How to use the crown molding angle calculator
Choose the spring angle of your crown, enter the angle between the two walls, and pick an inside or outside corner. Then pick the left or right piece, as you face the corner. The calculator gives the miter and bevel for cutting the crown flat on the saw table, or the single miter angle if you nest the crown against the fence. The saw view shows the table swing, the blade tilt and the keeper side.
The calculator works in imperial or metric. Use the Imperial | Metric switch at the top: in imperial you type feet and inches and the cut sheets read to the nearest 1/16 inch; in metric you type millimeters (or meters or centimeters) and the cut sheets give every board in millimeters, like 3,864 mm, the way metric trim carpenters mark a cut. A mark or unit you type always wins, so 3.2 m works in imperial and 10'6" works in metric. An inch is exactly 25.4 mm and a foot exactly 0.3048 m. Angles are degrees in both, so the miter and bevel never change with the units.
What the spring angle means
The spring angle is the angle between the back of the crown and the wall. A crown sold as 52/38 has a 38° spring angle, so its back makes 38° with the wall and 52° with the ceiling. A 45/45 crown makes 45° with both. If your profile is not marked, hold a piece in its installed position against a square corner and measure how far it reaches down the wall and out along the ceiling. The spring angle is the arctangent of the ceiling reach divided by the wall reach, and equal reach means 45°.
The math: a compound miter
A crown joint is cut in the vertical plane that splits the corner in half. Because the crown leans, that plane meets its back at a slant, so the saw needs a miter and a bevel at the same time. With spring angle S and wall angle W, where a square corner is 90°:
bevel = arcsin( cos S × cos(W ÷ 2) )
Here is a worked example. For a 38° crown in a square corner, sin 38° is 0.6157 and tan 45° is 1, so the miter is arctan(0.6157) = 31.6°. Next, cos 38° is 0.7880 and cos 45° is 0.7071, so the bevel is arcsin(0.5572) = 33.9°. A 45° crown in the same corner gives 35.3° and 30.0°. Both pairs match the settings printed in saw manuals.
| Angle between walls | 38° crown, flat (miter / bevel) | 45° crown, flat (miter / bevel) | Nested miter |
|---|---|---|---|
| 90° | 31.6° / 33.9° | 35.3° / 30.0° | 45.0° |
| 120° | 19.6° / 23.2° | 22.2° / 20.7° | 30.0° |
| 135° | 14.3° / 17.6° | 16.3° / 15.7° | 22.5° |
| 150° | 9.4° / 11.8° | 10.7° / 10.5° | 15.0° |
Flat or nested
Cutting flat means the crown lies on its back, face up, and you set both the miter and the bevel. It works on any compound saw and is steady for wide crown, but it depends on both scales being accurate. Nested means the crown stands upside down with its wall edge against the fence and its ceiling edge on the table, at its spring angle. The blade stays upright and only the miter changes, to 90° minus half the wall angle. That is 45° in a square corner. Nested needs a fence tall enough for the reach down the wall, shown in the results, and a crown stop or clamped strip so the crown cannot slip off its angle.
Which end is the keeper
Left and right always mean as you face the corner. A board that runs between two inside corners has its left end cut as the right piece of the left-hand corner, and its right end cut as the left piece of the right-hand corner. The keeper is the side of the blade that becomes the finished piece, and the other side is waste. In an inside corner the wall edge is the long point. In an outside corner the ceiling edge is the long point. Measure and mark your length at the long point, not at the middle of the cut.
Where the numbers come from
- The formulas. They come from plain trigonometry: the cut plane is the vertical plane that bisects the corner, expressed in the crown's own axes.
- Checked against published settings. We compared the calculator with the printed settings in saw manuals (31.6° and 33.9° for a 38° crown, 35.3° and 30.0° for a 45° crown) and with a published chart for corners from 86° to 93°. They agree to within a hundredth of a degree on the chart.
- Left, right and keeper. These follow the published cutting instructions from saw and tool makers for 90° corners, and the calculator extends them to any angle.
- No building code applies. Crown molding is trim, so no code sets its angles.
Other compound miters
A box, hopper or planter with leaning sides uses the same formulas. Replace the spring angle with how far each side leans from vertical, and replace the wall angle with the angle between sides seen from above, which is 180° minus 360° divided by the number of sides. Four sides leaning 15° from vertical give a miter of 14.5° and a bevel of 43.1° with the board lying face up on the table.
Tips for tight corners
- Measure each corner with an angle finder. Corners are rarely exactly 90°, and a 1° error leaves a visible gap.
- Check each measurement as you go. Cuts are only as accurate as your measurements, and if one end is 1/16" (about 1.5 mm) off, the other end needs an adjustment to look right.
- Mark the long point first, cut a little long, and trim to fit.
- Cut a test corner from scrap in the same orientation as the real piece.
- Many installers cope inside corners instead of mitering them, because coped joints stay tight if the walls move.
Questions
Can I use metric?
Yes. Pick Metric at the top and type your walls in millimeters, centimeters or meters (choose which under "Measure in"), and the crown size in millimeters. Board lengths come out in millimeters (3,864 mm), and the marks and the 3D walk-through read in metric. The miter and bevel are angles, so they are the same in both systems. The page remembers your choice.
My saw only tilts one way. Can I still use these settings?
Yes. Choose left only or right only under "My saw tilts" and the calculator picks the edge to put against the fence so the bevel goes the direction your saw can tilt.
Why is my corner not 90°?
Framing and drywall are rarely exact. Measure the corner with an angle finder and enter that number, then check the fit with a scrap corner.
Do I need a compound miter saw?
Flat cutting needs a bevel, so it needs a compound saw. A plain miter saw can still cut nested crown, because that method needs only the miter.
What if my crown has an unusual spring angle?
Use the measuring helper under the inputs, or enter the angle under Other. The formulas work for any spring angle from 1° to 89°.