Crown molding angle calculator

Flat means the crown lies flat on the saw table; nested means it sits upside down against the fence. The result also shows which way round each piece goes, and adding the length of your walls shows how much crown to buy. New to this? Words explained.

What the spring angle is

Crown molding does not sit flat on the wall. It bridges the corner between wall and ceiling, touching each one along a narrow flat on its back, with an empty triangle behind it. The spring angle is the angle between the back of the crown and the wall. Most crown sold in North America is 52/38: its back meets the ceiling at 52° and the wall at 38°, so its spring angle is 38°. The other common profile is 45/45.

If the label does not say, measure it. Set the crown upside down in a framing square, as if the square’s two legs were the wall and the ceiling, with both flats sitting flush. Measure how far down the wall leg it reaches (the drop) and how far along the ceiling leg (the projection). The spring angle is atan(projection ÷ drop): 3 in of projection over 3.84 in of drop is 38°. An angle finder laid along the back gives the same answer.

Two ways to cut crown on a miter saw

• Lying flat. The broad back of the crown lies flat on the saw table and you set both a miter and a bevel. It handles wide crown that will not stand up under the blade, and it is the method the table on this page gives you.
• Nested upside down. The crown leans between the fence and the table exactly as it will sit on the wall, but flipped: the edge that touches the wall goes against the fence, the edge that touches the ceiling rests on the table. Think of the fence as the wall and the table as the ceiling. The blade stays square and you only set a miter — 45° for a 90° corner. Both flats must sit flush; a strip of wood clamped to the table as a stop keeps the crown at its spring angle.

The formulas

With C the wall corner angle and S the spring angle, the flat settings are:

miter = atan( sin S ÷ tan(C ÷ 2) )
bevel = asin( cos S × cos(C ÷ 2) )
nested miter = (180° − C) ÷ 2, bevel 0°

Two checks show why they work. If the crown lay flat against the wall (S = 0), a square corner would need a plain 45° bevel, like baseboard cut lying down. If it lay flat on the ceiling (S = 90°), it would be a picture frame: a 45° miter and no bevel. Real crown sits in between, which is why a 90° corner with 38° crown lands at the familiar 31.6° miter and 33.9° bevel.

Settings for common spring angles

The full reasoning for the two most-searched corners is on Crown molding miter angle for a 90° inside corner (38° crown) and Crown molding angles for a 135° bay window corner.

Walls are rarely exactly 90°

Drywall corners are built up with tape and compound, and framing moves, so a “square” corner often reads 88° to 93°. Measure each corner with an angle finder at ceiling height, where the crown actually goes, and enter the reading as it is — the calculator halves it for you. If you set a nested cut by hand, split the reading yourself: a 92° corner is 44° on each piece. For an outside corner the finder may read over 180° from the room side; enter 360° minus that.

Which piece is which

The left-hand and right-hand results are the pieces as you stand facing the corner. The lying-flat directions follow the chart for a saw whose blade tilts to the left, which is most single-bevel saws; on a dual-bevel saw, tilt the blade to the left to use these settings exactly as written (tilting right mirrors the setup — follow your saw’s manual for that). Inside and outside corners use the same angles — only the edge against the fence, the swing of the table and the end you keep change.

How much crown to buy

Add up the length of every wall that gets crown and add 10–15% for the corner cuts and a test piece or two. Then check wall by wall: a joint in the middle of a wall (a scarf joint, two 45° cuts lapped over each other) is fine but takes time to hide, so a stick at least as long as the wall saves work. A 12 × 14 ft room has 52 ft of wall, about 60 ft of crown with 15% extra.

Tips for tight crown joints

• Cut a short test pair from offcuts and hold it in the real corner before cutting a full length. Write “L inside”, “R outside” and so on on the back of each test piece and keep them as templates.
• Cut pieces a little long, sneak up on the mark, and cut the joint end first.
• Many carpenters cope inside corners instead of mitering them: cut the first piece square into the corner, then cut the second to the profile. A coped joint hides small errors in the wall angle. Outside corners still need a miter.
• The compound math is the same as for a box with sloped sides — see Compound miter angles for a 4-sided planter with 15° sides and the compound miter calculator.