How To Measure And Install Prehung Interior Doors For Replacement

November 15, 2021

By: The Finished Space

Prehung doors are much easier to install than slab doors since the frame is already attached. Additionally, the doorknob hole is already cut, and the hinges are already mounted as well.

In other words, prehung doors come packaged and ready to go, making installation a breeze. However, you still need to know what you’re doing.

Let’s discuss how to measure for, and install, a prehung door so that you’re prepared for your next job.

Selecting the Right Prehung Door for Your Project

Masonite - Riverside - Yellow Bedroom

When you buy a prehung door, there are several things you need to consider, and measurements you need to: 

Measure the Rough Opening

You need to ensure the door frame will fit into the rough opening. You want the rough opening to be slightly larger than the dimensions of the prehung door. (More on this in a minute). 

Calculate the Jamb Width

You have to measure the jamb width, aka the width of the frame or the thickness of the wall. (More on how to do this below). 

Figure Out Your Door Swing

Take time to figure out the door swing, so you can determine whether you need a right-handed prehung door or a left-handed one. You can do this by going to the hinge-side of the frame and extending your arm in the direction the door should open. If you reach your left arm out, then you want a left-handed prehung door. If you reach your right hand out, you want a right-handed prehung door. 

Choosing Between Outswing and Inswing

Most interior doors swing inwards but be sure to decide which you want before buying a door. 

Know What Material You Want

Doors come in various materials, including wood, metal for exterior doors, MDF, and more. Be sure to know which material you want before you pick it out.

Measure Your Doorknob

The majority of prehung doors come with the doorknob holes precut, and they will fit most standard door knobs. However, some vintage or customized door knobs don’t fit the standard hole, so it’s always a good idea to get the doorknob first to make sure the prehung door will accommodate the desired knob.

How to Measure Prehung Interior Doors for Replacement

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Before you select your door, you need to know how to measure for prehung doors, because if you measure the rough opening or jamb width incorrectly, you could end up with an inoperable door.  

Measure the Rough Opening

You’ll first need to remove any trim from around the opening. 

Then, measure the width between the studs in three places: between the center points, between the highest points in the opening, and between the lowest points in the opening. Write down the smallest of the three measurements. 

Measure the height of the opening from the bottom to the top of the sill on both sides and write down the smallest of those two measurements. 

Use the two written measurements to compare against prehung door measurements. 

Not all rough openings are completely square and level, so you want to account for this by leaving a little extra space. The opening should be between one and two inches larger than the door so that you can shim the door into place.  

Measure the Jamb Width

The door jamb is simply the width of the prehung door’s frame. To measure the door jamb width, simply measure from the backside of the trim to the backside of the interior trim.

Then, make sure to specify this width or look for it when purchasing your interior door.

Measure the Size of The Door

You will need to have an idea of the door size you want. If you still have the old door, it’s really easy. Simply measure the width and height of the old door and round them up to the nearest inch.

If you don’t have the door to use as a reference, know that most doors are generally around 6 X 8 feet, with a width of two feet, six inches.

How to Install a Prehung Interior Door

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Once you have the new prehung door measured and ordered, it’s time to install it. Here’s how.

Start by removing the existing door and the frame (if applicable). Then, position the door into place by sliding it into the rough opening. Slide it until the jamb is even with the wall. Be sure the door opens in the right location and direction.

Then, use a level to ensure the door is straight, and add shims to make sure the jamb is aligned. Remember that the door frame will be slightly smaller than the rough openings, so you’ll need to slip shims under and between the side jambs and studs until the head jamb is level and the spaces between them are closed.

On the hinge side, begin with the top and bottom of the jamb, and shim between the studs and hinges, placing the shims where about half of it is above the hinge. On the latch side, place the shims around the latch level.

Once again, make sure the door is level and operable. Then, nail the door frame to the studs using two #8 finishing nails, and nail two nails into each shim around an inch from each edge of the jamb.

Finally, install the trim, and attach the doorknob and backplates. 

Choosing the Right Prehung Interior Door 

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That’s it! After completing these steps, your prehung door should be attached and ready to go. Remember to measure beforehand and continue testing the door’s operation and levelness throughout the installation process.

Do you need help choosing a door for your next project? Check out Option {M} today, our design tool to help contractors find the perfect finishing touches for their projects.