ToolwickMiter Angle Calculator
Use this miter angle calculator to find the exact saw setting for picture frames, baseboard, trim, planters and crown molding. Enter a corner angle or a number of sides and get the miter and bevel angles, plus the long-point cut length for each board.
Square corner = 90°. Measure real walls with an angle finder; few are exactly 90°.
4 = picture frame, 6 = hexagon, 8 = octagon.
Use the angle between the two walls (under 180°) for inside or outside corners.
Angle between the back of the molding and the wall. Common: 38° (52/38 type) or 45°.
Add width and length to get the long-point length of each mitered board.
Estimates only. By using this calculator you agree to our Terms.
How to Use This Miter Angle Calculator
Choose a tab: Corner angle for a single joint between two boards, Polygon / frame for any shape with equal sides, or Crown molding for compound miter and bevel cuts. Enter your numbers, optionally add the board width and inside length, and press Calculate. You get the saw setting, a breakdown table, and a long-point cut length rounded up so you can trim to a perfect fit.
Why This Matters
A one-degree error sounds trivial, but on a 3½-inch baseboard it opens a gap of about 0.06 inch at the face. Most of the time a miter gap comes from using the wrong number rather than a bad saw. The old "just set it to 45°" habit only works when the corner is exactly 90°, and plenty of real corners are 88° or 92°. In that case each board should be cut at 44° or 46°, and using 45° leaves a gap of roughly ⅛ inch.
Frame builders, trim carpenters, deck and planter builders, and woodworkers making hexagonal or octagonal projects all run into this. A six-sided garden bed needs 30° cuts, an octagonal planter needs 22.5°, and a five-sided shadow box needs 36°. Crown molding is the hardest case because the profile leans away from the wall, so both the saw table (miter) and the blade (bevel) have to be set. Getting the numbers right before you cut saves expensive stock: a single mistaken 12-foot piece of poplar crown can cost more than a good angle finder.
How It's Calculated
All three modes are plain geometry, not a building-code table. The results follow the standard trade rule of thumb: two boards meeting at a corner are each cut at half of the missing angle. No code edition applies to the cut angles themselves, and local requirements vary for anything structural such as stair parts, rafters or handrails. This tool is a layout aid and is not engineering, permit or inspection approval.
Corner angle mode. With W as the angle between the two boards, the saw setting measured from a square (90°) cut is:
Polygon mode. For n equal sides the interior angle is (n − 2) × 180° ÷ n, which gives:
θ = 180° ÷ n
Long-point length. If the board has width w, each mitered end adds w × tan θ to the length. For a board cut at both ends:
The calculator rounds the long point up (to 1/16 in, 1 mm or 1 mm in cm) so a board is never cut short; trim it back during dry fitting.
Crown molding (laid flat on the saw). With wall angle W and spring angle S:
sin(bevel) = cos S × cos(W ÷ 2)
For a 90° corner this reproduces the familiar values: 31.6° miter and 33.9° bevel for 38° crown, and 35.3° miter and 30° bevel for 45° crown.
Common Polygon Miter Angles
| Sides | Shape | Corner angle | Saw setting |
|---|---|---|---|
| 3 | Triangle | 60° | 60° |
| 4 | Square / frame | 90° | 45° |
| 5 | Pentagon | 108° | 36° |
| 6 | Hexagon | 120° | 30° |
| 8 | Octagon | 135° | 22.5° |
| 10 | Decagon | 144° | 18° |
| 12 | Dodecagon | 150° | 15° |
Worked Example
You are building an octagonal cedar planter from boards 3.5 in wide, and each side's inside (short-point) length is 10 in. In the Polygon tab, enter 8 sides, a width of 3.5 and a length of 10 in inches.
- Corner angle: (8 − 2) × 180 ÷ 8 = 135°.
- Saw setting: 180 ÷ 8 = 22.5° on each end of every board.
- Offset per end: 3.5 × tan 22.5° = 3.5 × 0.41421 ≈ 1.45 in.
- Long point: 10 + 2 × 1.4497 ≈ 12.899 in.
- Rounded up to 1/16 in: 12 15/16 in (12.9375 in).
- Total stock: 8 boards × 12.9375 in = 103.5 in before waste.
The tool shows these same numbers in its breakdown table.
Tips & Common Mistakes
- Don't confuse the corner angle with the saw setting. A 135° corner uses a 22.5° saw setting, not 135° or 67.5°.
- Measure the real corner. Use an angle finder, then enter the true angle. If one wall is out of square, split the difference between the two boards.
- Always cut a test joint on scrap. Saw scales and detents drift. Check the dry fit before cutting finished stock.
- Cut long and sneak up. The long-point result is rounded up on purpose; trim in small steps rather than cutting short.
- Verify crown spring angle. Place a piece against a square to measure it; guessing 38° versus 45° gives wrong compound angles.
- Stay safe. Clamp the work, keep hands clear of the blade path and wear eye and ear protection. Settings above about 45–50° usually exceed a saw's range, so use a jig or another method.
Frequently Asked Questions
What angle do I set my miter saw for a 90-degree corner?
Set the saw to 45° on both boards. Two 45° cuts add up to the 90° corner. This only works if the corner really is square, so check it with an angle finder first.
How do I find the miter angle for any corner?
Subtract the corner angle from 180° and divide by two. For a 135° corner that gives (180 − 135) ÷ 2 = 22.5°. Equivalent: 90° minus half the corner angle.
What is the miter angle for an octagon?
22.5° on each end of every side. An octagon has 135° interior corners, and 180° ÷ 8 sides gives the 22.5° saw setting.
What is the difference between miter and bevel angles?
A miter angle is the left or right swing of the saw table, while a bevel angle is the tilt of the blade. Flat trim only needs a miter. Crown molding laid flat on the saw needs both, which is why the Crown tab shows two numbers.
Why do my miter joints still have gaps?
The usual causes are out-of-square walls, a saw scale that is slightly off, or boards that are not held flat against the fence. Measure the actual corner, enter it above, and test on scrap. Small gaps can be closed with glue and a nail set.
Can I use this for structural work or building-code approval?
No. The results are geometry for cutting trim, frames and similar projects. Stair parts, rafters, handrails and other structural members must follow your local code and any engineer or inspector requirements.