Practical tools for measured work
Polygon & Bay-Window Miter Set Calculator
Find matching miter-saw settings and trim long-point offsets for polygon corners or measured bay-window turns.
Bay: finished inside angle in degrees. Polygon: whole number of sides.
Used to calculate the long-point offset.
Mark the same setting on both mating pieces, with opposite left/right cuts. Keep the chosen long-point or short-point convention consistent around the layout.
How it works
How to use it: choose a regular polygon and enter its side count, or choose a bay and enter the finished inside angle between adjacent trim runs. Enter the trim's face width to get the distance between its short and long points.
Formula. Direction change = 180 degrees minus the finished inside angle. An equal miter splits that change between the two pieces, so each saw setting is half the direction change. The long-point offset is face width × tan(saw setting).
Worked example. A common 135-degree bay joint changes direction by 45 degrees. Each mating end is cut at 22.5 degrees. Across 70 mm trim, the long point is 29.0 mm beyond the short point.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Layout type
- Bay / measured corner
- Included angle or side count
- 135
- Trim face width
- 70 mm
Primary result: Saw setting - each piece: 22.50 deg.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is for flat, equal-split miters with the material lying flat. It does not calculate compound bevels, unequal miter splits, or compensate for wall bows, saw calibration, kerf, or out-of-plumb jambs.
Measure the angle where the trim will sit, make a test pair from scrap, and fit the joint before cutting finished stock. Bay-window naming varies, so use the finished inside angle shown in the diagram - not the exterior turn angle.
Frequently asked questions
Is a 135-degree bay cut at 67.5 degrees?
No. On a saw whose zero is a square crosscut, set 22.5 degrees on each piece. A protractor may describe the cut line as 67.5 degrees relative to the board edge; these are complementary references.
Can I use this for an octagonal frame?
Yes. Choose regular polygon and enter 8 sides. The tool returns a 22.5-degree setting for every mating end.
What does long-point offset mean?
It is the along-the-board distance between the acute long point and short point of the angled cut, useful when transferring layout marks across wide casing.
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