One side of a standard 32 × 80 inch interior door takes about 16.8 linear feet of casing: two legs of 82 7/16 inches and a head of 36 7/8 inches with 2¼ inch casing and a 3/16 inch reveal. Crown molding runs the full perimeter of the room; chair rail, baseboard and shoe molding run the perimeter minus the doors. The calculator adds all of it up and packs every piece into the lengths you buy.
How the trim is calculated
Casing is measured at the long point of its 45° miters:
Door leg = door height + reveal + casing width
Door head = door width + 2 × (reveal + casing width)
A window cased picture-frame style has four pieces, each the jamb size plus 2 × (reveal + casing width). The reveal is the strip of jamb edge left showing inside the casing; Family Handyman marks it 3/16 inch from the jamb edge, This Old House works with 1/8 to 1/4 inch. Both cut the casing miters at 45°.
For the running trims:
Crown = 2 × (length + width)
Chair rail, baseboard, shoe = 2 × (length + width) − doors × door opening
Crown passes over the doors, so it keeps the whole perimeter. Chair rail, base and shoe stop at the casing; the default door opening of 36 inches covers a 32 inch door plus its casing.
Worked example: a bedroom with one door and two windows
A 12 × 14 ft room, one 32 × 80 inch door cased on both sides, two 36 × 48 inch windows, 2¼ inch casing, 3/16 inch reveal, buying 8, 12 and 16 ft lengths with 10% waste:
- Casing: door, two sides of 16.81 ft, 33.63 ft; windows, 15.63 ft each, 31.25 ft; together 64.9 linear feet. The plan cuts the four window heads and sills from one 16 ft length, a door leg and a window side from each of four 12s, and the two door heads from an 8: seven lengths with the spare.
- Crown: 2 × (14 + 12) = 52 linear feet, two 16s for the long walls and two 12s for the short ones, plus a spare 8: five lengths.
- Shoe molding: 52 − 3 = 49 linear feet, one 16, three 12s and a spare 8: five lengths.
Chair rail and baseboard are off in the defaults; switch them on and they use the same 49 feet as the shoe.
Casing per door and window
| Opening | Casing per side |
|---|---|
| 30 × 80 in door | 16.65 ft |
| 32 × 80 in door | 16.81 ft |
| 36 × 80 in door | 17.15 ft |
| 24 × 36 in window | 11.63 ft |
| 36 × 48 in window | 15.63 ft |
| 36 × 60 in window | 17.63 ft |
With 2¼ inch casing and a 3/16 inch reveal, windows cased on all four sides. Wider casing adds length at every corner: 3½ inch casing adds 5 inches to each door side.
Crown molding, chair rail and shoe
Crown is the trim most likely to need joints, because it runs the full length of every wall. With 16 ft lengths, any wall up to 16 feet is one piece; longer walls get a scarf joint. Its corners are compound cuts; the miter angle calculator gives the miter and bevel for 38° and 45° spring angles at any wall angle.
Chair rail stops at door casing like baseboard. If you are adding panels below it, the wainscoting calculator handles the panels and rails. Shoe molding covers the gap between baseboard and floor; Family Handyman fastens base shoe with 4d finish nails, the main baseboard with 6d.
Buying lengths and waste
Choosing a single length makes the plan simpler to cut; with the mix of 8, 12 and 16 ft the calculator compares plans using each length alone with a mixed plan and keeps the one that buys the least footage. The 10% waste allowance adds spare length for a miter that comes out short, and the calculator buys it as the shortest length that covers it.
What to work out next
- Baseboard with nails, corners and cost: the baseboard calculator.
- Caulking the trim before paint: the caulk calculator.
- Paint for the walls and trim: the paint calculator.
More finish tools are under floors, tile and trim.