Casing is held back from the edge of the jamb by a reveal, so the legs are longer than the jamb and the head is longer than the opening by two reveals and two casing widths. On 3 1/2 inch casing that is seven and three eighths inches on a 30 inch door, and seven of those inches are the casing alone. Cut the head at the width of the opening and it is scrap.
Casing pieces, cut lengths and stock
sides
in
in
sides
in
in
in
ft
lengths
Count sides, not openings. An interior door is cased on both sides, which is two heads and four legs for one door. A window is normally cased on one side, and a cased opening with no door in it is cased on both.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
A cased opening, with the reveal and the head
Worked example
Six door sides at a 30 by 80 jamb, three window sides at 36 by 48, 3 1/2 in casing, a 3/16 in reveal, 16 ft stock.
Legs. 80 + 3/16 = 80.188 in, twelve of them.
Heads. 30 + 2(3/16) + 2(3.5) = 37.375 in, six of them.
The gap. The head is 7.375 in longer than the 30 in opening.
Pieces. Nine sides, three pieces each: 27 pieces.
Run.133.8 linear ft.
Stock. Dividing says 9 lengths, packing needs 10.
Seven of those 7.375 inches are the two casing widths and only three eighths is the reveal. That is the part worth remembering: the reveal is a fiddly small number that changes the cut by a sixteenth, and the casing width is a large number that changes it by inches. People fuss over the first and forget the second.
The formula
Two lengths per side, and the head is the one that catches people:
leg = Hjamb + r head = Wjamb + 2r + 2c
H, W
jamb height and width, inside to inside
r
reveal, on all three edges. Your decision, not a standard
c
casing width
2c
because a mitred head runs to the OUTSIDE corner of each leg
3
pieces per side: two legs and one head
There is no bottom reveal, because the legs run to the floor rather than to a fourth edge. On a plinth block detail the legs stop on the block and the arithmetic changes; on a stool and apron window the head, legs, stool and apron are four different lengths and none of them is the jamb width.
Why sides, and why the head is its own row
A trim estimate that counts openings has already gone wrong. An interior door is cased on both sides, so six doors is twelve sides, twenty four legs and twelve heads. A window is usually cased on one side. A cased opening with no door in it is cased on both, and it is the one that gets left out because there is no door to remind anybody.
The head gets its own row on this page because it is the piece that gets cut wrong. It is not the width of the opening. It is not the width of the opening plus the reveals. It is the width of the opening plus two reveals plus two casing widths, because a mitred head has to reach the outside corner of each leg.
On 3 1/2 inch casing that is seven extra inches, which is more than a fifth of a 30 inch head. Cut it short and there is no recovery: it is a new piece.
The one thing on this page that is not geometry is the doorstop, which is the only piece of interior trim the residential code measures anything from.
Cut lengths for one jamb, at each reveal and casing width
Reveal
Casing width
Leg, for an 80 in jamb
Head, for a 30 in jamb
Head against the opening
1/8 in
2.25 in
80.125 in
34.75 in
4.75 in longer than the opening
1/8 in
3.5 in
80.125 in
37.25 in
7.25 in longer than the opening
3/16 in
2.25 in
80.1875 in
34.875 in
4.875 in longer than the opening
3/16 in
3.5 in
80.1875 in
37.375 in
7.375 in longer than the opening
1/4 in
2.25 in
80.25 in
35 in
5 in longer than the opening
1/4 in
3.5 in
80.25 in
37.5 in
7.5 in longer than the opening
Read 2026-09-09. This table is geometry rather than a source: no code or standard sets a reveal. The reveals listed are the ones finish carpenters actually set.
The head is longer than the opening, by twice the casing
Casing is not fitted flush with the inside edge of the jamb. It is held back by a reveal, normally an eighth to a quarter of an inch, on all three edges. A flush joint advertises every thousandth of jamb that is proud or hollow; a reveal hides all of it, and it is the line the eye reads as the opening.
That changes both cut lengths, and it changes the head far more than people expect:
leg = jamb height + reveal head = jamb width + 2 × reveal + 2 × casing width
The two casing widths are there because a mitred head runs to the OUTSIDE corner of each leg, not to the reveal line. Miss them and the head is short by twice the casing width, whatever the reveal is.
