Installing a door frame is one of those home projects that looks intimidating but follows a clear, repeatable process. A prehung door — where the door comes already mounted in its frame with hinges installed — simplifies the job enormously. You are essentially sliding a complete unit into the rough opening and shimming it plumb, level, and square. The whole project takes two to three hours with basic tools and produces a door that opens, closes, and latches properly.

Install a Door Frame

Prehung vs Slab Doors

A prehung door is the right choice for frame replacement or new openings. The door is pre-hung on hinges in a pre-built jamb frame. The hinge mortises are already cut, the bore hole for the knob is drilled, and the strike plate mortise is routed. You install the entire unit as one piece.

A slab door is just the door — no frame, no hinges, no hardware. You hang it in an existing frame. Only use a slab door when the existing frame is in good condition and you just want a different door style.

For frame replacement (the topic of this guide), buy a prehung door. Interior prehung doors cost $80 to $250 depending on material, style, and size. A basic 6-panel hollow-core prehung interior door runs $80 to $120 at any home center.

Tools and Materials

Prehung door unit (measured to fit your rough opening)
Cedar or composite shims
2.5-inch finish nails or brad nailer with 2-inch brads
3-inch screws (replace at least two hinge screws with these for structural support)
4-foot level
Tape measure
Hammer
Utility knife
Low-expansion spray foam (optional, for insulation around the frame)
Door trim/casing

Step 1: Measure the Rough Opening

The rough opening is the framed hole in the wall where the door goes. Measure its width (side to side at the top, middle, and bottom) and height (floor to header).

Standard interior prehung doors are sized for specific rough openings:

24-inch door: 26-inch wide rough opening
28-inch door: 30-inch wide rough opening
30-inch door: 32-inch wide rough opening
32-inch door: 34-inch wide rough opening
36-inch door: 38-inch wide rough opening

The rough opening should be 2 inches wider and 2 to 2.5 inches taller than the door itself. This gap provides room for the jamb thickness plus shimming space.

If your rough opening is not standard sized, you have two options: resize the opening (move studs) or order a custom prehung door. Resizing is faster and cheaper in most cases.

Step 2: Remove the Old Door and Frame

If replacing an existing door:

Remove the door from the hinges. Tap the hinge pins out from below with a nail and hammer.
Pry the door casing (trim) off both sides of the wall using a flat bar. Work carefully if you plan to reuse the trim.
Remove the old jamb. Cut the nails holding the jamb to the studs using a reciprocating saw with a metal-cutting blade slid between the jamb and stud. Pull the jamb pieces out.
Clean up the rough opening. Remove old shims, protruding nails, and debris.

Step 3: Test-Fit the Prehung Unit

Set the prehung door into the rough opening without fastening it. Check:

Does it fit with shimming space on both sides and at the top? (1/4 to 3/8 inch per side)
Is the floor level across the opening? If not, you will need to trim the jamb legs to different lengths.
Are the hinge-side and latch-side gaps even?

If the floor is not level (common in older homes), measure the difference and trim the longer jamb leg so the head jamb will be level when installed.

Step 4: Shim and Plumb the Hinge Side

The hinge side is your reference. Get it plumb and everything else follows.

Place the prehung unit in the opening. Center it roughly.
Starting at the top hinge, insert shims between the hinge-side jamb and the rough stud from both directions (cross-shim for stability). Tap the shims until the jamb is plumb. Check plumb with your 4-foot level on the face of the jamb.
Shim behind the middle hinge and bottom hinge, checking plumb each time.
Nail or screw through the jamb and shims into the stud at each hinge location. Use 2.5-inch finish nails or, better yet, remove the center screw from the top hinge and replace it with a 3-inch screw that reaches through the jamb, shims, and into the king stud. This single screw carries most of the door weight and prevents sagging over time.

Step 5: Shim the Head and Latch Side

Shim the head jamb (top) at two points — above each side of the opening. Adjust until the head jamb is level.
Close the door and check the gap between the door edge and the latch-side jamb. This gap (called the reveal) should be consistent — 1/8 inch from top to bottom.
Shim the latch-side jamb at the strike plate location, the top, and the bottom. Adjust shims until the reveal is even.
Nail through the latch-side jamb and shims at each shim point.
Shim behind the strike plate location and drive a screw through the jamb into the stud for structural support.

Step 6: Check the Door Operation

Open and close the door multiple times. Check for:

Consistent reveal on all three sides (1/8 inch)
No binding — the door should swing freely without rubbing
Proper latch engagement — the latch bolt should click into the strike plate smoothly
The door stays where you put it — if it swings open or closed on its own, the jamb is not plumb. Adjust shims.

If the door rubs at the top on the latch side, the hinge side needs to tilt slightly toward the hinge. Adjust the bottom hinge shims.

Step 7: Secure and Insulate

Score the shims flush with the jamb edges using a utility knife and snap them off cleanly
Optionally, apply low-expansion spray foam (use “window and door” formula — regular expanding foam generates enough pressure to bow the jamb and prevent the door from closing) in the gap between the jamb and rough opening for insulation and draft sealing
Let the foam cure fully before installing trim

Step 8: Install Casing (Trim)

Door casing covers the gap between the jamb and the wall surface:

Set the casing back 3/16 inch from the inside edge of the jamb (called the “reveal”). This small offset creates a shadow line and hides imperfections.
Miter the top corners at 45 degrees using a miter saw for a traditional look. Or butt the head casing against square-cut side casings for a Craftsman look.
Nail the casing to both the jamb and the wall framing with 2-inch finish nails or brad nails.
Set the nail heads with a nail set, fill with wood putty, sand, and paint or stain.

Step 9: Install Hardware

If the prehung door did not come with a knob installed:

Insert the latch bolt into the edge bore hole
Install the knob or lever through the face bore hole, threading through the latch mechanism
Tighten the mounting screws
Test the latch operation

For replacement hardware on existing doors, the process is the same.

Bottom Line

A prehung interior door installation is a three-hour project that transforms a room. Get the hinge side plumb first, shim patiently for consistent reveals, and replace at least one hinge screw with a 3-inch screw into the framing. That one structural screw prevents the most common door problem — top-corner sag — for the life of the door.