About the Compound Miter Calculator
This compound miter calculator gives the two saw settings — the miter (table) angle and the blade bevel (tilt) angle — needed for joints that are angled in two directions at once. Use crown molding mode for crown laid flat on a compound miter saw at any wall corner angle and spring angle, or box mode for frames whose sides lean outward, such as planters, hoppers, splayed-leg stools, lamp shades and shadow boxes with any number of sides.
It is meant for finish carpenters, woodworkers and DIYers who need corners other than the standard 90°, or crown with a spring angle the saw has no detent for. In crown mode it also shows the single miter angle for cutting crown “nested” upside down against the fence, which needs no bevel at all.
Angles are given as most miter saws read them, with 0° as a square cut. Cut a test joint in scrap first: real walls are rarely exactly 90°, and a fraction of a degree matters on wide molding. Measure an actual corner with an angle finder and enter it for the best fit.
With the default inputs, the miter (table) angle is 31.62 °. Change any value above to recalculate instantly.
How to use the compound miter calculator
- 1Choose crown molding or sloped-side box.
- 2For crown, enter the spring angle and measured wall corner angle.
- 3For a box, enter the number of sides and how far the sides lean from vertical.
- 4Set the saw’s miter table and blade bevel to the angles shown.
- 5Cut a test joint in scrap and fine-tune before cutting the real pieces.
Formula and method
A compound joint is the intersection of two inclined boards, so the cut has to be resolved into a rotation of the saw table (miter) and a tilt of the blade (bevel). For a box with N sides, each corner turns 360°/N, so the half-joint angle φ is 180°/N; the sides lean out by T from vertical. With vertical sides (T = 0) the formula gives a pure 45° bevel for a square box, and with flat sides (T = 90°) a pure 45° miter — the two familiar extremes.
Crown molding sitting against a wall is the same geometry: its spring angle (between the back of the molding and the wall) plays the role of the tilt, and the half-joint angle is 90° − C/2 for a wall corner of C degrees. For 52/38 crown in a 90° corner this gives the well-known 31.62° miter and 33.86° bevel; for 45/45 crown, 35.26° and 30.00°. Nested crown needs only a miter of 90° − C/2.
- T
- Side tilt from vertical (box) or crown spring angle
- φ
- Half the joint angle in plan: 180° ÷ N or 90° − C/2
- N
- Number of sides of the box
- C
- Wall corner angle for crown molding
Worked examples
52/38 crown in a 90° corner
With a 38° spring angle and a 90° corner, φ = 45°. Bevel = arcsin(cos 38° × sin 45°) = arcsin(0.5572) = 33.86°, and miter = arctan(sin 38° × tan 45°) = arctan(0.6157) = 31.62°.
45/45 crown around a 135° bay window corner
A 135° corner gives φ = 90 − 67.5 = 22.5°. Bevel = arcsin(cos 45° × sin 22.5°) = arcsin(0.2706) = 15.70°, and miter = arctan(sin 45° × tan 22.5°) = arctan(0.2929) = 16.32°. Nested, the same crown needs a plain 22.5° miter.
Four-sided planter with sides tilted 30°
A square box has φ = 45°. Bevel = arcsin(cos 30° × sin 45°) = arcsin(0.6124) = 37.76°, and miter = arctan(sin 30° × tan 45°) = arctan(0.5) = 26.57°.
Six-sided lamp shade with sides tilted 20°
A hexagon has φ = 30°. Bevel = arcsin(cos 20° × 0.5) = arcsin(0.4698) = 28.02°, and miter = arctan(sin 20° × tan 30°) = arctan(0.1975) = 11.17°.
Frequently asked questions
What are the miter and bevel settings for crown molding?+
For standard 52/38 crown laid flat in a 90° corner, set the miter to 31.62° and the bevel to 33.86°. For 45/45 crown, use a 35.26° miter and a 30° bevel. Most compound miter saws have detents at these angles.
What is the spring angle of crown molding?+
The spring angle is the angle between the back of the crown and the wall when it is installed. Common profiles are 52/38 (38° to the wall, 52° to the ceiling) and 45/45. Hold the molding in a framing square to check yours.
Do I need a compound cut for crown molding?+
Not if you cut it nested: place the crown upside down with its flats against the saw fence and table, then cut a simple 45° miter for a 90° corner. The compound setting is needed only when the crown is laid flat.
How do I cut a planter box with angled sides?+
Enter the number of sides and the side tilt from vertical. For a four-sided box tilted 15°, set the miter to 14.51° and the bevel to 43.08° and cut both ends of each side, flipping the board between cuts.
What if my wall corner is not exactly 90 degrees?+
Measure the corner with an angle finder or a protractor and enter the actual value. For example, a 92° inside corner with 52/38 crown needs about a 30.73° miter and 33.19° bevel instead of 31.62° and 33.86°.