How it works
Enter the room size and openings. Baseboard runs the room perimeter less about 3 feet for each door opening. Casing for a standard door is roughly 17 linear feet per side (two 7 ft legs and a 3 ft head with miters), and a typical window is similar.
Crown molding runs the full perimeter. Long pieces reduce the number of joints on each wall; 16 ft lengths cover most room walls in one piece.
Ten percent waste covers miter cuts at corners, scarf joints and the odd split board. Use 15% for rooms with many outside corners or when cutting crown for the first time.
Formula
Baseboard = 2 × (L + W) − 3 ft per door. Casing = 17 ft per door side + 17 ft per window. Pieces = linear feet × (1 + waste) ÷ piece length.
Questions people ask
How much baseboard for a 12 × 12 room?
The perimeter is 48 ft; less one 3 ft door opening that is 45 ft, or about 50 ft with 10% waste (four 16 ft pieces).
How much casing does a door need?
About 17 linear feet per side for a standard 3 ft × 6 ft 8 in door: two side legs of roughly 7 ft and a head piece of about 3 ft, with extra for miters.
Should I buy longer trim pieces?
Yes when you can transport them. Fewer joints means fewer visible seams and less filling. 16 ft lengths often cost the same per foot as shorter ones.