How to Install Crown Molding
Crown molding is the one trim job where the saw settings are not obvious and not guessable. The molding leans out from the wall, so every corner is a compound cut, and the two numbers you need depend on the profile as much as the room.
Start with the spring angle, because it sets everything else
Crown does not sit flat against the wall. It leans out at a spring angle, which is a property of the profile rather than the room, and on nearly everything sold it is 38 or 45 degrees.
If the stock is not marked, hold a short offcut into the corner the way it will sit, mark where it touches the wall and the ceiling, and measure the two legs. Equal legs is 45 degree stock. A longer wall leg than ceiling leg is 38.
Get this wrong and every cut in the room is wrong by about four degrees, which is a gap you cannot caulk out on a painted ceiling line.
The two settings, and the formula behind them
A compound cut needs a miter setting and a bevel setting, and neither of them is half the corner angle. Both come out of two lines of trigonometry:
miter = arctan( sin(S) / tan(theta / 2) )
bevel = arcsin( cos(S) x cos(theta / 2) )
S is the spring angle of the stock, theta is the measured corner. Worked out for the corners you actually meet:
| Corner measured | 38° stock, miter | 38° stock, bevel | 45° stock, miter | 45° stock, bevel |
|---|---|---|---|---|
| 85° | 33.9 | 35.52 | 37.66 | 31.42 |
| 86° | 33.43 | 35.19 | 37.17 | 31.14 |
| 88° | 32.52 | 34.53 | 36.21 | 30.57 |
| 90° | 31.62 | 33.86 | 35.26 | 30.0 |
| 92° | 30.73 | 33.19 | 34.33 | 29.42 |
| 94° | 29.86 | 32.51 | 33.4 | 28.83 |
| 95° | 29.43 | 32.17 | 32.94 | 28.54 |
Read the square corner row and you have the two numbers most jobs need: 31.62 and 33.86 for 38 degree stock, 35.26 and 30.00 for 45 degree.
The chart everybody quotes has one wrong cell, and it is the one you need
L.S. Starrett publishes a compound cut conversion table with its 505P protractor: 60 rows, four columns, 240 printed values covering both spring angles. We checked every cell against the formulas.
It matches everywhere except one cell, and that cell is the 90 degree corner. For 38 degree crown at a square corner it prints a miter of 31.8 where the geometry gives 31.62. Its own neighbouring rows bracket the correct figure, which is what makes it look like a typographical error rather than a different method.
0.18 of a degree is not a disaster on one cut. It is doubled at every corner, because both halves carry it, and it lands on the most common cut in the room.
The lesson is not that Starrett is unreliable. Their table is right 239 times out of 240, which is better than most published charts manage. The lesson is that a chart is a transcription and the formula is the source.
Measure the corner. It is not 90
Drywall corners are rarely square, and crown sits at the top of the wall where the taping is thickest. Measure each corner near the ceiling with an angle finder rather than assuming.
The sensitivity is easy to state: one degree of corner error is about half a degree of miter error. A corner at 92 rather than 90 moves the 38 degree miter from 31.62 to 30.73, and cutting it at 31.62 leaves a gap that opens at the top or the bottom of the joint depending which way the error went.
This is why two identical rooms take different times. Measuring four corners costs five minutes; recutting four corners costs the afternoon.
Flat or nested, and why the answer changes with the stock
There are two ways to cut crown, and they use different numbers.
Flat on the table uses the miter and bevel pair above. It works at any width, and it is the only option once the crown is too wide to stand against the fence.
Nested means holding the piece upside down against the fence at the angle it will sit on the wall, then mitering at half the corner angle with no bevel: 45 degrees on a square corner. It is faster and there is only one setting to get wrong, but it needs stock narrow enough to nest and a fence tall enough to hold it.
Starrett's manual also fixes the part people get wrong after the angles: which way each piece swings and which edge goes against the fence. For an inside corner it gives, verbatim: "Left Piece: Miter Swing Right, Bevel Swing Left, Work Piece Location Left of Blade, Molding Edge Against Fence: Top." The right piece mirrors it with the bottom edge against the fence.
Cope the inside corners if the walls are bad
A mitered inside corner is two compound cuts meeting. A coped corner is one piece cut square into the corner and the second profiled to sit over it.
The trade preference is not about beauty, it is about tolerance. A miter opens when the corner is out of square and again when the timber shrinks. A cope hides both, because only the profile line shows and it is cut to fit the piece it meets rather than to a number.
Outside corners get mitered either way, so the angles above still matter on every external return.
