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
Four references at one casing corner
Dimensions are in inches. On a small screen, scroll the drawing to inspect each endpoint.
When starting from jamb-inside measurements, the engine adds two reveals to the width. A three-piece door adds one reveal at the head to its height; a four-sided window adds one at each end. When starting from casing-inside measurements, do not add reveal again.
Draw four different boundaries
A door slab measurement will not give you a casing cut list. On a sketch, distinguish the rough opening in the wall, the inside edges of the jamb, and the inner and outer edges of the casing. Each outline answers a different question.
The reveal is the narrow strip of jamb left visible beside the casing. If you measure the jamb opening, the calculator needs that reveal as a separate input. If you mark the intended casing line first and measure between those marks, the reveal is already represented in the trim-inside dimensions.
Write the reference beside the numbers before leaving the opening. Switching between jamb-inside and trim-inside requires the measurements for the new reference; keeping the same numbers and changing only the mode moves the planned casing.
Measure a rectangle from more than one location
Measure width near the head and at the lower part of the opening. Record left and right heights separately. For a closed window frame, also check the intended bottom casing line. Keep any differences in your notes: the calculator describes a rectangle, while the actual opening may need fitting at individual corners.
Place the chosen profile in its installed orientation and measure its visible face from inside edge to outside edge. Use the actual width rather than the nominal product name. Mark which edge faces the opening on a sample; that mark makes the later long- and short-point drawing easier to follow.
Check thickness and projection against nearby hinges, blinds, tile and wall returns. The inside reference can be correct while a wider or deeper profile still fails to fit outside it.
Select the assembly before using its cut lengths
Look at the bottom of the opening before selecting an assembly. Door casing has two legs and a head, with no bottom member. A picture-frame window closes all four sides. A stool-and-apron arrangement adds a projecting stool and a separate trim piece below it.
The door worked example is useful for reading a head with two mitered ends beside legs with only one mitered end. Compare the window template to see why closing the bottom changes the side pieces. Use the assembly table to collect the additional dimensions before calculating.
| Assembly | Measurements to add | Read in the result |
|---|---|---|
| Three-piece door | Inside reference, reveal if needed, casing face, joint style | Head and legs with labeled ends and cutting references |
| Four-sided window | Width and height at the final inside reference; casing face | Top, bottom, and sides; long/short points for miters |
| Stool and apron | Stool depth, thickness, overhang; apron face and inset | Separate stool and apron blanks; jamb notches remain a field fit |
Understand the miter drawing before transferring a mark
The long point is the outside tip of a rectangular casing miter; the short point is the inside tip. Follow the piece name and the direction of each end cut in the enlarged drawing. Matching lengths do not make left and right pieces interchangeable in orientation.
On the door example, a leg meets the head at its upper end and stops at the specified lower boundary. A closed window side joins both the top and bottom. Check those end conditions before copying a dimension onto the profile.
Baird Brothers’ Door Trim Installation Tips demonstration shows reveal marking and contrasts mitered, square-cut and built-up trim. Its sample reveal is a design choice in that demonstration. Use the example to understand the marking sequence, then confirm your chosen line with a corner sample.
Separate repeated quantities and material profiles
Use a repeated quantity only after checking that the openings match. Measure the opposite face of a doorway separately before choosing both faces: a different floor stop, reveal or adjacent wall can require another assembly.
Casing, stool and apron may use different profiles and stock. Assign each to the material you will actually buy, then enter stock length, saw kerf, end trimming and spares. Compare the longest required blank with usable stock before relying on a total-length estimate.
The stool output is a blank that still needs its jamb notches and projection fitted on site. Arches, compound corners and unmeasured layers require additional fabrication details. Andersen’s linked flat-trim guide provides another example of measuring rectangular head, sill and side locations; its product-specific attachment and overlap details do not describe the wood miter cuts here.
- Confirm whether every measurement is jamb-inside or trim-inside.
- Label the profile’s inner edge and each piece’s head, left, right or bottom role.
- Check the intended reveal and outside clearance with the real profile.
- Record unequal sides and local corner fitting before using repeated quantities.
Questions about this layout
Do I add reveal to trim-inside measurements?
No. Trim-inside dimensions already locate the casing’s final inner edges. Reveal is entered separately when the starting dimensions describe the jamb’s inside opening.
Why is the casing head longer than the inside width?
A mitered head reaches from the inside short points out to the outside long points at both ends. Read the labeled cutting reference in the result; the inside opening width and the blank length are different dimensions.
Can a four-sided window cut list be used for a stool and apron?
Choose the stool-and-apron assembly instead. It needs separate stool and apron dimensions and material profiles, and the stool’s jamb notches remain a field fit.
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.