How much molding you need
Crown molding runs along the top of every wall, so the amount you need is the room’s perimeter plus waste for corner cuts:
- Perimeter = 2 × (length + width) for a rectangular room
- Length to buy = perimeter × (1 + waste)
- Pieces = length to buy ÷ piece length, rounded up
Worked example
A 14 × 12 ft room, with 12-foot pieces at $20 each and 15% waste:
- Perimeter: 2 × (14 + 12) = 52 ft.
- With waste: 52 × 1.15 = 59.8 ft.
- Pieces: 59.8 ÷ 12 = 4.98, rounded up to 5 pieces.
- Cost: 5 × $20 = $100.
- The room has 4 inside corners to cope or miter.
Choosing piece lengths
Crown molding is usually sold in 8, 12 and 16-foot lengths. Plan so each wall is covered with as few joints as possible:
- A 14-foot wall is covered by one 16-foot piece, or one 12-foot piece plus a short joint.
- A 12-foot wall fits a 12-foot piece only if the measurement is exact; corner cuts use a few extra inches, so a 16-foot piece is safer.
- Where you need a joint, make a scarf joint: two opposing 45° cuts that overlap and hide the seam better than a square butt joint. Place it over a stud and away from the main line of sight.
Waste allowance
| Situation | Waste |
|---|---|
| Rectangular room, experienced installer | 10% |
| Rectangular room, first project | 15% |
| Many corners, angled walls or a bay window | 20% |
Cutting the corners
- Inside corners are best coped: cut one piece square into the corner, then cut the profile of the second piece with a coping saw so it fits against the first. Coped joints stay tight as walls move with the seasons.
- Outside corners are mitered, with two cuts that meet at the point.
- On a miter saw, cut crown upside down and backward: the edge that touches the ceiling goes against the saw table and the wall edge against the fence. For standard 52/38° crown, compound miter settings of about 31.6° and 33.9° bevel let you cut it flat.
Installation tips
- Mark a level line on the wall at the molding’s bottom edge, since ceilings are rarely flat.
- Nail into studs and ceiling joists or the top plate, or use blocking behind large crowns.
- Fill nail holes, caulk the gaps along the wall and ceiling, and paint.
- Lightweight polyurethane or foam crown can be glued with construction adhesive and is easier for beginners.