How to Measure the Size of a Door: An Illustrated Guide

Measuring a door sounds like it should be a two-number situation: width and height, done. Then you walk into the store
and get hit with words like slab, prehung, rough opening, jamb depth, and the
mysterious “2/8 x 6/8” code that looks like a math problem you didn’t study for.

Don’t worry. This guide breaks door measuring down into a clean, repeatable process (with pictures you can actually
understand). You’ll learn exactly what to measure, where to put the tape, and how to write your numbers so you don’t
accidentally buy a door that fits your neighbor’s house perfectly.

First: What Are You ReplacingA Slab or a Prehung Door?

Before you measure anything, decide what you’re buying. This changes everything.

• Slab door: Just the door panel. You keep your existing frame/jamb. This is common for interior doors
  where the frame is still solid.
• Prehung door: The door already mounted in its frame (hinges installed). This is common for exterior
  doors, or when the frame is damaged/out of square.

If you’re replacing a slab, you mainly measure the door itself (plus hinge and lock
details). If you’re replacing a prehung unit, you mainly measure the rough opening
(the framed opening behind the trim).

Door Parts You’ll Hear About (Quick Glossary)

• Width & height: The size of the door slab (panel).
• Thickness: How thick the door is (important for hardware and exterior doors).
• Jamb: The frame the door closes into.
• Jamb depth (jamb width): Wall thickness the jamb must span (studs + drywall/plaster).
• Rough opening: The framed opening in the wall (stud-to-stud width and header-to-floor height).
• Handing / swing: Which side the hinges are on and whether the door swings in or out.
• Backset: Distance from door edge to center of the doorknob hole (common options exist).

Tools You’ll Need

• Tape measure (a stiff one makes life easier)
• Pencil + notepad (or your phone notes app)
• Small level or a straight board (helpful for checking squareness)
• Square or combination square (handy for hinge/lock measurements)
• Screwdriver or pry bar (only if you’ll remove trim to measure rough opening)

Part A: Measuring a Door Slab (Replacing the Door Panel Only)

This method is best when your frame is in good shape and you want the new slab to fit your existing hinges and latch.
Measure carefullyslab replacement is where “close enough” turns into “why won’t you CLOSE?!” pretty fast.

Step 1: Measure the Door Width (Do It 3 Times)

1. Keep the door closed (or remove it and lay it flat if you prefer).
2. Measure across the slab at the top, middle, and bottom.
3. Write down the widest measurement.

Doors can swell, warp, or get “character” over time. Taking three measurements helps you avoid ordering a door that is
perfect… for a door that no longer exists.

Step 2: Measure the Door Height (Again, 3 Times)

1. Measure from the top edge of the slab to the bottom edge.
2. Measure on the hinge side, center, and latch side.
3. Write down the tallest measurement.

Step 3: Measure Door Thickness

Measure the thickness at the edge of the door. Interior doors are often thinner than exterior doors. Thickness matters
for hinges, latch length, and certain locksets.

Step 4: Write the “Store Size” (Round the Right Way)

Doors are typically sold in “nominal” sizes (like 36" x 80") even if your old door measures slightly under
that. A common shopping method is to measure the existing slab and round up to the nearest whole inch for ordering.

Example: If your slab measures 35-3/4" wide and 79-1/2" tall,
you’d typically shop for a 36" x 80" door.

That said, don’t rely on rounding alone if your house is older or your door has been trimmedalways compare your slab
measurement to the opening and hinge layout too.

Step 5: Measure Hinge Locations (If You’re Matching an Existing Frame)

If your new slab needs to drop into the same hinges, measure hinge placement from a consistent reference point:

• Measure from the top of the door down to the top of each hinge mortise.
• Also note hinge height (commonly 3-1/2" or 4") and whether hinges are radius or square corner.

If the hinge positions don’t match, you can still make it workbut you’re signing up for some careful marking and
mortising.

Step 6: Measure the Lockset (Bore Hole Height + Backset)

If you’re keeping the same knob/lever location, measure:

• Bore hole height: From the top of the door to the center of the knob hole.
• Backset: From the door edge to the center of the knob hole (common sizes exist).

If you’re buying a slab that’s not pre-drilled, you can choose your own knob heightjust keep it consistent with
other doors in the house unless you want a “funhouse hallway” vibe.

Part B: Measuring for a Prehung Door (Measure the Rough Opening)

Prehung doors come mounted in their frame, so you need the opening in the wall to be right. The most accurate approach
is to remove interior casing/trim so you can measure the framing (studs and header). Yes, this feels dramatic. Yes,
it’s worth it.

Step 1: Measure Rough Opening Width (Stud to Stud)

1. Measure width at the top, middle, and bottom of the opening.
2. Use the smallest measurement (the tightest spot is what must fit).

Step 2: Measure Rough Opening Height (Header to Floor/Subfloor)

1. Measure on the left, center, and right.
2. For exterior doors, measure down to the subfloor (not the finished flooring) when you can. Flooring thickness can fool you.
3. Use the smallest height.

Step 3: Measure Jamb Depth (Wall Thickness)

Jamb depth should match the wall thickness from drywall to drywall (not including trim). If you don’t want to remove
trim, you can often measure jamb depth on the hinge side where it’s visible.

Common jamb depths exist (often tied to 2×4 vs 2×6 walls), but always measurewalls can have extra layers of drywall,
plaster, or paneling that change the number.

Step 4: Confirm Swing and Handing (So You Don’t Buy a Door That Opens Into the Fridge)

Write down:

• Inswing or outswing
• Left-hand or right-hand (hinges on left or right as you face the door from the outside, for most entry-door conventions)

Step 5: Check the Opening for “Sneaky Problems”

Even perfect measurements can fail if the opening is out of square.

