How this calculator works
Each cut creates a right triangle whose opposite side is the plank width. Subtract both longitudinal setbacks from the long edge to find the short edge. This is an inward-cut trapezoid, not a compound bevel or a three-dimensional knee joint.
Formula
left setback = t cot(alpha); right setback = t cot(beta); short edge = long edge - left setback - right setback; cut face = t / sin(angle).
Worked example
A 1 m long plank 100 mm wide with two 45 degree cuts loses 100 mm at each end of the short edge, leaving 800 mm. Each cut face is 141.421 mm long.
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Frequently asked questions
Is the angle a saw setting?
It is the cut angle to the long edge. Subtract it from 90 degrees to get a saw setting measured from square.
Does plank width mean material thickness?
Use the dimension across the plane of this cut. For a face mitre it is board width; for an edge bevel it is board thickness.
Does this add a kerf allowance?
No. The lengths are finished dimensions. Position the blade on the waste side and allow for kerf separately.