Worked layout
Picture-frame window casing
Example measurements
- Inside reference
- 36 × 48 in
- Actual casing face
- 2½ in
- Faces / quantity
- One face, one opening
Layout result
- Visible face
- 2.5 in
- Pieces
- 4
- Exact length
- 15 ft 8 in
Window casing: four mitered sides
Dimensions are in inches. On a small screen, scroll the drawing to inspect each endpoint.
| Piece | Outside long points | Inside short points |
|---|---|---|
| Head | 41 in | 36 in |
| Left | 53 in | 48 in |
| Bottom | 41 in | 36 in |
Read a closed frame at the lower corners
Trace the bottom member into both sides in the drawing. Those lower miters distinguish this assembly from a three-piece door casing. The left and right sides have an end cut at the bottom as well as the top, and the piece list names all four roles.
Opposite pieces may have equal lengths, but their inner edges and cut directions still need to face the opening correctly. Follow the enlarged long- and short-point detail and mark the profile’s inner edge before transferring a length.
The outside tips define a larger rectangle than the 36-by-48-inch inside reference. Check that footprint against blinds, cabinets and nearby wall returns. Keeping the inside opening unchanged does not preserve the outside clearance when the casing face changes.
Compare the four-sided frame with stool and apron
Look at how you want the window to finish at the bottom. This closed frame uses casing along the wall plane. A stool-and-apron arrangement introduces a ledge projecting into the room, with a separate trim member beneath it.
Select the stool-and-apron assembly to compare that option. Supply stool depth, thickness and overhang, plus apron face and inset. Renaming the existing bottom piece leaves out the dimensions that define the projection and lower trim.
Read the resulting stool and apron as separate blanks and material profiles. The stool’s jamb notches still need field fitting. Compare the visible lower edge and the required materials before choosing which assembly suits the window.
Measure the inside line and test a different profile
Replace the sample width and height using a clearly labeled reference. Trim-inside means the final casing’s inner line, with reveal already represented. Jamb-inside means the jamb opening, with the chosen reveal entered separately.
Measure the top and bottom widths and both side heights. Record differences rather than averaging them out of sight. This rectangular drawing helps plan the pieces; it does not establish that the physical jamb and wall plane are square.
With the inside reference held constant, try another actual casing face as the second comparison. Review the outside footprint, long and short points, and longest blank. This isolates the effect of the profile instead of changing the window size and profile together.
Match the miter detail to the material you buy
Dry-fit a sample corner at the chosen reveal and check the inner and outer edges together. Repeat the check at the lower corners, where the bottom member has to close the frame. Label every piece’s role so matching lengths do not obscure opposing cut directions.
Baird Brothers’ trim demonstration supports the reveal-marking and miter principles used here. Andersen’s linked flat-trim guide shows separate measurements at rectangular head, sill and side locations for its own product; its attachment details are not instructions for this mitered wood frame.
Enter the casing profile, raw stock, kerf, end trimming and spares before using the purchase result. Each continuous piece must fit usable stock. Curved heads, compound angles, layered backband and unusual mullion arrangements need additional measured construction details beyond this frame.
Questions about this layout
Is the bottom piece a window stool?
No. It is the fourth side of the casing frame. A stool projects into the room and needs its own dimensions; use the stool-and-apron assembly for that treatment.
What changes if I choose wider casing?
With the inside reference fixed, the outside footprint and miter long points change. Recheck nearby trim, blinds and wall returns as well as the recalculated blank lengths.
Can matching opposite lengths still need fitting?
Yes. Equal opposite lengths belong to the rectangular model. Check the actual jamb, wall plane and corner fit before treating the sample dimensions as final cuts.
Sources and assumptions
- Baird Brothers: Door Trim Installation TipsThe Door Trim Installation Tips tutorial demonstrates reveal marking and contrasts mitered casing, square cuts, and a built-up pediment. Its sample reveal is not a required dimension.
- Andersen: Straight Interior Flat Trim ApplicationRectangular opening measurements at the head, sill, and sides for Andersen flat trim. Its attachment method is product-specific; it is not a mitered wood-casing cut schedule.
- TrimLayout Calculation MethodsDefinitions of clear gaps, measurement references, finished pieces, and material allowances used in these examples.
- NIST: SI Units — LengthUnit relationships for converting measured lengths; this source does not prescribe decorative proportions.