Equal flat mitres at a corner

Gives the mitre saw settings for two equal-width pieces meeting in a flat corner at any angle, not just 90°.

θ is the included angle between the two finished runs, in degrees. α is the angle the cut line makes with the long edge of each piece. β is the same cut expressed as a saw setting from square, which is how most mitre saws are marked.

For a tight joint between pieces of equal width, each piece takes exactly half of the corner, so α = θ/2. A square cut sits at 90° to the edge, so the saw setting is the difference: 90° − θ/2.

A 90° corner gives 45° and 45°. A 135° corner gives 67.5° to the edge and 22.5° on the saw.

Watch out: θ is the smaller angle between the runs. Measure it with a bevel or angle finder rather than assuming 90°, and test on scrap.