• Diagonal check: Measure corner-to-corner diagonals. If they differ a lot, the opening is out of square.
• Plumb check: Put a level on the studs. Big lean = more shimming (or framing fixes).
• Floor check: Floors slope. That affects reveal gaps and thresholds.

Standard Door Sizes Cheat Sheet (So the Labels Make Sense)

You’ll see door sizes written like 3/0 x 6/8. That means 3 feet 0 inches (36")
by 6 feet 8 inches (80").

Common Interior Door Sizes

• Widths: 18", 20", 24", 28", 30", 32", 36"
• Heights: 80" is most common; 84" and 96" exist in many homes
• Thickness: often around 1-3/8"

Common Exterior Door Sizes

• Typical: 36" x 80"
• Thickness: often around 1-3/4"
• Also common: 30" or 32" wide for older homes; taller doors in modern builds

Special Situations (Because Doors Love Plot Twists)

Double Doors and French Doors

Measure the total width across both slabs (and note whether they’re equal). If one is “active” and one is “fixed,”
hardware and astragals can affect sizing, so measure carefully and follow the unit’s requirements.

Doors With Sidelights

Measure the door slab(s) and the sidelights separately where possible, and record overall unit width. Sidelights can be
narrow or wide, and the total unit matters when ordering.

Pocket Doors

Pocket doors still require width/height/thickness, but you also need to confirm the pocket frame opening and track
system. If the wall was framed for a specific kit, matching that kit’s specs prevents headaches.

Bi-fold and Sliding Closet Doors

These are usually measured by the opening and sold as sets. Measure opening width at top/middle/bottom
and height left/center/right, and use the smallest numbers (because closet openings are rarely perfectly polite).

A Simple Measurement Worksheet (Copy/Paste Friendly)

Conclusion: Measure Once, Shop Once (and Keep Your Receipt Anyway)

Measuring a door isn’t hardit’s just oddly specific. The winning formula is:
measure the right thing (slab vs rough opening), measure it three times (top/middle/bottom or left/center/right),
and write it down clearly (including swing/handing and jamb depth).

If you remember nothing else, remember this: slab replacement is about matching existing hardware, and
prehung replacement is about fitting the framed opening. Mix those up, and you’ll be doing the world’s
most expensive arts-and-crafts project with shims.

Experiences From the Field: The “Oops” Moments People Learn From (So You Don’t Have To)

A lot of door-measuring “experience” is really just the same lesson taught in different costumes: doors are picky, and
houses are… creatively inconsistent. One common story goes like this: someone measures only the door slab (36" x 80"),
orders a shiny new prehung unit, and then discovers the rough opening is tighter than expected because the framing is
slightly out of square. The door isn’t “wrong,” the opening isn’t “wrong,” but the combination is like trying to park a
pickup truck in a compact spot. That’s why measuring the rough opening at multiple pointsand using the smallest
measurementsaves a Saturday.

Another classic: forgetting about jamb depth. In theory, many homes use predictable wall thicknesses.
In reality, walls often have extra drywall layers, plaster, tile backer, or paneling added over decades. People order a
prehung door with a jamb that’s too shallow, and then spend the afternoon inventing new vocabulary while trying to make
trim cover a gap it was never meant to hide. Measuring drywall-to-drywall (not trim-to-trim) keeps the jamb depth
honest.

Interior slab replacements have their own “experience curriculum,” usually starring hinges. Someone buys a slab, eyeballs
hinge placement (“it’s basically the same”), and installs itonly to find the reveals look uneven or the latch barely
kisses the strike plate. Doors aren’t forgiving about hinge locations; a small mismatch can make the door rub at the
top or swing open by itself like it’s haunted. People who’ve done a few of these almost always recommend marking hinge
positions carefully from the top of the door, and double-checking hinge size and corner style before cutting mortises.
The good news is that once you do it right once, the process feels repeatable.

Hardware backset creates another surprisingly common “how did this happen” moment. A door arrives pre-drilled, the knob
looks fine, but the latch won’t sit correctly because the backset doesn’t match the existing hardware or the lockset
template someone assumed was universal. Experienced DIYers tend to measure from the door edge to the center of the knob
hole and confirm which common backset the lock supports. It’s a 20-second measurement that can prevent a long evening
of returning parts and explaining to customer service that “yes, I promise it’s the door, not me.”

Flooring changes can also fool otherwise careful measurersespecially with exterior doors. New tile, thicker underlayment,
or fluffy carpet can change where the door sits and how it clears the floor. People sometimes measure height from the
finished floor without realizing the door unit needs clearance for a threshold or needs to reference the subfloor. The
result can be a door that drags, a threshold that doesn’t seal, or a gap that invites every draft in the neighborhood
to move in rent-free. The “experienced” move is to note the floor build-up and measure to the subfloor when possible,
especially for prehung exterior units.

And then there’s the most relatable experience of all: writing measurements down in a way that makes sense later. Plenty
of people confidently walk into a store and announce, “I need a door that’s… uh… 34-ish?” The pros (and anyone who has
done this twice) record measurements like a mini report: W x H x T, rough opening, jamb depth, swing/handing, and any
hardware notes. When you do that, ordering becomes boringin the best way. The door fits, the trim goes back on, and you
get to brag about your “precision carpentry” even if your real secret was just taking notes like a responsible adult.

Final checklist before you buy: W, H, thickness, slab vs prehung, rough opening (if prehung), jamb depth, swing/handing, and backset.
That’s the whole magic trick.