Find the correct miter saw angle for any polygon, and calculate compound miter and bevel angles for crown moulding.
90°
Each corner of a regular 4-sided shape| Shape | Sides | Interior angle | Miter angle |
|---|---|---|---|
| Triangle | 3 | 60.0° | 60.0° |
| Square | 4 | 90.0° | 45.0° |
| Pentagon | 5 | 108.0° | 36.0° |
| Hexagon | 6 | 120.0° | 30.0° |
| Heptagon | 7 | 128.6° | 25.7° |
| Octagon | 8 | 135.0° | 22.5° |
| Decagon | 10 | 144.0° | 18.0° |
| Dodecagon | 12 | 150.0° | 15.0° |
Flat miter — For any regular polygon, the interior angle is (sides − 2) × 180° / sides. The miter saw is then set to (180° − interior) / 2, which is the rotation from square (90°). For a square frame (90° corners) that's 45°; for an octagon it's 22.5°.
Compound miter (crown moulding) — When a moulding sits at a spring angle S against the wall, both the saw's miter and bevel must be set:
where θ is half the corner angle. Most crown moulding is stamped with its spring angle (commonly 38° or 45°). If not, hold the moulding in the corner and measure the angle between the back face and the wall.