Calculator

Door & Window Casing Calculator

Door and window casing is bought per opening, not per linear foot of wall, and each opening needs two different piece lengths: legs that run vertically down each side, and a head — plus, on a window, a sill — that runs across the top. Neither length is the opening's raw dimension. A door leg has to clear the floor, run past the head casing above it, and leave enough at the top for a mitre; a head has to span the opening plus the reveal on both jambs plus the width of the leg casing it meets at each end.

This calculator works out those real cut lengths per opening — legs and heads for doors, legs and heads for windows — rather than estimating from a flat linear-foot rate around a rough opening size. It then packs those pieces into stock lengths the way a lumber yard actually sells casing, which is a separate question from how much total casing the room needs, because a stick too short for the longest leg still gets bought whole.

Calculate your quantity

Doors

Door opening width (in)

Door opening height (in)

Sides to case

2 for a doorway cased on both faces.

Windows

Window opening width (in)

Window opening height (in)

Casing width (in)

Jamb reveal (in)

Mitre allowance per piece (in)

Casing length sold

Casing sticks

14 8 ft sticks

Door leg length: 84.44 in
Door head length: 38.88 in
Window leg length: 62.38 in
Window head/sill length: 42.88 in
Longest single piece: 84.44 in
Stock length used: 8 ft
Leg pieces: 10 pieces
Head pieces: 7 pieces
Total casing: 87 lin ft
Mitre cuts: 14 cuts

A 7 ft stick is 0.44 in too short for the longest piece this job needs (84.44 in), so it cannot yield one at all. Counts above assume 8 ft stock instead.
A 0.1875 in reveal adds 0.375 in to every head and sill piece and 0.1875 in to every leg. Cutting to the bare opening size is the usual reason a mitre will not close.

Shopping summary

• 14 x 8 ft lengths of 2.25 in casing
• 2 box(es) of 2 in brad nails

This is an estimate — confirm structural work with a professional.

How this calculation works

A door leg's length is the opening height, plus the reveal (the small setback where casing sits proud of the jamb edge), plus the width of the head casing it butts into, plus one mitre allowance. A door head's length is the opening width, plus twice the reveal-and-casing-width sum (because it overlaps a leg's casing width at each end), plus a mitre allowance. Windows use the same shape of formula, but a window's legs only pick up the reveal once — there is no head casing sitting above them to clear the way a door's legs do.

Once every piece's exact length is known, the calculator sorts pieces into two pools — legs and heads (heads and sills share a length on a window) — and packs each pool into whichever stock length you're buying, fitting as many pieces per stick as will actually fit rather than assuming an even split. A stick that fits two heads but only one leg is common, because heads are almost always shorter than legs.

The reveal — the setback where the casing's inner edge sits back from the jamb, usually 3/16 to 1/4 in — adds to every single piece, legs and heads alike, because it changes where the mitre has to close on every corner.
A door's leg length includes the head casing's own width, since the leg has to run up to meet the underside of the head, not just to the top of the rough opening.
Doors cased on both faces of an opening (an interior doorway with no door slab, for instance) double every count for that opening — set the sides field rather than doubling the door count, since a doubled door count would also double the opening width calculations incorrectly.
Packing legs and heads into stock lengths is a separate step from totalling linear footage — a stick barely too short for one more leg is wasted footage on the shopping list even if the running-foot total says there's plenty of casing left.

The formula

door leg = height + reveal + casing width + mitre allowance; door head = width + 2×(reveal + casing width) + mitre allowance; window leg = height + 2×reveal + mitre allowance; window head/sill = width + 2×(reveal + casing width) + mitre allowance

reveal: The setback of the casing's inner edge from the face of the jamb, typically 3/16 to 1/4 in. It adds to every piece length because the mitre at each corner has to close at the reveal line, not at the bare jamb edge.
casing width: The face width of the casing stock itself, measured flat. It adds into head and door-leg lengths because those pieces have to clear the width of the casing sitting perpendicular to them at the joint.
mitre allowance: Extra length held on every piece for the 45-degree cut at each end, so a piece cut to the bare theoretical length doesn't come up short once the mitre's angle is accounted for.
doorSides: How many faces of the opening get casing — 1 for a door with trim on one side only, 2 for an interior opening cased on both faces. This multiplies every leg and head count for that opening, not just the linear footage.
stick length: The stock length being purchased. Legs and heads are packed into it separately, because the longest piece a stick can hold sets how many pieces of that type fit per stick — a length that fits two short heads may fit only one long leg.

