Compound Miter Calculator

1. Enter the corner angle in degrees. Standard rooms are 90 degrees. Bay windows are often 135 degrees.
2. Select the spring angle of your crown molding. Most stock crown is 38 degrees. Check the packaging or measure with a protractor.
3. Read the miter angle (the saw swing on the turntable) and the bevel angle (the blade tilt).
4. Set your compound miter saw to both angles and make a test cut on scrap before cutting finish stock.

Here is what each result line means:

For the amount of crown molding you need, see the crown molding calculator.

A compound miter is a cut that angles the workpiece in two planes at once: the miter (horizontal swing) and the bevel (blade tilt). It is used whenever a molding or board must wrap around a corner while sitting at a tilt, which is the case with crown molding, cove molding, and angled trim.

The “flat method” lays the molding flat on the saw table and uses both the miter and bevel adjustments to produce the correct joint. The alternative “nested method” holds the molding tilted against the fence and uses only a miter cut, but that requires a tall fence and careful alignment.

Compound miter cuts are standard in finish carpentry and are needed for inside corners, outside corners, and non-square corners such as bay windows and angled walls.

1. Compute K, the complement of the spring angle: K = 90 – spring angle.
2. Compute the miter angle: arctan(cos(K) / tan(corner / 2)).
3. Compute the bevel angle: arcsin(sin(K) x sin(corner / 2)).
4. Convert from radians to degrees for the saw dial readings.

These formulas assume the molding is laid flat on the saw table (the flat method). The miter angle controls the turntable swing and the bevel angle controls the blade tilt.

You are installing 38-degree spring crown molding in a room with standard 90-degree corners.

Step 1. K = 90 – 38 = 52 degrees.

Step 2. Half corner = 90 / 2 = 45 degrees.

Step 3. Miter = arctan(cos(52) / tan(45)) = arctan(0.6157 / 1.0) = arctan(0.6157) = 31.6 degrees.

Step 4. Bevel = arcsin(sin(52) x sin(45)) = arcsin(0.7880 x 0.7071) = arcsin(0.5572) = 33.9 degrees.

Set the saw turntable to 31.6 degrees and tilt the blade to 33.9 degrees. Make a test cut on scrap before cutting crown.

Corner Angle

Standard rooms have 90-degree corners, but bay windows (typically 135 degrees), angled walls, and non-standard construction produce different corner angles. Measure with a digital angle finder for accuracy, because even a 1-degree error shows as a visible gap in crown molding.

Spring Angle

The spring angle is the tilt of the crown molding away from the wall. Stock crown molding is usually 38 degrees (often written as 52/38, meaning 52 degrees from the ceiling and 38 degrees from the wall). Some moldings use 45/45. Check the molding packaging or lay a piece against the wall with a protractor.

Molding Profile

The compound miter formula works for any spring angle, but the physical fit of the joint depends on the profile shape. Asymmetric profiles can look different when flipped, so mark each piece with its orientation before cutting.

Saw Accuracy

Consumer miter saws have detents that may not land exactly on the calculated angle. Use the fine adjustment on your saw and verify with a digital angle gauge. A 0.5-degree error in the bevel setting is more visible than the same error in the miter setting.

Inside vs. Outside Corners

The same miter and bevel values apply to both inside and outside corners. The difference is which direction you swing the saw. Inside left uses the same setup as outside right, and inside right uses the same setup as outside left.

Miter and bevel settings for the three most common spring angles at selected corner angles. All values in degrees.

These values assume the flat cutting method. Always verify with a test cut on scrap.

Crown Molding Installation

Crown molding is the most common use. Every inside and outside corner needs compound cuts unless you use the nested method with the molding held against the fence.

Cove Molding

Cove molding sits at a spring angle like crown and needs the same compound settings.

Angled Trim Around Bay Windows

Bay window corners are not 90 degrees. A 135-degree or 120-degree corner with crown molding requires different miter and bevel settings from a standard room corner.

Cathedral or Vaulted Ceilings

When the ceiling meets the wall at an angle other than 90 degrees, the effective corner angle changes and compound miter values must be recalculated.

1. Using the Wrong Spring Angle

Assuming all crown is 38 degrees when your molding may be 45 degrees changes the bevel by several degrees and causes visible gaps. Always verify the spring angle before cutting.

2. Skipping the Test Cut

Calculated angles are precise but saws are not. A test cut on scrap reveals small calibration errors before you waste finish material.

3. Confusing Miter and Bevel

The miter is the turntable swing and the bevel is the blade tilt. Setting the miter dial to the bevel value (or the reverse) produces a joint that does not close.

4. Measuring the Corner Angle Incorrectly

A drywall corner can be 89 or 91 degrees instead of exactly 90. Use a digital angle finder for the actual reading, especially on older walls.

5. Forgetting Inside vs. Outside Orientation

The same two angles apply to both inside and outside corners, but the saw swings in the opposite direction. Cutting the wrong direction wastes the piece.

The calculator uses exact trigonometric formulas for a tilted flat workpiece meeting at a given corner angle. Results are as precise as the inputs.

Set the miter to 31.6 degrees and the bevel to 33.9 degrees. This is the most common setting for stock crown molding in standard rooms.

The spring angle is the angle the crown molding makes with the wall. Most stock crown molding has a 38-degree spring angle, written as 52/38 (52 degrees from the ceiling, 38 from the wall).

No. The miter and bevel values are the same for both. The difference is the direction you swing the saw. Inside left matches outside right, and inside right matches outside left.

Yes. Cove molding sits at a spring angle just like crown. Measure the spring angle of your cove profile and enter it along with the corner angle.

Measure the actual corner angle with a digital angle finder and enter it. Even a 1-degree difference changes the miter and bevel values enough to show a visible gap.

The flat method lays the molding flat on the saw table and uses both miter and bevel settings. The nested method holds the molding tilted against the fence and uses only a miter cut. This calculator covers the flat method.

A steeper spring angle changes how much the molding tilts away from the wall, which shifts the geometry of the compound cut. The formulas use the sine and cosine of K (90 minus the spring angle) to compute both settings.

Bay windows commonly meet at 135 degrees (for a three-panel bay) or 120 degrees (for an angled bay). Measure your bay corner before cutting.

The bevel setting is more critical than the miter. A 0.5-degree error in the bevel creates a larger gap at the joint than the same error in the miter, because the blade tilt affects the cross-section fit.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Nootan Crafts, Compound Miter Calculator (compound miter and bevel formulas for flat-method crown molding cutting, with spring angle presets).
• Wikipedia, Crown Molding (spring angle definition, 52/38 and 45/45 standard angles, installation methods).
• Calculate.co.nz, Angle Cut Calculator (miter and compound angle calculations for woodworking joints).