A 30 in by 80 in jamb, at each common reveal and two casing widths
Reveal
Casing width
Leg, for an 80 in jamb
Head, for a 30 in jamb
Head against the opening
1/8 in
2.25 in
80.125 in
34.75 in
4.75 in longer than the opening
1/8 in
3.5 in
80.125 in
37.25 in
7.25 in longer than the opening
3/16 in
2.25 in
80.1875 in
34.875 in
4.875 in longer than the opening
3/16 in
3.5 in
80.1875 in
37.375 in
7.375 in longer than the opening
1/4 in
2.25 in
80.25 in
35 in
5 in longer than the opening
1/4 in
3.5 in
80.25 in
37.5 in
7.5 in longer than the opening
Read the last column. On 3 1/2 in casing the head is 7.375 inches longer than the opening it goes over, and 7 of that is the casing width alone. Cut it at 30 and it is scrap.
The reveal itself is a decision rather than a standard. A wider reveal is more forgiving of a jamb that is not flush with the drywall and reads heavier; a narrow one reads tighter and leaves nowhere to hide. Set it once with a combination square and mark every jamb before cutting anything.
Packing, and why casing behaves better than baseboard
The lengths on this page are packed into stock rather than divided, the same method the baseboard calculator sets out at length. The result is usually different, and pleasantly so.
Casing pieces are short. A leg is under seven feet and a head is around three, so the remainder of a 16 ft length after two legs is genuinely long enough to be the next head. On baseboard, where the pieces are whole walls, the remainder fits nothing and the packing runs ahead of the division by a length or more.
So on casing the two answers often agree, and when they do the page says so rather than manufacturing a difference. The packing is still worth running, because the moment you have a tall opening or short stock it stops agreeing.
Order legs and heads from the same length where you can. Colour and grain vary along a board and between boards, and a head that came off a different stick from its legs shows it at the mitre once the stain is on.
Exterior trim is a different job with a different failure
A good share of the people looking for a trim calculator are pricing exterior trim, and the arithmetic above still applies to the lengths. What does not carry over is the risk.
Interior casing that is slightly wrong looks slightly wrong. Exterior trim that is slightly wrong lets water into the wall, and the failure is invisible for years.
The controlling detail on an exterior opening is not the trim, it is the flashing and the water-resistive barrier behind it, and the order those layers are lapped in. That is a code requirement rather than a matter of taste, and it is set out on the siding calculator where it belongs rather than being restated here.
The quantity difference outside is the sill. Exterior casing usually has a sloped sill or drip cap over the head, which is a fifth piece with its own length and its own angle, and it is not in the three-pieces-per-side count above. Add it separately.
Material matters more outside too. The same profile in finger-jointed pine, in cellular PVC and in a primed composite behaves completely differently at a mitre after two summers, and none of those differences show up in a linear foot.
Count sides, not openings
An opening is not a unit of casing, and this is where trim estimates go wrong before any arithmetic starts.
An interior door is cased on both sides. Two heads, four legs, one door. An exterior door is normally cased inside and trimmed differently outside, in a different material, so it is one set of interior casing plus a separate exterior job.
A window is usually cased on one side only, and often not with three pieces: a stool and apron treatment has a head, two legs, a stool that runs past the legs with horns on it, and an apron under the stool. That is five pieces on one window and none of them are the same length.
A cased opening with no door is cased both sides too, and it is the one people forget, because there is no door to remind them.
The calculator asks for sides rather than openings for exactly this reason. Count what you will actually cut.
The doorstop is the one piece of trim the code measures from
Interior trim is almost entirely unregulated. There is one exception, and it is a piece of trim so slight that people rip it off to fit a thicker carpet.
Egress door. Not less than one egress door shall be provided for each dwelling unit. The egress door shall be side-hinged, and shall provide a clear width of not less than 32 inches (813 mm) where measured between the face of the door and the stop, with the door open 90 degrees (1.57 rad). The clear height of the door opening shall be not less than 78 inches (1981 mm) in height measured from the top of the threshold to the bottom of the stop. Other doors shall not be required to comply with these minimum dimensions. Egress doors shall be readily openable from inside the dwelling without the use of a key or special knowledge or effort.
Read what the two dimensions are measured from. The clear width of 32 inches is measured between the face of the door and the stop, with the door open 90 degrees. Not the rough opening, not the jamb, not the door leaf. And the clear height of 78 inches runs from the top of the threshold to the bottom of the stop.
So a stop applied proud, or a head stop set low to close a gap, is the thing that decides whether the egress door passes. It is trim carpentry with a code consequence, which is not true of anything else in this trade.
