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How To Measure For A Prehung Door? (Easy Steps For A Perfect Fit)

How To Measure For A Prehung Door? (Easy Steps For A Perfect Fit)

Installing a pre-hung door is an important part of any home renovation project. It’s vital to take the time to measure accurately so that the right-sized door is ordered and ultimately installed.

Doing this well will not only ensure you get the replacement door you want but can also save money and time in the long run.

This article will look at step-by-step guidance for accurately measuring a prehung door. We’ll also provide tips on ensuring a perfect fit every time!

Understanding New Prehung Door Measurements

A prehung door is an entire unit with both the frame and door slab already connected.

The door slab will be pre-cut to fit the size of your opening, while the frame includes side jambs and a header. This makes door installation incredibly simple and efficient, as you don’t have to measure each piece separately.

Prehung doors come in interior and exterior styles and are available in various sizes, finishes, materials, and designs.  Interior doors are generally smaller and lighter than exterior doors. When selecting the right prehung door, consider your home’s style and the type of material you want to use (wood, steel, fiberglass).


All measurements must be precise in order to ensure the perfect fit. Before you start measuring for a prehung door, gather the necessary tools and materials. This includes:

  • Tape measure
  • Painter’s pry bar
  • Pencil
  • Notepad or paper to record measurements
  • Level for checking plumb (optional)

It is advised to have a helper during this process.

Step-by-Step Guide on How To Measure For A Pre-Hung Door

Step 1: Measuring the Slab Door

The first step when measuring for a prehung door is to measure the slab door itself. This includes the height of the door frame along with its width and depth.

Start by measuring the width of the existing door opening with your tape measure and record this measurement on your notepad. The standard size for an interior prehung door is 32 inches wide, but most manufacturers offer custom sizes.

Next, measure the height of your existing door slab and record this number as well. The standard door is 80 inches high, though some styles may vary by a few inches either way.

Finally, measure the depth of the existing door slab and record this measurement. Interior doors typically range from 1¾ inches to 2⅜ inches in depth. Exterior doors are even thicker, depending on your selected material and style.

Step 2: Measuring the Rough Opening

Rough openings are the empty spaces you’ll see when taking a door out of its opening. Measuring these accurately will ensure your pre-hung door is the correct size and fits perfectly.

Measure the width of the rough opening from one side to the other, taking note of any irregularities.

Measure this distance at both the top and bottom of the opening, as walls can be slightly out of plumb (not perfectly vertical). Then measure the height from top to bottom, taking note of any irregularities.

Step 3: Measuring Wall Studs and Header

Measuring Wall Studs and Header

Image Credit: inniskeenjoinery

Next, measure the width of the wall studs on each side of the rough opening. This will help you determine the size of the prehung door to purchase – the width of the door should be two inches smaller than the distance between the wall studs.

Finally, measure the height and thickness of the header (the beam above the rough opening) and record this on your notepad. The thickness of your prehung door’s frame must match that of the header for proper installation.

Step 4: Measuring Existing Door Jambs and Sill

If replacing an existing door, now is the time to measure the door jamb and sill. The jamb is the vertical frame on either side of the door, and the sill is the horizontal piece at the base.

Measure both widths and heights, including any molding or trim that is present.

Step 5: Deciding Door Swing

The door swing refers to the direction that your door will open and close. A right-handed door swings outward when opened, while a left-handed door swings inward.

To make the final decision, consider the existing setup and determine if a right-handed or left-handed door would be more practical.

Once you’ve determined your preferred door swing, measure from the side jamb to the center of where the knob will go and record this measurement. This will help you purchase a pre-hung door to ensure it’s the correct size.

Different manufacturers have different specifications for their doors, so take a moment to familiarize yourself with the manufacturer’s guidelines before purchasing.

Step 6: Purchasing a Prehung Door

Take all of your measurements with you when purchasing the door so that you can select one that fits your existing space perfectly. When selecting your prehung door, ensure it has at least one inch of extra space on each side for shimming (light adjustment) during installation.

