How To Trim a Window

Updated: Apr. 01, 2024

You don't even need a tape measure for perfect window casings!

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Most trim carpenters don't even use a tape to trim windows. It's all done by eye, with a sharp pencil, a miter saw and an 18-gauge nailer. Here's how they do it.

Tools Required

  • Air compressor
  • Air hose
  • Brad nail gun
  • Caulk gun
  • Hammer
  • Miter saw
  • Safety glasses
  • Utility knife

Materials Required

  • Extra pencils
  • Trim
  • Wood glue
  • Wood shims

Avoiding Headaches When Installing Window Casings

Here are a few tips to help you avoid a few hassles with interior trim when installing window casings:

  • Whenever you can, cut with the thick side of the trim against the miter saw fence. You’ll be less likely to tear out the narrow tapered edge that way.
  • Cutting right up to the pencil mark almost always leaves pieces too long, so remove the pencil line with the blade. You’ll most likely still have to shave off more.
  • Sneak up on cuts by starting long and dipping the saw blade into the wood while you work your way to the cutoff mark.
  • Trim out the biggest windows first. That way, you can reuse miscuts for the smaller windows and not run out of material.
  • When nailing 3/4-in.-thick trim, use 15-gauge 2-1/2-in. nails for the framing and 18-gauge 2-in. brads for nailing to the jamb.
  • To prevent splitting, avoid nailing closer than 2 in. from the ends.

Project step-by-step (11)

Step 1

Mark the Length

installing window moldingFamily Handyman

When installing interior window trim, start at the top. Cut a 45-degree angle on one end of the trim and hold it so the short end of the angle overhangs halfway, or 3/8 in., onto the jamb. Then mark the other end flush with the inside of the jamb. That’ll give you a 3/16-in. reveal.

Step 2

Get the Spacing Right

top window trim piece Family Handyman

Place the top piece and hold the trim 3/16 in. away from the jamb at both ends and along the base of the window casing. Nail the trim to the jamb with 1-in. brads spaced about every 6 in. Nail the thick part of the trim to the framing with 2-in. brads.

Step 3

Check the Fit, then Cut to Length

cut window trim sides to length Family Handyman

Cut a 45-degree miter on one end of the trim board. Adjust the miter as needed for a perfect fit. Then scribe the cut length 3/16 in. past the bottom of the jamb. Nail the trim onto the jamb first and then to the window casing framing, as you did with the top piece.

Step 4

Glue and Pin for a Solid Miter

fasten window trim cornersFamily Handyman

Fasten corners and glue and pin together the miter from both directions with 1-in. brads. Wipe the glue squeeze-out with a damp rag right away.

Step 5

Trim the Other Side

finish sides of window trimming Family Handyman

Repeat all the same steps on the other side of the window, fitting first the top miter, and then marking and cutting the bottom one. Nail the window casing trim into place.

Step 6

Fit the First Bottom Miter

bottom piece of window trimFamily Handyman

Start the bottom piece. Cut an overly long piece of window casing trim and cut a miter on one end. Overlap the far end to check the fit. Mark and recut the miter as needed for a perfect fit.

Step 7

Fit the Opposite Miter

adjust final miter cut window trimFamily Handyman

Cut a test miter on the other end and check the fit. Adjust the miter as necessary until you’re satisfied with the joint.

Step 8

Scribe for Length

mark final window trim pieceFamily Handyman

Mark the final cut. With the saw still set for the previous miter, flip the trim over and scribe the length for the end that has that miter. Transfer the mark to the front side and make the cut.

Step 9

Dealing with Problem Drywall: Projects More than 1/8 in.

smash drywall around window trimFamily Handyman

If the drywall projects more than 1/8 in., crush in the drywall with a hammer. Just be sure the crushed area will be covered by trim. In this situation, your miters won’t be 45 degrees. You may need to go as low as 44 degrees to get a tight miter.

Step 10

Dealing with Problem Drywall: Projects Less than 1/8 in.

window drywall projects less than 1/8 inchFamily Handyman

If the drywall projects past the jamb 1/8 In. or less, and is close to the window jamb, just chamfer the edge with a utility knife. Check to see if you’ve pared off enough drywall by holding a chunk of trim against the drywall and jamb. If it rocks and won’t sit flush against both surfaces, carve out some more.

Step 11

Dealing with Problem Drywall: Drywall Too Low

drywall to low on window trimFamily Handyman

If the drywall’s recessed behind the jamb, don’t nail the trim to the framing at first. Only nail it to the jamb and pin the mitered corners together. After the window is trimmed, slide shims behind each nail location to hold out the trim while nailing, then cut off the shims. Caulk the perimeter of the window casing trim to eliminate gaps before painting.