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Crown Molding Miter Calculator

Computes the saw settings for cutting crown molding flat on a compound miter saw from the wall corner angle and the molding's spring angle: miter = atan(sin(spring) / tan(corner/2)) and bevel = asin(cos(spring) × cos(corner/2)), plus the miter setting for cutting the molding nested (upside-down against the fence) and which end of each piece to keep.

When to use

You need miter and bevel settings for inside or outside crown molding corners, including non-square corners such as 135° bay-window walls.

Do not use when: The trim is flat stock such as baseboard or casing, where the miter is simply half the corner angle (45° for a square corner; use baseboard-trim for quantities), or you need general angle trigonometry (use trigonometry).

Formula

miter = atan(sin(spring_angle) / tan(corner_angle/2)); bevel = asin(cos(spring_angle) × cos(corner_angle/2)); nested miter = 90° − corner_angle/2 (bevel 0°); ceiling angle = 90° − spring_angle.

Standard compound-miter formulas for a molding lying flat on the saw table; for a 90° corner and a 38° spring angle they give the familiar 31.62° miter and 33.86° bevel, and 35.26° / 30.00° for 45° crown. The corner-piece instructions follow the usual saw-manual chart for a left-tilting saw; cut and test a short offcut pair before cutting to length.

Inputs

Parameter	Type	Unit	Required	Description
`corner_angle_degrees`	number	°	default 90	Angle between the two walls measured inside the room with an angle finder: 90 for a square corner, 135 for a typical bay-window corner. Drywall corners are rarely exactly 90°. Range: ≥ 60, ≤ 180
`spring_angle_degrees`	number	°	default 38	Angle between the back of the molding and the wall when installed: 38 for the common 52/38 profile, 45 for 45/45 crown, 52 for 38/52 crown (the number printed on the profile is sometimes the ceiling angle, so check with a square). Range: ≥ 0, ≤ 90
`corner_type`	enum: inside \| outside		default inside	Inside (concave) or outside (convex) corner; the angles are identical, only the fence side and the end to keep differ.

Outputs

Output	Type	Unit	Description
`miter_angle_degrees`	number	°	Saw-table (miter) setting for the molding lying flat: atan(sin(spring) / tan(corner/2)).
`bevel_angle_degrees`	number	°	Blade-tilt (bevel) setting for the molding lying flat: asin(cos(spring) × cos(corner/2)).
`nested_cut_angle_degrees`	number	°	Miter setting with the bevel at 0° when the molding is cut upside-down and nested against the fence at its spring angle: 90° − corner/2.
`ceiling_angle_degrees`	number	°	90° − spring angle: the angle between the molding and the ceiling.
`cutting_instructions`	string		Fence side, miter direction and which side of the blade to keep for the left and right pieces (left-tilting single-bevel saw).

Example

90° inside corner, 52/38 crown (38° spring): {"corner_angle_degrees":90,"spring_angle_degrees":38,"corner_type":"inside"} → {"miter_angle_degrees":31.62,"bevel_angle_degrees":33.86,"nested_cut_angle_degrees":45,"ceiling_angle_degrees":52}

135° outside corner (bay window), 45° crown: {"corner_angle_degrees":135,"spring_angle_degrees":45,"corner_type":"outside"} → {"miter_angle_degrees":16.32,"bevel_angle_degrees":15.7,"nested_cut_angle_degrees":22.5}

GET https://tttkmbb.com/api/v1/calculate/crown-molding-miter?corner_angle_degrees=90&spring_angle_degrees=38&corner_type=inside

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/crown-molding-miter (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/crown-molding-miter · Markdown: https://tttkmbb.com/construction/crown-molding-miter.md · JSON definition: https://tttkmbb.com/construction/crown-molding-miter.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="crown-molding-miter"
OpenAPI operationId: calculate_crown_molding_angles
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Crown molding (reference)
Wikipedia – Miter saw (compound miter saws) (reference)

FAQ

Which is the spring angle on 52/38 crown?

The angle between the molding's back and the wall, 38° for the common 52/38 profile (52° is the angle to the ceiling). Hold the piece against a framing square: the wall-side angle is the spring angle.

Flat or nested: which method should I use?

Nested (upside-down, at the spring angle against the fence) needs only a miter setting of 90° − corner/2 but requires the molding to fit under the fence; wide crown is cut flat with the compound settings given here.

My saw tilts right, not left.

Swap left and right in the instructions (mirror the fence side and the piece to keep); the miter and bevel values are unchanged.

Related calculators

Baseboard Trim Calculator — Quantity of baseboard or flat trim for the same room.
Trigonometry Calculator — Evaluate the sine, cosine and tangent used by the formulas.
Angle Converter — Convert between degrees and radians.