What a prehung door is and why it matters

A prehung door comes already mounted in its frame with hinges attached and the latch hole drilled. You buy the whole unit as one piece, set it into the opening, and find it. This is different from buying a slab door (just the panel) and a separate frame, which requires you to attach hinges and drill holes yourself. For most interior replacements, a prehung door cuts the work in half and reduces the chance of misaligned hinges or a door that sticks.

The trade-off is cost: prehung doors cost more than slabs. But if you are replacing a door in an existing frame, a prehung unit lets you skip the carpentry steps that trip up most first-timers. The frame it comes in may not fit your opening exactly, so you will need to shim it level and plumb, but that is simpler than building a frame from scratch.

Key Takeaways

  • Measure your existing door opening width, height, and depth before ordering; prehung doors come in standard sizes, and the frame thickness must match your wall.
  • Remove the old door and frame by cutting the nails that hold the frame to the studs, then pry the frame out piece by piece.
  • Set the new prehung unit in the opening, shim it level and plumb on all sides, and nail through the frame into the studs.
  • Install the doorknob and latch hardware after the frame is secured, then test the door swing and latch before finishing the trim.

Measuring your opening and choosing the right door

Measure the width of your current opening at the top, middle, and bottom. Measure the height on the left, center, and right. If the numbers vary by more than a quarter inch, the opening is out of square and you may need to shim more aggressively or have the opening squared before installation. Write down the largest width and height you find.

Prehung doors come in standard sizes: 24, 28, 30, 32, and 36 inches wide, and 80 inches tall for standard ceilings. The frame thickness (usually 4 9/16 inches for a 2-by-4 wall) must match your wall. If your wall is thicker or thinner—common in older homes or walls with extra insulation—you may need a custom frame or shims to fill the gap. Bring a photo of your wall cross-section to the supplier, or measure the thickness yourself by looking at the edge of the wall where a outlet or switch is cut out.

Check the door swing direction: does it open into the room or out? Does the knob go on the left or right? Prehung doors are sold as left-hand or right-hand, which refers to which side the hinges are on when you face the door from the room it opens into. Order the swing that matches your old door, or choose the swing that makes sense for your room layout.

Removing the old door and frame

Close the old door and remove the knob and latch hardware by unscrewing the bolts on the inside. If the bolts are stuck, spray them with penetrating oil and wait 15 minutes. Lift the door off its hinges by supporting it from below and pushing up gently on the hinge pin side. Set the door aside.

The frame is held to the studs with nails driven through the frame edge into the wood. Use a reciprocating saw or a handsaw to cut through these nails from inside the room. Start at the top corners and work down both sides, then cut the nails at the bottom. Once the nails are cut, pry the frame away from the studs with a pry bar, working from the bottom up. The frame will come out in pieces—the two sides first, then the head (top piece). Do not worry about damaging it; you are throwing it away.

Clean out any old shims, caulk, or debris from the opening. Look at the studs: if they are not straight or plumb, you will need to shim the new frame to compensate. A 4-foot level will show you how far off plumb the studs are.

Setting the prehung door in the opening

Unpack the new prehung door and set it in the opening without fastening it yet. The frame should rest on the floor or on shims you place under it. Check that the frame is level across the top by laying a 4-foot level on the head. If one side is higher, place shims under the low side until the head is level.

Check that each side (jamb) is plumb by holding a level against it vertically. If a jamb leans in or out, place shims behind it at the hinge locations and at the latch location (usually 12 inches from the top and bottom). Tap shims in from behind the frame until the jamb is plumb. Shims are thin tapered wedges; push them in as a pair from opposite sides so they do not bend the frame.

Once the frame is level and plumb, nail through the frame into the studs. Drive nails through the frame edge where you placed shims, so the nail goes through the shim and into the stud. Use 2.5-inch finish nails or casing nails, and drive at least two nails per side. Do not overdrive; the frame should sit snug but not crushed.

Installing the knob and latch hardware

Most prehung doors come with the latch hole already drilled in the door panel. The knob and latch bolt are usually sold separately. Slide the latch bolt into the hole in the edge of the door, with the bolt facing the frame. find it with the screws provided.

On the inside of the door, install the knob and interior trim ring according to the hardware instructions. On the outside, install the exterior knob or handle. Tighten all screws firmly so the knob does not spin.

Check that the latch bolt aligns with the strike plate hole in the frame. If it does not, loosen the strike plate and move it up or down slightly. The bolt should slide smoothly into the strike without binding.

Testing the door and finishing

Close the door and check that it swings freely without rubbing on the frame. If it binds, the frame may not be plumb or the hinges may need adjustment. Open the door fully and make sure it does not swing shut on its own (a sign the frame is tilted). Test the latch: it should catch when you close the door and release smoothly when you turn the knob.

Once the door operates correctly, you can install trim around the frame. Caulk the gap between the frame and the wall, then install baseboard and casing to match the rest of the room. If you are painting, do that before caulking so paint does not seal the caulk joint.

Common mistakes and how to avoid them

The most common error is not checking that the frame is level and plumb before nailing. A frame that is tilted will cause the door to swing open or closed on its own, and you cannot fix that by adjusting hinges. Always use a level and shims before driving any nails.

Another mistake is ordering the wrong door swing. Before you buy, stand in front of your old door and note which side the hinges are on and which way it opens. Take a photo if you are not sure. Returning a prehung door because the swing is wrong is expensive and time-consuming.

Do not force the frame into an opening that is too small. If the frame does not fit, stop and check your measurements. Forcing it will bend the frame and make the door stick. It is better to return the door and order the correct size.

Frequently Asked Questions

Can I reuse the old frame if it is still in good condition?

Reusing the frame is possible but not recommended for most interior doors. The old frame is likely warped or out of plumb after years of use, and trying to fit a new door into it often results in binding or gaps. A prehung unit with a new frame costs more upfront but saves frustration and rework later.

What if my opening is slightly smaller than the prehung door I ordered?

If the difference is less than a quarter inch, you can carefully trim the sides of the frame with a circular saw or handsaw. Mark a line on the frame edge and cut slowly. Do not remove more than a quarter inch total or the frame will be too weak. If the opening is more than a quarter inch too small, return the door and order the next smaller size.

Do I need to remove the door from the frame to install it?

No. Most prehung doors come with the door already hung on the hinges. You can install the whole unit as one piece. Removing the door makes it easier to fit the frame into a tight opening, but it is not necessary if you have room to maneuver.

How long does the installation take?

A straightforward replacement in a square opening takes two to four hours, including measuring, removing the old door, setting and shimming the new frame, and installing hardware. If the opening is out of square or you need to adjust the frame multiple times, add another hour or two.

What should I do if the door sticks after installation?

First, check that the frame is plumb with a level. If it is tilted, the door will bind. If the frame is plumb, the hinges may need adjustment. Loosen the hinge screws slightly and tap the door into position, then retighten. If the door still sticks, the frame may be warped and you may need to remove it and start over.