Published August 15, 2026 · Updated August 23, 2026 · By Tupep
Casing is counted per opening, never per wall
Baseboard follows walls. Casing does not follow anything. It wraps openings, and every opening is a fixed package of three or four pieces whose lengths are set by the jamb, the reveal and the casing width. That makes casing the easiest trim to estimate accurately and the easiest one to get badly wrong, because the whole job is a multiplication problem and one error in the per-opening figure multiplies across thirty openings.
The per-opening formula
For mitered casing on a door, three sides:
Leg long point = floor to underside of head jamb + head reveal + casing width
Head long point = jamb opening width + (2 x side reveal) + (2 x casing width)
LF per side = (2 x leg long point + head long point + 3 in of cut allowance) / 12
The reveal is the sliver of jamb left showing inside the casing, commonly 3/16 in, sometimes 1/8 or 1/4. Pick one number for the whole house and mark it with a combination square or a reveal gauge on every jamb before you cut anything. An inconsistent reveal is the single most common reason miters look open when they are actually cut correctly.
Worked example: a standard interior door
A 32 in door, head jamb underside at 82 in above the finished floor, 2-1/4 in casing, 3/16 in reveal.
- Leg long point: 82 + 0.1875 + 2.25 = 84.44 in
- Head long point: 32.125 + 0.375 + 4.5 = 37.0 in
- Per side: (84.44 x 2) + 37.0 + 3 = 208.9 in, or 17.4 LF. Call it 17.5.
Look hard at that leg number. 84.44 in is longer than a 7 ft stick. Casing is stocked in 84 in, 92 in, 96 in and 14 ft lengths, and the 84 in stick, which is the one most people grab for a door, is half an inch too short to make a mitered leg on a standard 80 in door. It is not close, and it is not fixable. That single fact wrecks more casing orders than anything else.
The remodel schedule
Build a schedule of openings, not a footage total. Doors get cased on both sides unless they are exterior. Cased openings get both sides plus jamb material. Windows get four sides if you are picture framing, or two legs, a head, a stool and an apron if you are not.
| Opening | Qty | Sides cased | LF per side | Total LF |
|---|---|---|---|---|
| Interior door, 32 x 80 | 8 | 2 | 17.5 | 280 |
| Exterior door, 36 x 80, inside only | 2 | 1 | 18.0 | 36 |
| Window, 36 x 48, picture framed | 9 | 1 | 16.0 | 144 |
| Cased opening, 60 x 84 | 3 | 2 | 20.5 | 123 |
| Net total | 583 LF |
The window figure comes from the same formula run four-sided: legs at 48 + 0.375 + 4.5 = 52.875 in each, head and sill trim at 36 + 0.375 + 4.5 = 40.875 in each, plus cut allowance, which totals about 16 LF. The cased openings are 84 in tall and 60 in wide, giving legs of 86.4 in and a head of 65 in, about 20.5 LF a side.
Add 10 percent for miscuts, split miters and boards you reject for grain, and the order is about 641 LF. On stain grade we would go to 15 percent, because you are also culling for color.
Buy the right lengths, not the right footage
641 LF is meaningless at the counter. What matters is which stick makes which piece.
| Stock length | What it yields | Drop |
|---|---|---|
| 84 in | 2 door heads, or 2 window heads. No door legs. | about 6 in |
| 92 in | 1 door leg | about 6 in |
| 96 in | 1 cased opening leg, or 1 door leg plus scrap | about 8 in |
| 168 in (14 ft) | 1 door leg + 1 door head + 1 window head | about 3 in |
That last row is the one to build the order around. A 14 ft stick cut into an 85-1/2 in leg, a 38 in door head and a 42 in window head uses 165-1/2 of 168 in. Buying 14 footers and cutting to a plan is how a casing order lands at 8 percent waste instead of 25.
Details that decide whether the miters close
- The jamb has to be flush with the drywall. If the jamb sits proud, the casing rocks and the miter opens at the outside. If it sits shy, the casing bridges and the miter opens at the inside. Plane down a proud jamb or shim behind the casing before you blame your saw.
- Back-cut the miter a degree or two. Removing a hair off the back face lets the two front edges meet under pressure. Glue the miter face, pin it from both directions so the pins cross, and it stays closed through a heating season.
- Plinth blocks change the math. A plinth at the floor lets the leg butt into it square, which drops the leg to about 78 in and puts 84 in stock back in play. It also removes the leg-to-floor scribe on an out-of-level floor.
- Stool and apron are separate. If your windows get a traditional stool, that is a different profile in a different thickness, sized to the opening plus horn projection on both sides. Do not fold it into the casing count.
One practical sequencing note: on a full remodel we set all the jambs and mark all the reveals in one pass, then cut casing in one pass. Batching the setup is what keeps thirty openings looking like they were built by the same person on the same day.
Related guides
- How Much Baseboard for a 3-Bedroom House
- Why Trim Waste Is Higher Than You Expect
- Crown Molding Linear Feet: Measuring With Miters
- Why Flooring Waste Is 7% Straight and 15% Diagonal
Sources and Further Reading
Run your own numbers with the Baseboard & Trim Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
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