Buying: perimeter, waste and where the joints land
Quantity is the room perimeter plus an allowance, then divided by the stock length, which is where people under-buy. Crown comes in fixed lengths, commonly 8, 12 and 16 ft, and a wall longer than the stock needs a joint in the middle of the run.
| Wall length | Joints with 8 ft stock | With 12 ft stock | With 16 ft stock |
|---|---|---|---|
| 8 ft | 0 | 0 | 0 |
| 10 ft | 1 | 0 | 0 |
| 12 ft | 1 | 0 | 0 |
| 14 ft | 1 | 1 | 0 |
| 16 ft | 1 | 1 | 0 |
| 20 ft | 2 | 1 | 1 |
| 24 ft | 2 | 1 | 1 |
That joint is a scarf, not a butt: both ends mitered at 45 degrees in the same direction so they overlap. A butt joint opens into a visible line as soon as the timber moves; a scarf stays closed because the overlap slides rather than parting.
So the longest stock the room and the van will take is the right answer. A 16 ft wall in 8 ft lengths has a scarf in it. In 16 ft stock it does not.
The 10% waste allowance on our calculator is ours, not a manufacturer's figure, and the page says so. Corners eat offcuts, and a mis-set bevel eats a whole length.
The order of the day
Cut order matters more than speed. A sensible sequence for a rectangular room:
- Measure every corner, write the number on the wall under it.
- Confirm the spring angle from an offcut.
- Mark the ceiling line by holding a piece in place, so you can see where the crown will actually land and where it will cross a bad ceiling.
- Start on the wall opposite the door, where a joint is least visible.
- Cut one piece at a time and offer it up before cutting its partner.
- Fasten into framing where you can find it, and into the ceiling and wall plates where you cannot.
- Leave scarf joints until last on a long wall, so the two pieces can be adjusted together.
Corners are what the job costs. The linear feet barely move with the shape of the room, but a room with a bay window has fourteen compound cuts against a plain rectangle's eight, and each one is a separate setup on the saw.
What this guide does not cover
It does not price the work. No source we could read cleared the bar: the most detailed figures refuse automated access, and the two articles that are readable give a single project figure or a range with no method behind it. The calculator page explains that in full.
It does not cover built-up crown, lighting coves or curved walls, all of which are different geometry.
And it does not tell you the spring angle of your stock. Measure it, because a chart cannot see what you bought.
Run the numbers
Frequently asked questions
- What angle do you cut crown molding?
- For 38 degree stock in a corner that measures exactly 90, miter 31.62 and bevel 33.86. For 45 degree stock, miter 35.26 and bevel 30.00. Those are settings for cutting the molding flat on the saw table, and they change with the measured corner.
- What is a spring angle?
- The angle at which the crown leans out from the wall when it is installed. It is a property of the profile, normally 38 or 45 degrees, and it is what makes a corner cut compound instead of a simple miter.
- Is the published crown molding angle chart reliable?
- Starrett's 240 value table matches the geometry 239 times. The exception is 38 degree crown at a square corner, where it prints a miter of 31.8 against a true 31.62, with the rows either side correct.
- Why does my crown molding corner not close?
- Usually because the corner is not 90 and the cuts assumed it was. One degree of corner error is about half a degree of miter error, which shows as a gap at the top or the bottom of the joint.
- Can I cut crown without a compound miter saw?
- Yes, by cutting it nested: held upside down against the fence at the angle it will sit on the wall, mitered at half the corner angle with no bevel. It needs stock narrow enough to nest and a tall fence.
- Should I cope or miter inside corners?
- Cope where the walls are out of square, because only the profile line shows and it is cut to fit rather than to a number. Miter where the corners are good and speed matters. Outside corners are mitered either way.
- How do I join two pieces on a long wall?
- With a scarf joint: both ends mitered at 45 degrees in the same direction so they overlap. A butt joint opens into a visible line as the material moves.
- How much crown molding should I buy?
- The room perimeter plus a waste allowance, then divided by the stock length and rounded up per wall. A wall longer than one stock length forces a joint, so longer stock means fewer joints.
Check these numbers yourself
- A Perfect Crown Molding with your Starrett 505 Protractor, user manual PKG08595 UM505APThe L.S. Starrett Company. The 60 row compound cut conversion table for 38 and 45 degree crown, the saw swing and fence orientation for all four pieces at inside and outside corners, and the one cell that disagrees with the geometry. Read 2026-09-09.
- How To Install Crown MoldingWritten by Joseph Truini, This Old House, updated 8 October 2024. Read 2026-09-09. Checked as a price source and not used for prices: it gives a single project figure with no method. Cited here for the installation sequence only.
Figures here last checked against the source documents on 2026-09-16. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Every guide on this site: Guides.