Where these numbers come from

2 1/4 in standard casing width: Trade convention for a common mid-weight profile — colonial and clamshell casing both run in this range as a standard stock width. Wider casing (3 1/4 in and up) is common in taller-ceilinged or more formal rooms, which is why width is an input rather than a constant.
3/16 in reveal: Trade convention, near-universal in North American finish carpentry — a small enough setback to be barely visible but consistent enough that a jamb that's slightly out of plane doesn't telegraph through the casing line.
7 and 8 ft stock lengths: Trade convention. 7 ft (84 in) covers most standard 80 in door legs with room for reveal and a modest mitre allowance; taller openings or a generous mitre allowance push the requirement past 7 ft, which is why 8, 12 and 16 ft options exist alongside it.
2 in mitre allowance: Working default from finish-carpentry practice, not a manufacturer spec — enough margin to true up a mitre that doesn't close on the first cut without running a piece short. Confident installers on square openings sometimes trim this back.

Worked examples

A typical 3-bedroom home's doors and windows, 8 ft stock

Inputs
Doors	3
Door opening width	32 in
Door opening height	80 in
Sides to case	1
Windows	2
Window opening width	36 in
Window opening height	60 in
Casing width	2.25 in
Jamb reveal	0.1875 in
Mitre allowance per piece	2 in
Casing length sold	8 ft

Result
Casing sticks	14 8 ft sticks
Door leg length	84.44 in
Door head length	38.88 in
Window leg length	62.38 in
Window head/sill length	42.88 in
Longest single piece	84.44 in
Stock length used	8 ft
Leg pieces	10 pieces
Head pieces	7 pieces
Total casing	87 lin ft
Mitre cuts	14 cuts

Three doors and two windows come to 87 linear feet of casing total, but the shopping number is 14 sticks of 8 ft stock, not the 87 ÷ 8 = 11 sticks a running-foot estimate would suggest. The gap is the leg length: at 84.44 in, a standard 80 in door's leg is longer than a 7 ft stick and eats nearly all of an 8-footer, so legs pack one to a stick almost everywhere, and it's the leg count — 10 pieces — rather than the total footage that drives most of the order.

Heads pack two to a stick instead, since the longest head here (42.88 in for the windows) leaves enough of a 96 in stick to fit a second one. That difference — legs one per stick, heads two per stick — is exactly why totalling linear feet and dividing by stock length undercounts: it assumes every foot of stock is used as efficiently as the heads are, when the legs are the pieces actually setting the order.

A great room opening cased on both faces, plus four large windows, 12 ft stock

Inputs
Doors	2
Door opening width	36 in
Door opening height	80 in
Sides to case	2
Windows	4
Window opening width	48 in
Window opening height	72 in
Casing width	3.25 in
Jamb reveal	0.25 in
Mitre allowance per piece	2 in
Casing length sold	12 ft

Result
Casing sticks	22 12 ft sticks
Door leg length	85.5 in
Door head length	45 in
Window leg length	74.5 in
Window head/sill length	57 in
Longest single piece	85.5 in
Stock length used	12 ft
Leg pieces	16 pieces
Head pieces	12 pieces
Total casing	159.7 lin ft
Mitre cuts	24 cuts

Casing both faces of two open doorways doubles the leg and head count for those openings before a single window is added — 16 leg pieces and 12 head pieces here, against 10 and 7 in the first example, even though the first example actually had more doors. Doorway count alone is a poor predictor of casing volume; sides-cased and opening size matter more.

