How to Calculate Cut Angles for Miter Saws

This guide explains how to calculate the correct cut angles for miter saws and table saws when you need to join two pieces of material at a corner. You enter the number of sides of your project (or the corner angle), and the calculator returns the miter angle to set on your saw and the bevel angle if you are cutting a compound joint. The method works for picture frames, baseboards, crown molding, and any polygonal frame.

Formula

For a flat miter joint (two pieces meeting edge-to-edge at a corner), the miter angle is:

Miter angle = 90° – (Corner angle / 2)

Where:

• Corner angle is the interior angle at the corner, measured in degrees. For a square frame, it is 90°; for a pentagon, it is 108°; for a hexagon, it is 120°.
• Miter angle is the angle you set on your miter saw, measured from the square (0°) position. It is the angle you cut on each piece so that two pieces fit together to form the corner.

For a compound miter (used when the material is tilted, like crown molding), the bevel angle is:

Bevel angle = arctan( tan(Corner angle / 2) × cos(Miter angle) )

Where the bevel angle is the tilt of the saw blade. This formula assumes the material is laid flat on the saw table and you tilt the blade. For crown molding, you usually use a different method (see FAQ).

Worked Example

Let’s calculate the miter angle for a standard 4-sided picture frame. The corner angle is 90° (the default in the calculator).

1. Divide the corner angle by 2: 90° / 2 = 45°.
2. Subtract that from 90°: 90° – 45° = 45°.
3. So the miter angle is 45°.

That matches the calculator output: miter = 45.00°.

Now, for a 5-sided frame (pentagon), the corner angle is 108°. The miter angle is 90° – (108°/2) = 90° – 54° = 36°. You would set your saw to 36°.

When to Use This

Use this calculation whenever you are joining two pieces of material at a corner that is not 90°. Common projects include:

• Picture frames (4 sides, 90° corners)
• Hexagonal or octagonal frames (6 or 8 sides)
• Baseboard or trim corners that are not square
• Boxes or trays with angled sides
• Roof trusses or other angled framing (but always double-check with a framing square)

The formula works for any regular polygon (all sides equal) and also for any single corner if you know the angle between the two pieces. For irregular corners, measure the actual corner angle with a protractor or bevel gauge.

Common Mistakes

• Using the outside angle instead of the inside angle. The corner angle is the angle between the two pieces on the inside of the joint. For a square frame, the inside angle is 90°, not 270°.
• Forgetting to subtract from 90°. Some people think the miter angle is simply half the corner angle. That works only for a 90° corner (45° = 90°/2), but for a 120° corner, half is 60°, but the correct miter angle is 30°.
• Mixing degrees and radians. The formula uses degrees. If you use a calculator in radian mode, you will get nonsense. Always set your calculator to degrees.
• Cutting both pieces with the same miter setting but opposite directions. For a frame, you cut one piece with the saw set to 45° left and the other with 45° right (or you flip the piece). If you cut both the same way, they won’t fit.
• Ignoring the thickness of the material. For very thick material, the miter cut may need a slight adjustment because the saw blade’s pivot point is not at the edge. Usually, a test cut on scrap is best.

Tips

• Always make a test cut on scrap wood before cutting your actual pieces. Wood can be slightly warped, and your saw may not be perfectly calibrated.
• Use a digital angle finder or protractor to measure the actual corner angle, especially for walls that are not square.
• When cutting a frame, cut the pieces slightly long (1/16 inch) and then trim them to fit. This avoids gaps.
• For compound angles (like crown molding), the formula above gives the bevel angle, but many saws have a “compound miter” chart. Alternatively, you can use a crown molding jig to hold the molding at the correct angle.
• If your miter saw has a detent at 45°, it may be slightly off. Use a square to check the saw’s accuracy.
• For a perfect fit, sand the cut edges lightly to remove any saw marks.

Troubleshooting

• The pieces don’t fit together. Check your corner angle measurement. If the corner is not exactly 90°, the miter angle must be adjusted. Also, check that you cut both pieces with the saw set to the same angle but in opposite directions.
• There is a gap at the front or back of the joint. This usually means the saw blade is not perpendicular to the table (bevel angle is off). Check the bevel setting with a square.
• The calculated angle seems too small. For a 120° corner, the miter angle is 30°, which looks small. That is correct. The pieces are cut at 30° from the square, so they form a 120° angle when joined.
• My saw only goes to 45°. For a corner angle less than 90°, the miter angle is more than 45°. For example, a 60° corner requires a 60° miter angle. In that case, you need a saw that can tilt beyond 45°, or you can build a jig to hold the piece at an angle.
• The bevel angle formula gives a negative number. That happens when the corner angle is greater than 180° (reflex angle). For standard frames, that doesn’t occur. If you have a reflex corner, you need to cut the opposite angle.

Frequently Asked Questions

How do I find the corner angle for a non-square wall?

Use an angle finder tool or a protractor. Place it in the corner and read the angle. For a regular polygon, use the formula: interior angle = (n-2) × 180° / n, where n is the number of sides. For a pentagon, (5-2)×180/5 = 108°.

Does this formula work for crown molding?

This formula works for flat material. Crown molding is tilted at a spring angle, so you need a different calculation that accounts for the spring angle. Many saws have a crown molding chart, or you can use a dedicated crown molding calculator.

What is the difference between miter angle and bevel angle?

The miter angle is the horizontal rotation of the saw blade (left/right). The bevel angle is the vertical tilt of the blade (up/down). For a flat miter joint, you only need the miter angle. For a compound joint, you need both.

Can I use this for cutting baseboards at a corner?

Yes, if the baseboard is flat against the wall. Measure the corner angle (usually 90° but often not exactly), then calculate the miter angle. For inside corners, you often use a coping cut instead of a miter, but the miter calculation still helps for the initial cut.

Why do I get a different answer if I use the outside angle?

The outside angle is 360° minus the inside angle. For a square, the outside angle is 270°, which would give a miter angle of 90° – (270/2) = -45°, which is the same as 45° but in the opposite direction. For other angles, it gives a different (wrong) result. Always use the inside angle.

How accurate does my saw need to be?

For most projects, an accuracy of ±0.5° is fine. For fine furniture, you may need ±0.1°. Check your saw’s calibration with a digital angle gauge. Even a small error can cause gaps in a frame.

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in sides | Number of sides | - | 4 | 3 | 100
in corner | Corner angle | deg | 90 | 0 | 360
out miter | Miter angle per piece | deg | 90 - corner / 2 | 2
out bevel | Bevel angle (for compound) | deg | atand(tand(corner / 2) * cosd(miter)) | 2