What is a Miter Angle?
A miter angle is the angle at which each piece of material must be cut so that two pieces join together to form a desired corner angle. The miter saw or table saw blade is set to the miter angle to make the cut. For a standard 90-degree corner, each piece is cut at 45 degrees, but for non-standard angles the calculation becomes more complex.
Miter joints are among the most common joints in woodworking, trim carpentry, and picture framing. They create clean, professional corners by hiding the end grain of the material. Understanding miter angles is essential for creating regular polygons like hexagonal planters, octagonal frames, and other multi-sided projects.
Miter Angle Formula
For regular polygons:
Miter Angle = 180° ÷ n
Where n = number of sides.
For crown molding with a spring angle:
Bevel = arcsin(cos(spring) × sin(corner/2))
Common Polygon Angles
| Shape | Sides | Corner Angle | Miter Angle |
|---|---|---|---|
| Equilateral Triangle | 3 | 60° | 30° |
| Square | 4 | 90° | 45° |
| Pentagon | 5 | 108° | 36° |
| Hexagon | 6 | 120° | 30° |
| Octagon | 8 | 135° | 22.5° |
| Decagon | 10 | 144° | 18° |
| Dodecagon | 12 | 150° | 15° |
Crown Molding Angles
Crown molding is installed at an angle (the spring angle) between the wall and ceiling, making miter calculations more complex. The most common spring angles are 38 degrees and 45 degrees. For a 90-degree corner with 38-degree spring angle, the miter is approximately 31.6 degrees and the bevel is approximately 33.9 degrees.
Tips for Accurate Miters
- Measure the actual angle: Walls rarely meet at exactly 90 degrees; use a digital angle finder
- Test cuts: Always make test cuts on scrap material first
- Sharp blade: A dull blade causes tearout and inaccurate cuts
- Support the workpiece: Use a stop block and hold the piece firmly
- Account for the kerf: The saw blade has width that removes material
Worked Example
Building a hexagonal planter (6 sides):
Miter Angle = 120° ÷ 2 = 30°
Set your miter saw to 30° for each cut.
Frequently Asked Questions
What miter angle do I need for a 90-degree corner?
For a standard 90-degree corner, set your miter saw to 45 degrees. Each piece gets a 45-degree cut, and the two 45-degree angles combine to form the 90-degree corner.
How do I cut a miter for an angle that is not 90 degrees?
Measure the actual corner angle with an angle finder, then divide by 2 to get the miter angle. For example, if the corner is 135 degrees, set your saw to 67.5 degrees (or equivalently, 22.5 degrees from perpendicular on most saws).
What is the difference between a miter and a bevel?
A miter cut is angled across the face (width) of the board, while a bevel cut is angled through the thickness. Crown molding often requires both a miter and a bevel cut simultaneously, which is called a compound miter.
Why do my miter joints have gaps?
Gaps usually result from walls not meeting at exactly 90 degrees, inaccurate saw calibration, or the material moving during cutting. Always measure the actual wall angle rather than assuming 90 degrees, and calibrate your saw before cutting.