corner angle C: miter = atan(sin(S) / tan(C/2)); bevel = asin(cos(S) × cos(C/2)). Evaluate trigonometric functions in degrees. As the corner approaches a straight 180°, both settings approach zero. Multiplying by tan(C/2) instead of dividing only agrees at a 90° corner.

Worked examples for 38° spring crown

At a 90° corner, use miter 31.62° and bevel 33.86°. At 93°, they become 30.30° and 32.85°. At 120°, they are 19.57° and 23.20°. These magnitudes agree with the SBE Builders crown molding tables. Keep the actual measured spring angle; rounding it to a whole degree changes the result.

Setup and test cuts

The results do not specify left/right saw directions or which offcut to keep. Those depend on inside/outside, the end being cut, profile orientation and saw capabilities. Follow the setup chart for your saw and make a mirrored test pair in scrap. The Bosch GCM12SD manual includes separate crown setup directions for each corner and end.

Scope

This model assumes two pieces of the same crown profile and spring angle meeting at a wall corner below a level ceiling. Unequal spring angles, sloping ceilings and arbitrary compound assemblies require a different geometric model. A numerical result does not confirm that your saw can reach or accurately reproduce the settings.