Linear feet, then pieces
Trim is sold in 8, 12 and 16-foot sticks priced by the linear foot. The calculation: <strong>room perimeter minus door openings, × 1.1 for miter waste, ÷ piece length = pieces to buy</strong>. Corner miters eat several inches per corner, and the piece you measure wrong is always the last one.
Crown needs more than the math
Crown molding runs on a compound angle — the spring angle means the wall and ceiling cuts differ, and inside/outside corners flip the miter direction. Cut test pieces from offcuts before touching the good stock. Baseboard and casing are square-cut at the wall, far more forgiving.