This will ensure a perfect fit in the frame and prevent unnecessary gaps.

You will also need to consider the thickness of the wall when purchasing a prehung door and your best interior and exterior trim styles. Make sure these match the measurements you took earlier for a perfect fit.

Step 7: Installing a Prehung Door

Installing a pre-hung door is relatively simple but requires patience and precision. With the right tools and some elbow grease, you can do it in no time.


Begin by removing any existing trim or molding, and then use a utility knife to cut away the carpet and drywall around the opening.  Make sure all existing nails are removed from the rough opening using a pry bar or hammer. Then check to ensure the door’s inside and the backside is plumb (vertical).

Once you’ve completed these steps, you’re ready to install your pre-hung door.

  • Start by placing the door in position and then use a level or straight edge to check that it is perfectly aligned. You may need to adjust the door using shims to get it exactly right.
  • Next, attach the side jambs of the prehung door to the wall studs on either side of the opening. Use wood screws and two-inch nails to secure them and ensure they are flush with the rough opening. After that, you can attach the header and then the sill.
  • Once everything is in place, drill pilot holes for the hinges and attach them to the latch side jamb of the door frame with screws. Attach the knob or handle on the outside door (if needed), hang any molding or trim you plan to install, and you’re done!

Step 8: Finishing Touches

You may consider installing a deadbolt lock with the knob or handle for added security. You can choose from various styles and finishes to match your door.

Ensure any weatherstripping has been installed correctly to keep out drafts and moisture. Make sure there are no gaps between the door and the jamb, sill, or header. You can use a caulking gun to fill gaps and ensure the weatherstripping is flush with the door frame.

Take extra care when painting or staining your new pre-hung door –  follow all manufacturer’s instructions and allow proper drying time between coats.

Tips and Tricks for Installing a Pre-hung Door

  • Measure twice and cut once! Accurate measurements are key to getting the perfect fit for your pre-hung door.
  • Always use quality hardware for utmost strength and durability.
  • Consider using shims to adjust the door in place and ensure a secure fit.
  • For added security, consider installing a deadbolt lock with the door handle or knob.
  • Use weatherstripping to ensure a snug fit between the door and jamb, sill, and header.
  • Take extra care when painting or staining your door to get the perfect finish.

Maintenance Tips

To keep your pre-hung door looking and functioning at its best, you must perform regular maintenance.

  • Check that all hinges and screws are tight and free from rust or corrosion.
  • If the door has a knob or handles, ensure it turns smoothly and does not stick.
  • You should also inspect the weatherstripping regularly to check for gaps or tears. If any are found, replace them as soon as possible and ensure no debris is blocking the seal.
  • Take some time every year to clean and wax your door. This will protect the finish and keep it looking great for years to come.

Things to Avoid

  • Never use old hardware for your pre-hung door. Always get new, quality materials for a secure fit.
  • Avoid drilling too large of a hole in the door frame or jamb. This could weaken the structure and make it easier for an intruder to break in.
  • Do not use the wrong type of nails when attaching the door frame. Make sure they aren’t too short or long, as this could cause problems down the line.
  • Never paint or stain a pre-hung door until the manufacturer’s instructions have been followed and proper drying time has been allowed. Failing to do so could lead to an uneven finish.
  • Finally, never attempt to force the door into place if it doesn’t fit properly. Take the time to adjust it correctly using shims, otherwise, you could end up with an uneven door.

Final Thoughts

Measuring a prehung door is an important part of the installation process. By taking accurate measurements, your new door will fit perfectly in place and give you years of satisfaction.

Measure both the rough opening and the prehung door itself and the thickness of your wall. Then double-check all measurements to make sure they are correct before you begin the installation.

Installing a pre-hung door will give your home an instant upgrade and provide you with extra security. With patience and precision, you can easily install your pre-hung door and enjoy its beauty for years.