Replacing an interior door does not always mean replacing the entire door frame. If the existing jamb is in good condition and the replacement door is compatible with it, a new door slab may be all you need.
The challenge is getting the measurements and configuration details right. A replacement slab has to fit the existing opening, align with the hinges, accommodate the existing hardware, and work with the way the door swings.
Before ordering a replacement interior door, take a few minutes to document the existing door carefully. This guide walks through the measurements and details you should record so you can determine whether a replacement slab is appropriate and what information you will need when ordering.
Tip: Do not rely on a standard door size based only on the opening. Measure the existing door and record its configuration before ordering a replacement.
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Caiman Slab |
Caiman Prehung |
Slab vs. Prehung: What Are You Measuring?
The first step is understanding what you are replacing.
A door slab is the door itself, without the surrounding jamb or frame. A slab can be supplied unfinished or finished and may be machined for hinges, bores, or other hardware depending on the order.
A prehung door includes the door and its jamb assembly. If your existing frame is damaged, out of square, difficult to match, or unsuitable for the replacement door, a prehung configuration may be more appropriate.
This guide is primarily for situations where you want to replace the existing door slab while keeping the existing frame.
Before measuring, inspect the existing jamb.
Look for:
- Cracked or damaged jamb components
- Loose or damaged hinges
- Significant warping
- Evidence that the opening is out of square
- Damaged hinge mortises
- Hardware that will be difficult to reuse
- An existing frame that does not properly support the replacement door
If the frame is in good condition, carefully matching the replacement slab to the existing configuration can avoid unnecessary frame replacement.
What You Need to Measure
For a basic replacement, start by recording:
- Door height
- Door width
- Door thickness
- Hinge locations
- Hinge size and preparation
- Door handing and swing
- Bore location
- Edge preparation
- Door style and panel layout
- Finish requirements
The exact information required can vary depending on whether the new slab will be factory-machined or prepared in the field.
Step 1: Measure the Door Height
Measure the existing door slab itself, not the rough opening.
Place your tape measure along the vertical edge of the door and measure from the bottom edge to the top edge.
Record the measurement as accurately as possible.
Do not assume that a door described as a nominal size will have exactly that measurement. Existing doors can have dimensions that differ from what you might expect based on the nominal size of the opening.
If the door has been altered previously, record the actual measurement you find.
Check the measurement in multiple places
If the door appears warped or damaged, measure at more than one location.
Compare the dimensions and note any meaningful differences. A significant difference can indicate that the existing door or frame is not square.
For a straightforward slab replacement, the goal is to document what is physically installed rather than estimating what the original door size was supposed to be.
Step 2: Measure the Door Width
Measure the width of the existing slab from one vertical edge to the other.
Again, measure the door itself, rather than the framed opening.
Record the measurement carefully.
If the door has unusual dimensions, do not round it to a common nominal size when documenting your measurements. The actual slab dimensions provide more useful information when determining the replacement configuration.
Also check whether the door edges are straight and whether the slab appears to have been trimmed.
Tip: If you are replacing only the slab, the existing door is one of your most useful sources of information. Photograph it and keep your measurements with the photos before removing anything.
Step 3: Measure the Door Thickness
Door thickness is another important measurement that should not be overlooked.
Measure the thickness along the edge of the existing slab.
Record the actual dimension rather than assuming it based on the door's style.
Thickness can affect the compatibility of the replacement with the existing hinges, hardware, jamb, and other components.
This is especially important when replacing a door with a different construction or when changing between different door types.
Step 4: Document the Hinges
The hinge side of the door contains some of the most important information for a slab replacement.
A hinge mortise is the recessed area cut into the door edge or jamb so the hinge leaf can sit at the intended depth. Proper hinge preparation helps the door align within the opening.
Record:
- Number of hinges
- Hinge locations
- Hinge size
- Hinge shape
- Hinge corner style
- Mortise dimensions, if relevant
- Distance of each hinge from the top or bottom of the door
The replacement slab needs to align with the existing jamb preparation if you plan to reuse the existing hinges and frame.