And read the sentence most people have never noticed. "Other doors shall not be required to comply with these minimum dimensions." The 32 inch rule is for the one required egress door per dwelling unit. It is not a general minimum for interior doors, and it is repeated as one constantly.
Also in the same section: the egress door must be side-hinged, and readily openable from inside without a key, special knowledge or effort. A double cylinder deadbolt on the front door is the classic failure of that last clause.
The source, and what is not in it
Read 2026-09-09. Seattle Residential Code, Chapter 3, Building Planning, Section R311.2, City of Seattle, adopting the 2018 International Residential Code. Sets the egress door's clear width and clear height, and measures both from the STOP rather than from the jamb or the frame. Also states that other doors are not required to comply with either dimension.
Seattle is used because codes.iccsafe.org returns HTTP 403 and cannot be fetched, and a city that publishes its adopted code as a PDF states its edition and jurisdiction on the document itself. Your jurisdiction may amend this.
Everything else on this page is geometry rather than regulation. No code sets a reveal, a casing width, a mitre, a species or a fastening schedule for interior trim. The reveal is a decision, the widths are what the yard stocks, and the arithmetic that turns them into cut lengths is arithmetic you can check with a tape.
Which is worth saying plainly, because a page that quoted a code section at you about casing would be inventing one.
Frequently asked questions
How do you calculate door casing?
Three pieces per side. Each leg is the jamb height plus one reveal. The head is the jamb width plus two reveals plus two casing widths, because a mitred head runs to the outside corner of each leg. Count sides rather than doors: an interior door is cased on both sides.
What is a reveal on door casing?
The setback between the inside edge of the jamb and the inside edge of the casing, normally 1/8 to 1/4 inch, held the same on all three edges. It exists because a flush joint shows every thousandth of jamb that is proud or hollow and a reveal hides all of it. No code or standard sets one; it is a decision, and it should be set once and marked on every jamb before anything is cut.
How long should the head casing be?
The jamb width plus two reveals plus two casing widths. On a 30 inch jamb with a 3/16 inch reveal and 3 1/2 inch casing that is 37 3/8 inches, which is 7 3/8 longer than the opening. Cutting it at the opening width is the most common casing mistake and it costs a whole piece.
How many pieces of casing per door?
Six for an interior door, because it is cased on both sides: four legs and two heads. Three for a single side, which is what an exterior door gets on the inside. A cased opening with no door is six as well.
Does the building code say anything about interior trim?
Almost nothing, and what it does say is about the doorstop. IRC R311.2 requires the egress door to give a clear width of not less than 32 inches measured between the face of the door and the stop, and a clear height of not less than 78 inches measured from the top of the threshold to the bottom of the stop. The datum for both is a piece of trim.
Do all interior doors have to be 32 inches wide?
No. R311.2 sets that clear width for the required egress door and then says, in terms, that other doors are not required to comply with these minimum dimensions. It is repeated as a general minimum constantly and it is not one. Accessibility standards other than the residential code may say otherwise for a particular project.
How much trim do I need for a whole house?
Count the sides rather than the openings, then run this page once with your typical door jamb and once with your typical window. Most houses have two or three repeated sizes and a couple of oddities, and estimating from the repeated sizes plus a list of the oddities is faster and more accurate than an average.
Is a stool and apron window the same calculation?
No. A stool and apron treatment has a head, two legs, a stool that runs past the legs with horns on it and an apron beneath, which is five pieces at four different lengths. The three-pieces-per-side arithmetic here is for a picture framed opening.
Check these numbers yourself
Seattle Residential Code, Chapter 3, Building Planning, Section R311.2City of Seattle, adopting the 2018 International Residential Code, read 2026-09-09. Sets the egress door's clear width and clear height and measures both from the STOP rather than from the jamb or the frame, which makes the doorstop the only piece of interior trim the code has an opinion about. Also states that other doors are not required to comply with either dimension. Seattle is used because codes.iccsafe.org returns HTTP 403 and because a city PDF states its edition and jurisdiction on the document.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The reveal, the casing width and the mitre are decisions, not requirements. No code sets any of them, and this page says so rather than dressing a convention up as a standard. The one code value here is IRC R311.2, quoted from the Seattle Residential Code as read on 9 September 2026, and it governs the egress door rather than the casing. Your jurisdiction may amend it. The spare lengths are this page's own allowance and are an input. Saw kerf is not modelled. Exterior trim depends on flashing and the water-resistive barrier behind it, which is not a quantity question and is not answered here.