How this calculator works
The calculator treats each cased opening as two vertical legs plus one head piece on each casing side. Width and height are converted from inches to feet, multiplied by the number of openings and wall faces, then increased by the waste allowance. Board count is rounded up from total casing length divided by stock board length.
Example: six 30 by 80 inch openings with casing on both sides need about 209.0 linear feet after 10% waste. With 12 ft stock boards, that rounds to 18 boards. A simple mitered casing layout has four miter cuts per wall face, or 48 planned miter cuts for this example.