Miter angles for frames and boxes
To join pieces of molding or board into a closed shape with equal sides, each end is cut at the same angle so that two cuts together turn the full corner. For a regular polygon with n sides:
Miter saw setting = 180° ÷ n
| Shape | Sides | Saw setting |
|---|---|---|
| Triangle | 3 | 60° |
| Square or rectangle | 4 | 45° |
| Pentagon | 5 | 36° |
| Hexagon | 6 | 30° |
| Octagon | 8 | 22.5° |
| Dodecagon | 12 | 15° |
The setting is the angle read on most miter saw scales, where 0° is a square cut. The angle between the cut and the edge of the piece is 90° minus that setting.
Miter angles for wall corners
For baseboard, chair rail or trim around a wall corner of angle A, each of the two pieces is cut at:
Miter = (180° − A) ÷ 2
A square corner of 90° takes two 45° cuts. A bay window corner of 135° takes two 22.5° cuts. Walls are rarely exactly square, so measure the real angle with an angle finder or a protractor and test the fit on scrap.
Inside length of each piece
With the outside length of each side and the width of the molding, the calculator gives the inside length:
Inside length = outside length − 2 × width × tan(miter angle)
For a square frame, the inside length is simply the outside length minus twice the width.
Example
A hexagonal frame with sides 12 in long outside, made from molding 2 in wide, needs every end cut at 30°. Each piece measures about 9.69 in on its inside edge, and the frame uses 72 in of molding plus waste for the cuts.
Tips
- Cut a test frame from scrap first: an error of half a degree is multiplied by the number of joints.
- For crown molding, which sits at an angle against the wall, you also need a bevel angle; use the crown molding calculator.
- Mark the long and short edges on each piece before cutting.