How this trim calculator works
Baseboard runs the room perimeter minus the openings — a standard interior doorway takes about 3 feet (90 cm) out of the run. The waste allowance covers miters, coping and the offcuts that every corner produces; 10% suits a simple rectangular room, 15% a room with many corners, closets or angles.
Buy long, cut once
Longer sticks mean fewer joints: a 16 ft wall trimmed with one 16 ft piece looks better than two 8 ft pieces spliced mid-wall. Where a splice is unavoidable, use a scarf joint (two 45° cuts overlapping) over a stud, not a butt joint — scarf joints stay closed as the wood moves.
Corners: cope inside, miter outside
Outside corners get mitered at 45 degrees. Inside corners look better coped — one piece runs square into the corner and the other is cut to its profile — because coped joints do not open up when the house moves seasonally. The same math and waste rules apply to quarter round, shoe molding, chair rail and crown, so run this calculator once per trim type.