Miter Angle Calculator

How to use it: for a regular shape (a frame, planter or table with equal sides - like a square, hexagon or octagon), choose Regular shape and enter the number of sides. For an irregular corner - an out-of-square wall, an odd-shaped frame - choose Custom corner angle and enter the actual angle between the two pieces.

Formula. Every two-piece corner has an interior corner angle. The miter angle - the number you set on the saw, measured from a square (0°) cut - is half of what's left after the corner angle: miter angle = (180° − corner angle) / 2. For a regular polygon of n equal sides, the corner angle is (n − 2) × 180° / n, so the two modes agree: a 4-sided frame has a 90° corner and a 45° miter; a hexagon has a 120° corner and a 30° miter.

Worked example. With the default (regular shape, 4 sides): corner angle = (4−2) × 180 / 4 = 90°; miter angle = (180 − 90) / 2 = 45° - the familiar 45° setting for a square picture frame. For an 8-sided shape: corner angle = (8−2) × 180 / 8 = 135°; miter angle = (180 − 135) / 2 = 22.5°.

Rabbet/opening, stock length and the corner guide. Enter a moulding face width (the rabbet-to-outside width of the profile) to draw an actual-size printable corner guide - two moulding pieces meeting at the corner, cut at the miter angle shown, with the square (0°) reference marked for contrast. Add an opening width (and, for a 4-sided frame, an opening height) to also calculate the outer frame size and the stock length to cut for each piece. Each mitered end adds moulding width × tan(miter angle) beyond the inside (rabbet) length - this follows from the same regular-polygon side/apothem relationship used for the miter angle itself, and reduces to the familiar "outer = inner + 2 × width" rule at 45°. A 4-sided frame totals as two width pieces plus two height pieces; any other regular shape totals as sides × the per-piece stock length.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Corner type

Regular shape (equal sides)

Number of sides, or corner angle

4

Units

Inches

Moulding face width (optional)

2

Opening width (optional)

8

Opening height (optional, 4-sided frames only)

10

Primary result: Miter angle: 45.00 deg.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This calculates the miter angle for a flat, single-bevel corner joint only - it does not compute compound miter/bevel pairs for angled (non-vertical) material such as crown moulding run at a spring angle, which needs a second, tilt-plane angle as well.

"Regular shape" mode assumes every side is the same length and every corner is identical, which is true for a regular polygon but not for an arbitrary irregular frame - measure each corner separately with the custom-angle mode if your shape isn't regular.

The stock-length figures assume a constant moulding width all the way around and add no glass/mat clearance, saw kerf or trim allowance - add your own margin before cutting. For a non-4-sided shape, the total assumes every side uses the same opening length; measure and total irregular shapes side by side instead. The corner-guide diagram is drawn to the entered moulding width for a true-size check - always verify the printed calibration mark before trusting it as a cutting reference.