How much crown molding do I need?
Start with the perimeter: for a plain rectangular room that's 2 × (length + width), and for anything with bays, alcoves or a bumped-out chimney breast you're better off measuring each wall and adding them up. Unlike baseboard you don't subtract anything, because crown runs unbroken around the ceiling with no doorways to interrupt it. Then add waste and round up to whole stock lengths, since molding is sold in fixed pieces and the offcut from one wall is rarely long enough to be useful on the next.
Be generous with the waste allowance, because crown is the least forgiving trim in the house. It sits on an angle between wall and ceiling, so every inside corner should be coped rather than mitred, and coping is a skill that costs you a few practice pieces before it clicks. Outside corners are compound mitres that have to be cut with the stock held upside down and backwards in the saw — get the orientation wrong and the piece is scrap. Wherever you can, plan each wall to be a single uninterrupted run and buy stock long enough to do it; a scarf joint in the middle of a wall is far more visible on crown than on baseboard because of the shadow line the profile casts. A room with four simple corners is fine at 10%, but a room with several outside corners, or your first ever crown job, deserves 15% or more.
Estimates for planning only. Available stock lengths and prices vary by profile and material (finger-jointed pine, MDF, hardwood, polyurethane) — confirm with your supplier before ordering.