Photograph the hinge side
Take clear photographs of every hinge location.
A photo can help document details that are easy to overlook when measurements are being taken separately.
If the replacement door will be factory-machined, the hinge information should be verified against the hardware being used. Darpet's supplied glossary guidance specifically recommends matching hinge size, corner requirements, mortise depth, quantity, and hinge locations to the specified hardware.
Step 5: Determine Door Handing and Swing
Do not guess whether a door is left-hand or right-hand.
Door handing identifies the hinge side and operating orientation of a door, while door swing describes the direction in which the hinged door travels when opened.
Handing conventions can vary depending on the viewing side, so use the supplier's specified convention when ordering.
Record:
- Which side the hinges are on
- Which direction the door opens
- Whether it opens into or out of the room
- Which side the knob or lever is located on
Also check the surrounding space.
Furniture, walls, adjacent doors, and other objects can affect the practical swing of the door. The existing configuration should be documented before the slab is removed.
Tip: Take a photo of the door while it is open. A photo showing the room, hinge side, and swing direction can be much easier to reference later than a written note alone.
Step 6: Measure and Document the Bore
If the existing door has a knob or lever, document the location of the door bore.
The bore is the prepared opening used for the lockset or latch hardware.
Record the location of the bore in relation to the top and edge of the door. Also document the existing hardware if you plan to reuse it.
Different hardware systems can require different preparation dimensions, so the existing hardware should not be treated as interchangeable with every replacement door.
Take photographs of:
- The front of the hardware
- The edge of the door
- The bore
- The latch
- Any additional preparation
If you are changing hardware at the same time, note that the new hardware specifications may determine the required door preparation.
Step 7: Check the Door Edge
Look closely at the edges of the existing slab.
Record whether the door has:
- Square edges
- Beveled edges
- Existing hinge preparation
- Latch preparation
- Other machining
The edge condition can matter when matching a replacement slab to an existing jamb and hardware package.
If the replacement door will be factory-machined, these details should be confirmed before ordering.
Step 8: Identify the Existing Door Style
Measurements are only part of the replacement process.
You also need to identify the door's design.
Record the existing:
- Panel configuration
- Number of panels
- Panel shape
- Rail-and-stile layout
- Flush construction
- Louver configuration
- Glass or other inserts
- Decorative details
If the goal is to replace one damaged or outdated door while keeping the rest of the home's doors, matching the design may be just as important as matching the dimensions.
A replacement slab that fits physically but has a noticeably different design may not produce the appearance you want.
Step 9: Determine the Door Construction
If possible, identify the construction of the existing door.
For example, determine whether it is hollow core or solid core.
A solid-core door has a dense internal core that gives it greater weight and rigidity than a typical hollow-core door. It is commonly selected when greater durability, a more substantial feel, or improved sound reduction is desired.
This information matters because changing the door construction can affect:
- Weight
- Hinge requirements
- Overall feel
- Sound control
- Hardware compatibility
If you are replacing an existing solid-core door with another solid-core door, the existing hinges and frame should still be checked rather than assumed to be compatible.
Step 10: Check the Existing Frame and Jamb
If you intend to reuse the frame, inspect it before ordering the replacement slab.
The jamb includes the vertical and horizontal components surrounding the door. The hinge jamb is the side where the hinges connect the door to the frame, while the opposite side is the strike jamb.
Check that:
- The jamb is structurally sound
- The hinge side is secure
- The existing hinge preparation is usable
- The door closes properly
- The frame is reasonably square
- The strike and latch align
- The existing stop is in good condition
If the existing jamb is significantly damaged or the replacement requires a different configuration, replacing the complete prehung unit may make more sense than replacing only the slab.
What About the Rough Opening?
For a slab replacement, the rough opening is usually not the primary measurement.
A rough opening is the framed opening prepared to receive a door assembly. It is normally larger than the finished opening to allow for installation, shimming, leveling, and adjustment.