At 12 ft stock, legs still only fit one per stick (an 85.5 in leg leaves too little of a 144 in stick for a second), but heads now fit two per stick instead of the tighter fit in the first example, because the wider 57 in window heads still leave enough 12 ft stock for a pair. Moving to longer stock buys back some efficiency on heads without changing anything about how legs pack — the leg count is what a longer stick can't fix.

Common mistakes

Cutting legs to the bare opening height with no reveal or mitre allowance added, then discovering the mitre at the head won't close because the leg was cut to the theoretical minimum rather than the buildable length.
Estimating casing by a flat linear-foot rate per door or window rather than working out legs and heads separately — a tall door and a wide window cost casing in very different proportions even at the same total opening perimeter.
Doubling the door count instead of setting the sides-cased field for an opening trimmed on both faces. Doubling the count doubles the opening width math along with it, which is wrong — only the sides field should change for that case.
Buying stock length based on the average piece rather than the longest one. A stick that fits most of your heads but not your longest leg still has to be bought at the length the leg needs, or a second, longer stock line has to be added.
Assuming 7 ft stock always covers a standard door. At default reveal, casing width and mitre allowance, an 80 in door's leg needs about 84.4 in — a hair over the 84 in a 7 ft stick provides, so many suppliers stock 8 ft for exactly this reason.

Shopping summary

The sticks-needed figure already accounts for how legs and heads pack differently into your chosen stock length — buy that number rather than rounding total linear footage up to the nearest stick, since the two rarely agree once legs are the longest piece in the order.

A box of 2 in brad nails covers roughly a hundred pieces of casing at typical nailing spacing; scale it against the leg-and-head piece count this page returns rather than the room count. If the same rooms are also getting base trim, price it with the baseboard calculator — baseboard runs wall to wall rather than around an opening, so it doesn't share this page's leg-and-head arithmetic at all.

Rooms getting casing at doors and windows plus crown at the ceiling are common together; the crown moulding calculator prices that separately, per wall rather than per opening. And where casing meets a wainscot cap below chair-rail height instead of running to the floor, check the panel layout on the wainscoting calculator first — the stile nearest a door opening usually has to land at the casing's edge, which can shift where the panels fall.

FAQ

Why is my leg length longer than the actual door opening height?

Because a leg has to reach past the top of the rough opening to meet the underside of the head casing, and pick up the reveal and mitre allowance along the way — three additions stacked on the raw framed height, not one. Skip any of the three and the piece comes up short by exactly the amount left out, so cut to the leg length this page returns rather than the tape-measure opening height.

My supplier only stocks 7 ft casing — will that cover an 80 in door?

It depends on how close your actual reveal and mitre allowance sit to the working defaults, so check the leg length figure above for your specific inputs before assuming yes. Where the margin is tight or negative, ask for 8 ft stock instead — the extra length costs little next to the risk of a leg cut a hair short on a door that's already hung, where the casing can't be un-cut and re-run.

Why do heads pack two to a stick more often than legs do?

Because heads are shorter than legs on almost every standard opening — a head only spans the opening's width plus two casing widths, while a leg spans the full height plus a head casing's width. On typical stock lengths, that difference is often enough that two heads fit where only one leg does, which is why the head and leg totals on this page rarely track the same stick count.

Does a wider casing profile change more than the head length?

Yes — casing width feeds into both the head length (twice, once per side) and the door leg length (once, for the head casing it meets). Moving from a 2 1/4 in profile to a 3 1/4 in one adds roughly two inches to every door head and about an inch to every door leg, which on a house with several openings adds up to real extra footage, not just a wider-looking trim line.

How much extra should I buy for a mistake on my first mitre?

This page's mitre allowance already assumes one attempt might not close cleanly and holds back length for a second try on the same piece. What it doesn't cover is a piece cut too short entirely — for that, one spare stick of your longest-running size (usually the leg length) is a reasonable hedge on a first casing job.

Do sidelights next to a front door count as windows or as part of the door opening?

Treat a sidelight as its own window entry rather than folding it into the door's width — it has its own head and legs meeting the door's casing at a butt joint rather than sharing the door's mitred corners, and pricing it as part of the door opening will understate both the leg count and the linear footage needed.

Where to go next

The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.