Figure out how much trim you need — baseboard, crown, door and window casing.
Enter room dimensions in feet. Works for baseboard and crown — both run the full perimeter.
Why add waste?
Miter cuts use up the ends of each board. 15% is a safe default for most rooms — bump it to 20% for rooms with lots of corners or angled walls.
Works out the linear feet of baseboard, crown, or casing a room needs, and turns that into the number of boards you actually have to buy.
Baseboard and crown follow the room's perimeter — 2 × (length + width) — with door openings subtracted for baseboard. Casing is calculated per opening from its height and width. The linear footage is then divided by the length of the boards sold at the store and rounded up, with extra added because mitered corners waste material.
Example
A 12 × 14 foot room has a 52-foot perimeter. Take out a 3-foot doorway and you need 49 feet of baseboard — which is five 12-foot boards once you allow for waste at the miters.
Add up the perimeter of the room, subtract the width of each door opening, then add about 10 percent for waste at the miters. Divide by the length of board you are buying and round up.
At least 10 percent, and 15 percent if the room has lots of corners or you have not cut many miters before. Trim from the same batch matches; a board bought three weeks later may not.
Cope them. Mitered inside corners look fine on day one and open into a visible gap as the house expands and contracts. A coped joint absorbs that movement.