Door and Window Casing Calculator

How it works

Casing frames an opening a small reveal back from the jamb edge. Family Handyman's window-trim guide (Ryan Van Bibber, Aug. 8, 2024) sets that reveal at 3/16 in, the default here. Each leg runs the height of the opening plus the reveal at the head; the head runs the width plus a reveal and a casing width on each side.

With mitered corners every piece is measured to the long point of its miter, so a leg is one casing width longer than with a butt joint, where the head sits across the tops of the legs. A door cased with butt joints takes 2 × (height + reveal) + (width + 2 × reveal + 2 × casing width): a 36 × 80 in door with 2¼ in casing and a 3/16 in reveal is 201.25 in, about 16.8 ft, per side.

A picture-framed window is cased on all four sides with four miters, so each piece is its side plus two reveals and two casing widths, and the total is the opening's perimeter plus 8 reveals and 8 casing widths. A stool-and-apron window has two legs and a head above a stool; the apron below the stool is cut from casing to the outer width of the casing line, the head's length (the rule in USDA Agriculture Handbook 73, Wood-Frame House Construction, 1955), and the stool runs past the casing by the horn at each end and is listed separately.

The casing total, with a waste allowance, is divided by the stock length and rounded up. Every piece must come out of one length, so a stock length shorter than the longest piece is rejected.

Formula

m           = casing width (mitered) or 0 (butt joint)
door side   = 2 × (height + reveal + m) + (width + 2 × reveal + 2 × casing width)
door casing = doors × sides × door side
frame window = 2 × (width + 2 × reveal + 2 × casing) + 2 × (height + 2 × reveal + 2 × casing)
stool window = 2 × (height + reveal + m) + 2 × (width + 2 × reveal + 2 × casing)   (head + apron)
stool       = width + 2 × reveal + 2 × casing width + 2 × horn
lengths     = total casing × (1 + waste % ÷ 100) ÷ stock length, rounded up

Example

Three 36 × 80 in doors cased on both sides with 2¼ in casing, a 3/16 in reveal and mitered heads take 82.44 in legs and a 40.88 in head, 205.75 in per side, or 1,234.5 in (102.875 ft) for six sides.

Two 36 × 48 in picture-framed windows take 2 × (40.88 + 52.88) = 187.5 in each, 375 in (31.25 ft) together. The total of 134.125 ft is 147.5 ft with 10% waste: 11 lengths of 14 ft.

Assumptions and limitations

• Lengths are pooled: offcuts are counted toward other pieces. In practice each leg needs most of a length, so short offcuts may be left over; the waste allowance (10%, a typical figure) and the longest-piece check are the only allowances for that.
• Pieces are measured to the long point of each miter. Back-band, plinth blocks, rosettes and decorative head caps are not included.
• The stool is listed in linear feet only, since it is usually a different profile from the casing.
• The casing width, horn and stock length defaults are illustrative; the reveal default is the 3/16 in from Family Handyman's guide.

Frequently asked questions