Rough-opening measurements become especially important when ordering a new prehung door or installing a new frame.
If you are keeping your existing jamb, focus first on the existing slab and the existing frame configuration.
What If the Wall Thickness Has Changed?
If you are replacing the entire prehung unit rather than just the slab, wall thickness becomes important.
A jamb extension can add width to a jamb so it accommodates a wall that is deeper than the base jamb dimension. This can be necessary when drywall, paneling, trim, or other wall finishes affect the finished wall thickness.
For a prehung replacement, measure the finished wall thickness and account for the wall finishes.
This is another reason to distinguish between a slab replacement and a complete prehung replacement before taking measurements.
Interior Door Replacement Measurement Checklist
Before ordering, make sure you have documented the following:
| Measurement or Detail | What to Record |
|---|---|
| Door height | Actual slab height |
| Door width | Actual slab width |
| Door thickness | Actual slab thickness |
| Hinges | Quantity, size, locations and preparation |
| Handing | Hinge side and operating orientation |
| Swing | Direction the door opens |
| Bore | Location and existing hardware |
| Edge | Edge profile and preparation |
| Style | Panel, flush, louver or other design |
| Core | Hollow core, solid core or other construction |
| Frame | Condition and compatibility |
| Finish | Existing or desired finish |
Keeping all of this information together makes it easier to discuss the replacement with a door supplier, contractor, or installer.
Common Measuring Mistakes to Avoid
Measuring the opening instead of the slab
If you are replacing only the door, the finished opening does not tell you everything you need to know about the existing slab.
Measure the door itself first.
Assuming the nominal size
Do not assume the existing slab has a standard dimension simply because it appears to be a common door size.
Record the actual measurements.
Forgetting thickness
Two doors with the same height and width can still have different thicknesses.
Always record it.
Ignoring hinge locations
A replacement slab needs to work with the existing hinge configuration if the frame is staying.
Guessing the handing
Handing and swing should be documented rather than assumed.
Measuring only one detail
A door can have the correct width and height but still require different hinge preparation, bore location, edge preparation, or hardware.
A replacement is a configuration, not just a set of dimensions.
When Should You Replace the Slab vs. the Whole Door?
A slab replacement can be a practical option when the existing frame is in good condition and the replacement can be properly matched to the opening.
A complete prehung replacement may be more appropriate when:
- The jamb is damaged
- The existing frame is unsuitable
- The configuration needs to change
- You are substantially remodeling the opening
- The existing hardware or hinge preparation cannot be reused
- The new door requires a different frame configuration
The decision should be based on the condition of the existing assembly and the requirements of the replacement—not simply on the age of the door.
What to Send Your Door Supplier
Once your measurements are complete, organize the information before requesting a replacement.
A useful package can include:
- Door height and width
- Door thickness
- Photos of both faces of the door
- Photos of the hinge side
- Photos of the existing hardware
- Hinge measurements and locations
- Bore measurements
- Door handing and swing
- Door style or collection
- Any finish requirements
- Photos of the existing jamb and opening
Providing this information up front can make it easier to determine whether a slab replacement is appropriate and what machining or configuration is required.
The more accurately you document the existing door, the less guesswork is involved in specifying its replacement.
Final Thoughts
Measuring an existing interior door for replacement is more than measuring height and width.
If you want to keep the existing frame, the replacement slab needs to work with the jamb, hinges, hardware, swing, and overall configuration. Taking the time to document these details before ordering can help prevent a replacement door from arriving with the wrong dimensions or preparation.
For homeowners, contractors, and builders, the simplest approach is to treat the existing door as a specification reference. Measure the slab, photograph the hardware and hinge preparation, document the swing and handing, and inspect the existing frame before deciding whether you need a slab or a complete prehung door.
Darpet offers both door slabs and prehung configurations, giving projects the flexibility to replace the door itself or coordinate a complete door-and-frame assembly when needed.