Miter Angle Calculator

For flat trim such as baseboard or casing, each piece is cut at half of the turn the wall makes: 45° for a square 90° corner and 22.5° for a 135° bay corner. Crown molding is harder because it leans off the wall; cut lying flat, 52/38 crown in a square corner needs a 31.62° miter and a 33.86° bevel, 45/45 crown a 35.26° miter and a 30° bevel.

How the miter angle is calculated

For trim standing flat against the saw fence:

Miter = (180° − wall angle) ÷ 2

The same number works for inside and outside corners; only the side of the blade you keep changes. A corner that measures 92° needs 44° on each piece, one at 88° needs 46°.

For a closed frame with equal sides, every joint turns 360° ÷ sides and each piece carries half of that:

Frame miter = 180° ÷ number of sides

Picture frames are 45°, hexagons 30°, octagons 22.5°.

Crown molding angles: miter and bevel

Crown touches the wall and the ceiling with two flats on its back and leans out at its spring angle, measured here from the wall. When it lies flat on the saw table, the cut needs both a miter and a bevel:

Miter = atan(sin spring ÷ tan(wall angle ÷ 2))

Bevel = asin(cos spring × cos(wall angle ÷ 2))

These reproduce the numbers printed by saw makers: DeWalt's DWS780 manual gives 31.62° miter and 33.86° bevel for standard US crown, which it describes as 52° against the ceiling and 38° against the wall, and a reversible 30° bevel stop for 45/45 crown. They also match Joe Fusco's crown chart from the Mastering the Miter Saw DVD, published on Gary Katz's site, at every wall angle to within 0.01°.

Miter / bevel, molding flat on the table. Check the spring angle of your molding before you trust a column: hold it in a framing square the way it sits on the wall and read the angle between its back and the wall.

Inside and outside crown corners

With the molding flat, its back on the table and the ceiling edge against the fence, DeWalt's manual sets the saw like this for a square corner:

• Inside corner, left piece: bevel left, miter right, keep the left end. Right piece: bevel right, miter left, keep the right end.
• Outside corner, left piece: bevel right, miter left, keep the left end. Right piece: bevel left, miter right, keep the right end.

The calculator prints the same table for any wall and spring angle. There is also the nested method: stand the crown upside down between fence and table at its spring angle, the way it sits on the wall. Then no bevel is needed and the miter is the flat-trim value, (180° − wall) ÷ 2. DeWalt highly recommends its crown molding fence accessory for that method, to hold the molding at its angle.

Worked example: an octagon frame

A frame with 8 sides from 2 inch molding, each side 10 inches along its inner edge:

• Miter: 180° ÷ 8 = 22.5° on both ends of every piece.
• Each corner of the octagon is 180° − 360° ÷ 8 = 135°.
• Long point: 10 in + 2 × 2 in ÷ tan(67.5°) = 11.66 in, 11 11/16 inches to the nearest sixteenth.
• Eight pieces use about 7.77 feet of molding before saw kerfs.

Getting tight joints

Few rooms are square, so measure each corner with an angle finder and enter the real angle. On inside corners, This Old House recommends coping one baseboard piece to the profile of the other, which it says typically gives the best results; outside corners are cut at 45°, dry-fitted and glued. Make test cuts on scrap: the crown angles above are precise to the hundredth of a degree and hard to set exactly on a saw scale.

What to work out next

• How many lengths of base to buy for the room: the baseboard calculator.
• Casing, crown and chair rail for a whole room: the trim calculator.
• Sealing the gaps at the top of the trim: the caulk calculator.
• Solid wood for a picture or mirror frame: the board foot calculator.

More tools for floors and finish work are under floors, tile and trim.