Worked layout
Three-piece mitered door casing
Example measurements
- Inside reference
- 36 × 80 in
- Actual casing face
- 2½ in
- Faces / quantity
- One face, one opening
Layout result
- Visible face
- 2.5 in
- Pieces
- 3
- Exact length
- 17 ft 2 in
Door casing: two legs and one head
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 | 82 1/2 in | 80 in |
Follow the head and the two legs
Read the head first: its two end cuts meet the legs at the upper corners. Then follow the left and right legs down to their lower stops. There is no bottom casing member, and the legs are not window-frame sides with a second lower miter.
The enlarged corner identifies the inner short point and outer long point. Match those edges to the actual molding profile before marking a cut. Keep left and right labels even when the lengths match, because their upper cuts face opposite ways.
Use the longest blank in the result to check stock. Total trim length can help with a budget, but it cannot make a leg fit into a board that is too short after trimming.
Change the reference without adding reveal twice
The 36-by-80-inch reference describes the intended inside casing line, not a promise that a door slab with those dimensions will fit this plan. If those are the lines you measure, keep trim-inside mode and do not add reveal again.
If you measure the jamb’s inside opening instead, select jamb-inside and enter the reveal separately. Replace the sample dimensions with that reference’s measurements before comparing the result. Changing the reference label alone would describe a different casing position.
Check width at more than one height and measure both legs to their intended lower stops. A finished floor and a plinth block create different stopping conditions. Also check the profile’s outside edge beside any wall return and measure its actual face and thickness.
Compare mitered and square head-over-legs trim
With the measured inside reference held fixed, compare the mitered assembly with square head-over-legs trim. The latter places the head across the tops of the legs. Its role lengths must be recalculated; turning the saw to a square cut while retaining the miter list does not produce the same assembly.
As a separate comparison, review one face and both faces. Doubling is appropriate only when the second physical face really has matching dimensions and details. Different floor levels, reveals or profiles call for another measured assembly.
Baird Brothers’ door-trim demonstration shows reveal marking alongside mitered, square-cut and built-up arrangements. Use it to understand the joint choices. Its demonstrated reveal is not a required dimension for this sample or your door.
Check the sample and purchase groups
Make a corner sample with the selected profile and hold it against your marked reveal. Check the inside line, outside projection and joint orientation together. This is where a profile that looked suitable in the diagram can reveal a conflict beside the actual opening.
Assign the casing to the material you intend to buy and enter stock length, kerf, end trim and spares. Review any stock warning before ordering. A continuous head or leg needs enough usable length for that member.
This template is for a rectangular, three-piece assembly. A pediment, backband, plinth block or jamb extension needs its own measured detail; none is hidden in the sample’s three-piece quantity.
Questions about this layout
Why are the leg ends different?
Each leg meets the mitered head at the top and stops at its specified lower boundary. Unlike a closed window frame, this door assembly has no lower casing piece to receive another miter.
Can I use the door’s listed size as the inside reference?
Measure the selected reference directly. A door slab size does not establish the jamb-inside or final trim-inside dimensions needed for this casing layout.
Does selecting both faces survey the other side?
No. It repeats the assembly for matching faces. Measure the opposite side first and create a separate assembly if its dimensions, reveal, profile or lower stop differ.
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.