Miter Angle Calculator — Flat, Compound & Polygon Cuts
Three cuts, one calculator. A flat miter for any measured corner. The compound miter and bevel for crown molding laid flat on the saw, from its spring angle and the real wall angle — with the nested setting shown next to it, in case holding the crown against the fence is the easier way. And the segment angle for any regular polygon, from a three-sided planter to a twelve-sided table top. Every angle in the diagrams is drawn at its true value.
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Compound miter chart — crown molding cut flat on the table
Miter and bevel for the three standard spring angles, by wall corner. The last column is the single setting you need if you cut the crown nested against the fence at its spring angle instead. Every cell is computed from miter = arctan(sin S ÷ tan(C/2)) and bevel = arcsin(cos S × cos(C/2)), where S is the spring angle and C the wall corner — the same formulas the calculator uses, so the chart cannot drift from the tool.
| Wall corner | 38° spring (miter / bevel) | 45° spring | 52° spring | Nested miter |
|---|---|---|---|---|
| 45° | 56.1° / 46.7° | 59.6° / 40.8° | 62.3° / 34.7° | 67.5° |
| 60° | 46.8° / 43.0° | 50.8° / 37.8° | 53.8° / 32.2° | 60.0° |
| 90° | 31.6° / 33.9° | 35.3° / 30.0° | 38.2° / 25.8° | 45.0° |
| 120° | 19.6° / 23.2° | 22.2° / 20.7° | 24.5° / 17.9° | 30.0° |
| 135° | 14.3° / 17.6° | 16.3° / 15.7° | 18.1° / 13.6° | 22.5° |
| 150° | 9.4° / 11.8° | 10.7° / 10.5° | 11.9° / 9.2° | 15.0° |
- Spring angle is measured between the back of the crown and the wall. A 52/38 crown springs 38° off the wall and meets the ceiling at 52°; the 38° figure is the one these formulas want.
- Bevels past 45° are beyond the stop on most compound miter saws — the 45° corner row is the one that runs into this. Cut those joints nested instead, where the setting is a plain miter and the blade stays vertical.
- A flat (non-sprung) miter for any corner is (180° − corner) ÷ 2, which is the nested column above. Both halves of the joint take the same setting, mirrored: for an inside corner the long point of each cut goes to the back of the board, for an outside corner it goes to the face.
- Regular polygons: saw miter = 180 ÷ sides. Triangle 60°, square 45°, pentagon 36°, hexagon 30°, octagon 22.5°, decagon 18°, twelve sides 15°. Any error is multiplied by the number of joints, which is why the last joint of a many-sided ring is the one that shows a gap.
How to calculate a compound miter for crown molding
- Find the spring angle. Hold the crown against a framing square the way it will hang and read the angle its back makes with the wall leg. 38° (a "52/38" crown) and 45° (a "45/45") cover almost everything sold in North America.
- Measure the actual corner. Use an angle finder on the corner itself. Framed walls are rarely the 90° they were drawn at, and half of any error ends up in each of the two cuts.
- Read the miter and the bevel. Those are the two settings for cutting the crown lying flat on the saw table: swing the table to the miter, tilt the blade to the bevel. A 38° crown in a 90° corner is 31.6° and 33.9°.
- Or cut it nested instead. Held against the fence upside down at its spring angle, the same joint needs only the plain miter — 45° at a 90° corner — and no bevel at all. Use whichever your saw and the width of your stock allow.
Frequently Asked Questions
- What are the miter and bevel angles for crown molding?
- For the common 38° spring crown in a 90° corner, cut flat on the table: miter 31.6°, bevel 33.9°. For a 45° spring crown in the same corner: miter 35.3°, bevel 30.0°. Both numbers change as soon as the wall is not square, which is why measuring the real corner matters more than memorising the pair.
- What is a spring angle?
- The angle between the back of the moulding and the wall when it is installed. A "52/38" crown sits at 38° to the wall and 52° to the ceiling; a "45/45" splits the difference. It is a property of the moulding profile, not of your room, and it is what makes a crown cut compound instead of simple.
- Should I cut crown flat or nested?
- Nested — held against the fence at its spring angle, upside down — needs one setting and no compound saw, and it is faster once you have a stop block. Cut flat needs both a miter and a bevel, but it handles moulding too wide to stand up against the fence, and it is the only option on a saw with a short fence.
- How do I calculate the miter angle for a corner that is not 90 degrees?
- Each piece gives up half the turn: the saw miter is (180° − corner) ÷ 2. A 135° corner is cut at 22.5°, a 60° corner at 60°. Measure the corner with an angle finder rather than assuming, and cut a scrap pair to test before you cut the real moulding.
- What angle do I cut for a hexagon or octagon frame?
- The saw miter for a regular polygon is 180 ÷ number of sides, on both pieces at every joint: 60° for a triangle, 45° for a square, 30° for a hexagon, 22.5° for an octagon, 15° for a twelve-sided ring. That is what makes the cuts add up to a full 360° turn so the last joint closes.
- My compound joint has a gap at the ceiling. What went wrong?
- Usually one of three things: the wall corner is not the angle you assumed, the crown was not held at a consistent spring angle for both cuts, or the two settings were swapped. Cut a short scrap pair, hold them up in the corner, and adjust the corner angle you entered until the test pair closes — then cut the real stock.
Use Cases
- Get crown molding miter and bevel for a wall corner that is not square
- Cut baseboard, casing and chair rail miters for any measured corner
- Work out segment angles for a hexagonal planter or an octagonal frame
- Decide whether to cut crown nested against the fence or flat on the table
- Check a compound miter chart against the real angle of your own walls