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Compound Miter Calculator

Miter and bevel settings for crown moulding and sloped corners.

Your measurementsEvery assumption editable01

The angle between the two walls or faces, measured in plan. 90 for a square corner.

For crown moulding this is 90 minus the spring angle — 52° for standard 38° crown, 45° for 45° crown.

Saw settingsCalculated live02

Result

31.62° / 33.86°miter / bevel

Set the mitre to 31.62° and the blade to 33.86°

Corner anglemeasured in plan between the two faces
90°
Tilt from verticalequivalent to a 38° spring angle
52°
Mitre — saw scalewhat most mitre saws read, measured off square
31.619°
Mitre — from the facethe complement, if your tool reads that way
58.381°
Bevel — blade tiltmeasured off vertical
33.863°
If it were laid flatthe simple mitre with no tilt, for comparison
45°

Two settings, and they are not independent. Tilting a piece changes the mitre it needs as well as adding a bevel, which is why a compound cut cannot be worked out by adjusting a flat mitre — the whole thing has to be solved together.

Mitre saws read off square: a square crosscut is zero and a flat 90° corner mitre is 45. The headline figure uses that convention. The complement is given alongside for protractors and tools that measure from the face instead.

Cut a test piece in scrap before committing. Spring angle varies between crown profiles, walls are rarely the angle they appear, and a compound cut that is a degree out shows badly at the joint.

Two settings that are not independent — tilting a piece changes the mitre it needs as well as adding a bevel. Enter the corner angle and how far the piece tilts from vertical, and this solves both together.

Why use this tool?What it does differently03

Why use this tool?

Both settings at once

Mitre and bevel solved together, because adjusting a flat mitre for tilt does not work — the two interact.

Any wall angle

Not just 90°. Bay windows, octagonal rooms and out-of-square walls all work.

Convention stated

Mitre saws read off square. The complement is given alongside for tools that measure from the face.

Spring angle translated

For crown, tilt is 90 minus the spring angle — 52° for standard 38° crown.

How this worksThe method04

How this compound miter calculator works

The saw's half-angle is 90 less half the corner, not half the corner itself. Those happen to be equal at a 90° corner, which is why crown figures look right either way — but at a 135° octagon corner one gives 22.5° and the other 67.5°, and only one of them cuts.

From that half-angle and the tilt, the mitre is the arctangent of the cosine of the tilt times the tangent of the half-angle, and the bevel is the arcsine of the sine of the tilt times the cosine of the half-angle.

With no tilt the bevel goes to zero and the mitre collapses to the ordinary flat figure, which is a useful sanity check on any compound calculation.

How to use itStep by step05

How to use it

  1. Step 1: Measure the corner

    The actual angle between the walls in plan. Assume nothing — walls are rarely square.

  2. Step 2: Work out the tilt

    For crown, 90 minus the spring angle printed on the profile. For a hopper or splayed box, the angle the sides lean from vertical.

  3. Step 3: Set both

    Mitre on the table, bevel on the blade. Both are needed; either alone will not fit.

  4. Step 4: Test in scrap

    Always. Spring angles vary between profiles and a degree out shows badly at the joint.

Example usageWorked figures06

Example usage

Standard crown, square corner
A 90° corner with 38° spring crown — 52° of tilt — gives a mitre of 31.62° and a bevel of 33.86°. Those are the numbers printed on most crown charts.
45° spring crown
The same corner with 45° spring gives 35.26° mitre and 30° bevel. Spring angle alone moves both settings several degrees.
An octagonal bay
A 135° corner with the same 38° crown gives 14.31° mitre and 46.72° bevel — nothing like the square-corner figures, which is why guessing does not work.
Frequently asked questionsCommon questions07

Frequently asked questions

What are the settings for 38 degree crown molding?

In a square 90° corner, 31.62° mitre and 33.86° bevel. Those are the standard chart figures, and this reproduces them from the geometry rather than from a lookup.

What is spring angle?

The angle between the wall and the back of the moulding when it sits in position. 38° and 45° are the common profiles, and it is usually printed on the moulding or its packaging.

Why can I not just adjust the flat mitre?

Because tilting a piece changes the mitre it requires as well as introducing a bevel. The two settings are coupled and have to be solved together.

My walls are not 90 degrees — does that matter?

Considerably. Enter the actual measured corner. A wall out by three degrees changes both settings enough to leave a visible gap.

Which number goes on the saw table?

The mitre. Saws read it off square, so a square crosscut is zero. The complement is shown in case your tool measures from the face instead.

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