Compound Miter Calculator

How it works

Crown molding sits at an angle between the wall and the ceiling, so a corner joint is a compound cut: the blade must swing (miter) and tilt (bevel) at the same time. Cutting the molding flat on the saw table is the usual way to make that cut on a compound miter saw, and this calculator gives the two settings.

The spring angle is the angle between the back of the molding and the wall. Common crown is sold as 38/52 or 45/45; the first number is the wall angle and is what you enter here. The corner angle is the angle between the two walls, 90° for a square room; measure it, because real corners are rarely exact.

The results are the sizes of the two settings. For an inside corner, the left-hand piece is cut with the miter swung right and the bevel tilted left, with the top of the molding against the fence; the right-hand piece mirrors it. For an outside corner the miter swings the other way. Make a test cut on scrap before cutting the real piece.

Formula

θ     = corner angle ÷ 2
miter = atan( sin(spring) ÷ tan(θ) )
bevel = asin( cos(spring) × cos(θ) )

Example

For 38/52 crown (spring angle 38°) in a square 90° corner, θ is 45°. The miter is atan(sin 38° ÷ tan 45°) = atan(0.6157) = 31.62° and the bevel is asin(cos 38° × cos 45°) = asin(0.5572) = 33.86°.

For 45/45 crown in the same corner the settings are 35.26° miter and exactly 30.00° bevel, the pair printed on many saw tables.

Assumptions and limitations

• The molding is cut lying flat on the saw table, face down. If you cut it nested upside-down against the fence instead, use a miter of half the corner's supplement (45° for a square corner) and no bevel.
• The walls and ceiling are flat and the molding is installed at exactly its spring angle. Pushing it up or down from that position changes the joint.
• Both results are given as magnitudes. Which way to swing and tilt the saw depends on which piece you are cutting and whether the corner is an inside or outside corner.
• Measure the actual corner angle rather than assuming 90°; out-of-square corners are the most common cause of an open joint.

Frequently asked questions