Short answer
Measure the opening width between the side jambs and the height from the floor to the head jamb. For mitered casing, each leg is the height plus the reveal plus the casing width, and the head is the width plus two reveals plus two casing widths, both to the long point. One side of a 32 x 80 in door with 2-1/4 in casing takes about 17 linear feet.
Step 1: measure each opening
Width
Measure between the side jambs, inside face to inside face, near the top. Write it down to the nearest 1/16 in.
Height
Measure from the finished floor to the underside of the head jamb on each side. The two sides often differ, so record both.
Casing width
Note the width of the casing you will buy, for example 2-1/4 or 3-1/2 in, and the reveal you will use, typically 3/16 in.
Sides
Note whether you are trimming one side of the door or both.
Step 2: turn it into lengths
| Piece | Mitered head | Butted head |
|---|---|---|
| Each leg | H + reveal + casing width (long point) | H + reveal (square cut) |
| Head | W + 2 reveals + 2 casing widths (long point) | W + 2 reveals + 2 casing widths + overhang |
| Per side | 2 legs + 1 head | 2 legs + 1 head |
Example: a 32 x 80 in door with 2-1/4 in casing and a 3/16 in reveal. Each leg is 80 + 3/16 + 2-1/4 = 82-7/16 in. The head is 32 + 3/8 + 4-1/2 = 36-7/8 in. One side totals about 201-3/4 in, or 16.8 ft. The Home Depot guide works the same way: a 36 in opening with a 3/16 in reveal gives a 36-3/8 in head between short points (Home Depot guide).
Step 3: what to buy
- Stock length matters. A mitered leg on an 80 in door with 2-1/4 in casing is about 82-1/2 in, which fits a 7 ft (84 in) piece. With casing wider than about 2-3/4 in it no longer fits; buy 8 ft stock.
- Heads pack together. Two heads for 30 or 32 in doors fit in one 7 ft piece.
- Add spares. 10 percent extra, or one spare piece per few doors.
- Standard doors? A trim kit skips most of the math.
Step 4: measure again at the saw
Totals tell you what to buy. When you cut, measure each piece in place: fit the head first, then mark each leg against it at the floor. Fine Homebuilding cuts the legs long and marks them in place for this reason. Full steps: how to install door casing.
Questions people ask
How do you measure door trim length?
Measure the opening between the jambs and from the floor to the head jamb. Add the reveal (often 3/16 inch) and the casing width to get each leg to the long point of its miter, and add two reveals and two casing widths to the opening width to get the head to its long points.
Do I measure from the inside or outside of the door jamb?
Measure inside to inside, between the faces of the jambs, which is the width of the opening. The reveals and casing widths are then added to that.
How many linear feet of trim per door?
About 17 feet per side for a standard 80 inch tall door with 2-1/4 inch casing, a little more for wider doors and wider casing. Fixr suggests planning on at least 17 linear feet per door.
Sources
Last reviewed October 12, 2026. Profiles, sizes and listings change; check the product details before you buy, and follow the maker’s instructions.