The conventional saw scale measures rotation from a square cut. Setting = |180° − included corner angle| / 2. A 90° corner needs 45°, a 135° corner 22.5°, a 92° corner 44°, and a straight 180° joint 0°.
Why the miter angle is always half the corner
The conventional saw scale measures rotation from a square cut. Setting = |180° − included corner angle| / 2. A 90° corner needs 45°, a 135° corner 22.5°, a 92° corner 44°, and a straight 180° joint 0°.
Inside vs. outside corners
The miter angle number is identical for inside and outside corners at the same measured angle — a 90° inside corner and a 90° outside corner both take a 45° miter. What changes is orientation: an inside corner's long point (the tip of the angled cut) sits on the back, wall-facing edge of the board, while an outside corner's long point sits on the front, room-facing edge. Cutting the correct orientation for the wrong corner type is one of the most common trim-carpentry mistakes, even when the angle itself is right.
Measuring real-world corners accurately
Walls settle, drywall corners aren't always taped perfectly square, and older homes especially can be off by a degree or more. A digital angle finder or a simple protractor against the wall gives a far more reliable reading than assuming 90°. The Out-of-Square Walls tab shows how a degree or two of measurement error shifts the resulting miter angle, so you can judge whether your measurement precision is good enough for a clean-looking joint.
Limits and edge cases
This calculator handles the simple, single-plane miter — two flat pieces of trim meeting at a corner angle in one plane. It does not calculate the compound miter and bevel angles needed for crown molding, which sits at a spring angle against both the wall and ceiling and requires a second angle (see the Compound Miter Calculator for that case). It also assumes the trim profile itself is symmetric; asymmetric or hand-shaped moldings may need a test cut to confirm the fit.