How to use the miter and bevel angle calculator
- Choose your job: trim at a corner, a frame or polygon with a set number of sides, or crown molding.
- For a corner, enter the angle between the walls, which is 90 for a square corner. For a polygon, enter the number of sides. For crown molding, enter the wall angle and pick the spring angle printed on the molding.
- Read the miter angle for your saw. Crown molding also shows a bevel angle.
- Make test cuts in scrap first and adjust for walls that are not perfectly square.
Formula
Corner or frame: miter = (180 - corner angle) / 2
Polygon with n sides: miter = 180 / n
Crown on its back: bevel = asin(cos h x cos s)
Crown miter = 90 - acos(cos h x sin s / sqrt(sin^2 s + sin^2 h x cos^2 s)), where h is half the wall angle and s the spring angle
The crown formulas come from the geometry of the cut plane through a corner with the molding resting on its back against the saw fence. They reproduce the familiar settings for a square corner: 31.6 degrees miter and 33.9 degrees bevel for 38 degree spring, and 35.3 and 30 for 45 degree spring.
Worked example
A square 90 degree corner needs (180 - 90) / 2 = 45 degrees on each piece. A 120 degree corner needs (180 - 120) / 2 = 30 degrees.
A hexagon has six sides, so the miter is 180 / 6 = 30 degrees, and each inside angle is 120 degrees. A pentagon is 36 degrees and an octagon is 22.5.
Crown molding with 38 degree spring on a 90 degree wall gives a 31.62 degree miter and 33.86 degree bevel. With 45 degree spring the settings are 35.26 and 30 degrees.
Miter and bevel in plain words
A miter angle is the left or right swing of the saw, measured from a square crosscut. A bevel angle is the tilt of the blade. Plain trim and picture frames need only a miter cut. Crown molding sits tilted between wall and ceiling, so its cut needs both at once, which is called a compound cut.
The two pieces meeting at a corner are cut as mirror images. If the angles are right, the cut faces meet in a clean line with no gap. Walls are rarely exactly 90 degrees, so measure the actual corner with an angle finder and enter the real number.
Practical cutting tips
Careful setup avoids wasted trim.
- Cut a pair of scrap pieces and hold them in the corner before cutting the real stock.
- Cut crown molding a little long and trim to fit. Inside corners are often coped instead of mitered, and that is a reasonable choice when walls are uneven.
- Check the spring angle on the package. The common sizes are 38 degrees (also written 52/38) and 45 degrees (45/45).
- Some saws show angles in different directions on the scale, so confirm the direction on scrap before you cut.
Assumptions and limits
- The corner and polygon results are for pieces cut with the blade square to the face and the board lying flat.
- Crown results assume the molding is cut lying on its back against the fence, with the wall angle measured inside the room.
- Angles are shown in degrees to two decimals, but most saws read to half a degree at best.
- The calculator does not tell you which way to swing the saw. Test on scrap first.
Frequently asked questions
What angle do I set a miter saw to for a 90 degree corner?
Set it to 45 degrees for each piece, swung left for one piece and right for the other. In general, subtract the corner angle from 180 and divide by two.
How do I find the miter angle for a hexagon or an octagon?
Divide 180 by the number of sides. A hexagon is 30 degrees, an octagon is 22.5 degrees and a pentagon is 36 degrees. Both ends of each piece are cut at that angle.
What are the miter and bevel settings for crown molding?
For a square corner with the usual 38 degree spring, about 31.6 degrees miter and 33.9 degrees bevel when the molding lies on its back. For 45 degree spring, about 35.3 degrees miter and 30 degrees bevel. Other corners change the numbers, which this tool works out.
Why do my miters not close up even with the right numbers?
The corner is probably not exactly the angle you assumed. Measure it with an angle finder, enter the real value and test again. Small gaps can also come from a saw that is slightly out of calibration.