How this calculator works
The diagonal joining corresponding inside and outside corners is found from the board widths and corner angle. A saw reads zero for a square cut, so its setting is the complement of the angle between the cut line and the long board edge. Negative settings mean cutting to the opposite side of square.
Formula
alpha = atan2(wA sin(theta), wB + wA cos(theta)); beta = theta - alpha; saw A = 90 degrees - alpha; saw B = 90 degrees - beta.
Worked example
For a 90 degree joint between equal widths, each saw setting is 45 degrees. At 120 degrees, equal boards need 30 degrees on each end.
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Frequently asked questions
Why do unequal boards need different cuts?
The shared seam must connect both pairs of edges, so it no longer bisects the corner.
Is this for crown moulding?
This page models flat boards. Compound mitres need a spring angle and a bevel calculation.
How should I check the setup?
Make a trial joint from offcuts and verify both edges and the corner before cutting finished stock.