Miter Angle Calculator

How to Calculate Miter Angles

A miter joint connects two pieces at an angle. For a regular polygon (all sides equal), the miter angle is simply 180 divided by the number of sides. A square frame has 4 sides, so the miter angle is 180 ÷ 4 = 45 degrees.

For non-square corners (like a bay window or custom frame), you can enter the corner angle directly. The miter angle is half the supplement of the corner angle: miter = (180 − corner angle) ÷ 2.

On most miter saws, the angle scale reads from 0° (straight cut) to 45° or beyond. The "miter angle" result from this calculator is the angle to set on your saw fence.

For a frame with N sides, each miter cut angle = 90° − (180°/N), or equivalently 90 × (1 − 2/N). A 4-sided frame: 90° × (1 − 2/4) = 90° × 0.5 = 45°. A 6-sided hexagonal frame: 90° × (1 − 2/6) = 90° × 0.667 = 60°. A 5-sided pentagon: 90° × (1 − 2/5) = 90° × 0.6 = 54°.

Miter saws are set to the miter angle from 90° (perpendicular to blade travel). For a 45° miter (4-sided frame), set the saw to 45°. The two cut ends of the joint sum to 90°. The bevel angle is different — bevel cuts are rotations of the blade toward you or away; miters rotate the fence/table horizontally.

Compound miter (crown molding): requires both a miter angle and a bevel angle simultaneously. The combination depends on crown molding spring angle (angle at which the molding sits against the wall — typically 38° or 45°) and the inside/outside corner angle of the room (usually 90°).

Always make test cuts in scrap wood before committing to final material. Miter joints close gaps when clamped but telegraph any discrepancy once glue dries. For frames, cut all pieces in sequence and test-assemble dry before gluing.

See also: Crown Molding Calculator · Board Foot Calculator