Window trim — the casing that frames the window on the interior wall — takes a beating. Sun exposure warps and cracks it, moisture rots it (especially on bathroom and kitchen windows), and paint buildup makes it look lumpy and tired. Replacing window trim is one of those projects that has an outsized visual impact. New crisp casing around even old windows makes a room look renovated. The project takes one to two hours per window and costs $15 to $40 per opening in materials.

Window Trim Replacement

Choosing the Trim Style

Traditional Mitered Casing

Four pieces of molding mitered at 45 degrees at each corner, framing the window like a picture frame. This is the most common residential window trim treatment and matches most home styles.

Profile choices: Colonial casing (rounded profile, $0.80 to $1.50 per linear foot) is the default in most homes. Ranch casing (flat with a slight edge detail) gives a cleaner, more modern look for $0.60 to $1.20 per foot. Craftsman casing (flat stock with no profile) suits Arts and Crafts and modern homes.

Craftsman/Farmhouse Style

Flat casing on the sides and top with a thicker head piece (header board) across the top. The header is a wider, flat board (often a 1x4 or 1x6) that extends past the side casings by 1/4 to 1/2 inch on each side. No miters needed — all cuts are square, making this the easiest style to execute.

Stool and Apron (With Window Sill)

Traditional windows have a stool (the interior sill that extends into the room) with an apron beneath it (a short piece of trim under the stool). The side casings rest on top of the stool. This is the classic treatment for double-hung windows and provides a small shelf for plants or decoration.

Tools and Materials

Miter saw (for mitered casing) or hand saw for square cuts
Brad nailer with 1.5 to 2-inch brads
Flat pry bar
Utility knife
Tape measure and pencil
Combination square
Caulk and caulk gun
Wood putty
Sandpaper (120, 220 grit)
Paint or stain
Replacement casing material

Removing Old Trim

Score the paint line between the casing and wall with a utility knife. This prevents tearing the wall paint and drywall paper when you pry.
Start at one end of the casing. Slide a thin putty knife behind the casing to separate it from the wall.
Follow with a flat pry bar. Work slowly along the length, pulling the casing away from the wall and jamb. Pry at nail locations to minimize wall damage.
Remove all nails from the wall and jamb. Pull protruding nails through the back of the old casing with pliers (pulling forward through the face splinters the wood).
Scrape any remaining caulk or paint buildup from the window jamb edge.

If the window jamb itself is damaged (rotted, water-stained, warped), address that before installing new casing. A jamb extension or repair with epoxy wood filler may be needed.

Measuring and Cutting

Reveal

Set the casing back 3/16 inch from the inside edge of the window jamb. This small offset — called the reveal — creates a shadow line that hides minor imperfections between the casing and jamb. Mark the reveal on all four jamb edges using a combination square set to 3/16 inch. Run the square along the jamb edge and mark with a pencil at several points. Connect the marks.

Mitered Corners

Measure between the reveal marks on the head jamb (top). This is the short point measurement for the head casing.
Cut the head casing with 45-degree miters on each end, measured short point to short point.
Nail the head casing in position, aligning with the reveal marks.
Measure from the reveal mark at the bottom of the side jamb to the short point of the head casing miter. This is the side casing length.
Cut each side casing with a 45-degree miter at the top and a square cut at the bottom.
Dry-fit each side piece. If the miter joint is not tight, adjust the angle on the miter saw by half a degree and re-cut.

Tip: Measure and cut each piece individually. Never assume opposite sides are the same length — they rarely are in older homes where settling has shifted the framing.

Square-Cut (Craftsman) Style

Cut the head board to the window width plus the two casing widths plus 1/2 inch of overhang on each side
Cut the side casings to fit between the window stool (or bottom reveal mark) and the underside of the head board
Nail the head board first, then butt the side casings under it

No miters, no angle cuts. This is why Craftsman style is popular with DIYers.

Installation

Nailing Pattern

Shoot brads in two rows:

One row through the casing into the window jamb (thin edge, closer to the window)
One row through the casing into the wall framing (outer edge, into the jack stud and header behind the drywall)

Space nails 12 to 16 inches apart. Use 2-inch 18-gauge brads for standard 5/8-inch thick casing. Pre-drill if nailing hardwood casing to prevent splitting.

Gluing Miter Joints

Apply a thin layer of wood glue to both faces of each miter joint before nailing. The glue bond is stronger than the brad nails and prevents the miter from opening as the wood shrinks seasonally. Pin the joint from both directions with brads to hold it while the glue cures.

Dealing with Gaps

Walls are never perfectly flat. The back of the casing may not sit flush against the wall along its entire length. Minor gaps (under 1/8 inch) are filled with caulk in the finishing step. Larger gaps indicate a wall bulge or depression — shim behind the casing at nail locations to keep the face straight, even if the back is not fully contacting the wall.

Stool and Apron (If Replacing the Sill)

Cut the stool from 1x4 or 5/4 x 4 stock. Length equals the window width plus the two casing widths plus 1 inch of horn (extension past the casing on each side).
Notch the stool to fit around the window jamb legs — the stool extends into the room about 3/4 inch past the casing face.
Level and nail the stool to the rough sill.
Install the side casings, resting on the stool surface.
Cut the apron (same profile as the casing) to the length between the casing outside edges.
Nail the apron under the stool, flush with the casing edges.

Finishing

Set all nail heads below the surface with a nail set
Fill nail holes and any miter gaps with wood putty. For painted trim, lightweight spackle works and dries faster.
Sand filled areas flush with 220-grit
Caulk the joint between the casing and the wall with paintable latex caulk. Run a thin, consistent bead and smooth with a wet finger.
Do not caulk the joint between the casing and the window jamb — this joint is already tight against the reveal and caulk there looks messy.
Prime with a stain-blocking primer and apply two coats of semi-gloss or satin trim paint.

For stained trim, apply stain before installation (staining installed trim results in stain bleeding onto the wall). Apply finish coats after installation, masking the wall.

Bottom Line

Window trim replacement is a one-to-two-hour project per window that makes a visible difference in any room. Use the Craftsman flat-stock style if you want the easiest installation with no miter cutting. Use mitered colonial casing for a traditional look. Either way, set a 3/16-inch reveal, nail into framing, glue the miters, and caulk the wall joint. Total cost per window: $15 to $40. An entire house worth of windows can be re-trimmed over a few weekends with a miter saw and a brad nailer.