The calculation
A 90° corner uses two 45° saw settings
For an equal-width, symmetric flat joint, the two pieces share the turn at the joint. An inside corner of 90° turns through 90°, so each saw setting is 45°. An inside corner of 120° turns through 60°, so each setting is 30°.
The setting is measured away from a square crosscut. It is not simply half the inside corner angle. Confusing those conventions gives the wrong answer for every corner except 90°.
Regular frames follow the side count
A regular polygon turns through 360° divided by its number of sides at each corner. Each matching cut takes half that turn. A square has four sides and needs 45° miters; a hexagon has six sides and needs 30° miters; an octagon has eight sides and needs 22.5° miters.
The polygon mode assumes equal sides and equal corner angles. It does not solve an irregular room or a frame with unequal angles. Measure the actual geometry when it is not regular.
Miter is not bevel
Miter rotates the cut across the face of flat stock. Bevel tilts the blade. This tool keeps bevel at zero and does not calculate compound cuts, crown molding settings or unequal-width joint geometry.
A valid geometric answer can exceed a saw’s travel. Check your saw’s capabilities and make a test joint in scrap. Use the diagram to verify which included angle you entered.
Questions from the workbench
Why is a 120° corner a 30° saw setting?
The outside turn is 180° − 120° = 60°. Two symmetric pieces share that turn, so each miter is 30° away from a square crosscut.
Does this determine left versus right?
No. It reports the magnitude for each end. Mirror the cuts as the project requires and verify the orientation with a scrap test.
See how we calculate and validate results. All entries are processed in your browser; the calculator does not upload or save them.