Miter Angle Calculator (Corners, Polygons and Compound Miters)

Set the saw right the first time. This calculator gives the miter setting for a corner of any angle (frames, boxes, trim on out-of-square walls), the miter for any regular polygon from a triangle to a 20-sided planter, and the compound miter and bevel for crown molding laid flat on the saw or for a box whose sides lean in. It also works out the width and perimeter of a polygon from its side length.

Miter angles calculator

How it works

Corner: miter = (180° − corner angle) ÷ 2. A 90° corner needs two 45° cuts; a 135° bay-window corner needs 22.5°.
Regular polygon: miter = 180° ÷ sides; interior angle = 180° × (sides − 2) ÷ sides. Across the flats = side ÷ tan(180° ÷ sides); across the corners = side ÷ sin(180° ÷ sides).
Compound (crown flat, or sloped sides): miter = arctan(sin S ÷ tan(C ÷ 2)) and bevel = arcsin(cos S × cos(C ÷ 2)), where C is the corner angle and S is the crown’s spring angle or the side’s tilt from vertical.

The compound formula reduces to the plain miter when the piece is flat (S = 90°) and to a plain bevel when it stands upright (S = 0°), which is a good way to check it.

Worked example

Common 38/52 crown molding (38° spring angle) meeting at a square 90° inside corner, cut lying flat on the saw:

What are you cutting?: Crown molding cut flat on the saw (compound)
Corner angle: 90 degrees
Crown spring angle: 38 degrees

The calculator gives:

Miter setting: 31.62°
Bevel (blade tilt) setting: 33.86°
Corner angle: 90°
Spring angle: 38°

Miter 31.6° and bevel 33.9°, the same pair printed on most crown charts. For 45/45 crown the settings are 35.3° and 30°. If your corner measures 92° instead of 90°, the numbers shift by about a degree, which is enough to open a joint in a wide molding: measure the corner first.

Notes and limits

On a compound cut the miter direction and which end of the molding is up change between left and right pieces and between inside and outside corners; the angles do not. Cut a test pair from scrap.
Nested crown (upside down, against the fence at its installed angle) needs only a plain miter of half the corner angle and no bevel.
For a sloped box, tilt is measured from vertical: 0° is a straight box, 15° is a typical planter. Some charts measure from horizontal; 15° from vertical is 75° from horizontal.
Polygon side length is the outside length. Cut each side a little long and creep up on the final length with all sides in a strap clamp.
Digital angle finders and a scrap test are cheaper than a second sheet of good plywood.

Frequently asked questions

What angle do I cut for a 45 degree miter?

Set the saw to 45° for each piece of a 90° corner. For any other corner, cut each piece at half the angle between the pieces: 22.5° for a 135° corner.

What is the miter angle for an octagon?

22.5° at each end of each side (180 ÷ 8). A hexagon is 30°, a pentagon 36°, and a 12-sided piece 15°.

What are the miter and bevel for 38 degree crown molding?

For a 90° corner with the crown flat on the saw: miter 31.6°, bevel 33.9°. For 45° spring crown: miter 35.3°, bevel 30°.

What is the difference between miter and bevel?

The miter angle turns the blade left or right across the face of the board; the bevel tilts the blade through its thickness. A compound cut uses both.

Why does my mitered corner have a gap?

Usually the corner is not 90°, the saw is not calibrated, or the pieces are different lengths. Measure the actual corner with an angle finder and enter it here; check the saw against a square.

Sources

NIST Special Publication 811, Guide for the Use of the International System of Units (unit conversion factors)

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.