| To measure a door, record the rough opening width at three heights (use the smallest) and the height at two points (use the smallest). Subtract 2 inches from each dimension to get your finished door size. For an existing door, measure width at top, middle, and bottom (use the largest) and height along both sides (use the largest). Always measure thickness at the door edge and record jamb depth separately. |
By Jeremy Barker, Founder and CEO of Murphy Door®
Published July 2025 | Reviewed by Jeremy Barker
A door that is off by even a quarter inch can bind in the frame, leave visible gaps, or refuse to latch correctly. Whether you are ordering a standard replacement slab, a pre-hung unit, or a custom Murphy Door® hidden door, accurate measurement is the single most important step before you spend a dollar. Getting it right the first time saves you from costly returns, shimming hacks, and unnecessary labor charges.
This guide walks you through every measurement a door project requires: rough opening dimensions, existing door dimensions, thickness and jamb depth, and the clearances that determine whether your door actually functions after it is installed. The steps apply to any interior or exterior door, and we have added specific notes for hidden doors and bookcase doors throughout, since those applications have tolerances that leave no room for guesswork.
Read through the full guide once before you pick up a tape measure. Understanding why each number matters makes the physical process faster and far less likely to produce errors.
What tools do you need to measure a door?
Door measurement requires only a handful of items, but using the right versions of each makes a real difference in accuracy.
- 25-foot locking tape measure: A cheap, narrow tape can bow under its own weight on tall openings. Use a tape with a rigid blade at least 1 inch wide.
- Pencil and notepad (or phone notepad): Record every reading immediately. Memory is unreliable when you are taking a dozen measurements.
- 6-foot level or laser level: Walls and floors are rarely perfectly plumb or level. A level tells you how far out of square your opening is, which affects which dimension to use.
- Speed square or framing square: Useful for checking corner angles when you suspect the opening is racked.
- Step stool or stable ladder: Required for reaching the top of a standard 80-inch or taller opening safely while keeping the tape straight.
Optional but helpful: a digital caliper for measuring door thickness with precision, and a stud finder if you are removing casing to expose the rough opening.
How do you measure the rough opening for a new door?
Measuring the rough opening is the correct approach when you are installing a door where no door currently exists, or when you have removed the old door and casing completely. The rough opening is the framed hole in the wall before any jamb, casing, or shims are applied.
- Measure the width at three points. Hold your tape measure horizontally and record the width near the top, at the midpoint, and near the bottom. Walls bow and studs shift, so the three readings will often differ slightly.
- Use the smallest width measurement. The smallest number represents the tightest point the door unit must pass through. Using a larger number risks ordering a door that cannot be fitted.
- Measure the height at two points. Run your tape from the subfloor to the underside of the header on the left side, then on the right side. Record both numbers.
- Use the smallest height measurement. Same logic: the shortest point controls what will actually fit.
- Subtract for finish flooring. If finish flooring (hardwood, tile, carpet) has not yet been installed, subtract its thickness from your height figure. Hardwood and tile typically add 3/4 inch to 1 inch; carpet with pad can add 1 inch to 1.5 inches.
- Subtract 2 inches from each dimension to find the door slab size. A standard pre-hung unit needs roughly 1 inch of clearance on each side and above for the jamb and shimming. That accounts for the full 2 inches per dimension.
- Round decimals up, not down. If your calculated door width is 35.6 inches, record 36 inches. Rounding down risks a door that is too small for the opening.
Example: rough opening measures 38 inches wide (smallest of three readings) by 82 inches tall (smallest of two readings), with 3/4-inch hardwood flooring to be installed. Subtract 3/4 inch from height, then subtract 2 inches from each: door slab should be 36 inches wide by 79.25 inches tall, rounded to 36 inches by 80 inches (a standard size).
How do you measure an existing door for replacement?
If you are replacing a door that is currently in place and functioning, you can measure the door slab itself. This method is faster but carries a caveat: the existing door may have been cut or planed at some point, so cross-check your readings against the rough opening whenever possible.
- Measure the door width at three points. With the door closed, run your tape horizontally at the top, middle, and bottom of the door face, measuring from edge to edge (not including the stop molding).
- Record the largest width reading. Unlike the rough opening method, you use the largest measurement here because the door slab must cover the full width at its widest point.
- Measure the door height on both sides. Run your tape from the bottom edge of the door to the top edge, on the hinge side and on the latch side.
- Record the largest height reading. Same principle: use the largest to ensure full coverage.
- Measure the door thickness. Open the door and measure the thickness at the top edge using your tape or a digital caliper. Standard interior slabs are 1-3/8 inches thick; exterior and many interior pre-hung doors are 1-3/4 inches thick.
- Note the handing. Stand outside the room (or outside the building for exterior doors) and observe which side the hinges are on. Hinges on the left mean a left-hand door; hinges on the right mean a right-hand door. Record this before you order anything.
How do you measure the door jamb and wall thickness?
The jamb depth must match your wall thickness or the door unit will either protrude on one side or sit recessed, preventing the casing from lying flat. This measurement is separate from the door slab dimensions and is just as important.
- Remove a small section of casing on one side of the existing door, or simply measure from the face of the drywall on one side of the wall to the face of the drywall on the other side.
- Measure straight across the wall opening. For a standard 2x4 framed wall with 1/2-inch drywall on each side, total wall thickness is typically 4.5 inches. A 2x6 framed wall with 1/2-inch drywall on each side is typically 6.5 inches.
- Compare to available jamb depths. Most pre-hung doors are offered in 4-9/16-inch jamb depth (for 2x4 walls with 1/2-inch drywall) and 6-9/16-inch jamb depth (for 2x6 walls). Custom jamb extensions are available for non-standard wall thicknesses.
- Account for any wall cladding variations. Tile wainscoting, shiplap, or double drywall add to the effective wall depth. Measure the full depth, not just the framing.
What clearances do you need to measure for door swing and function?
A door that fits the opening perfectly can still fail if the swing clearance is wrong. Measure these before finalizing your door selection.
- Swing clearance: An in-swing door needs clear floor space equal to the door width as it arcs open. Measure from the hinge edge of the opening and trace the arc across the floor. Mark any obstacles (cabinets, stairs, counters) that fall within that arc.
- Undercut clearance: Building codes and HVAC practice often require a gap under interior doors for air return. Standard undercut is 1/2 inch to 3/4 inch above finish flooring. Measure existing doors before ordering to match the undercut.
- Header clearance: For any door with an overhead closer or a bookcase door that must align with ceiling trim, measure from the top of the rough opening to the ceiling. You need at least 1 inch of clearance above the head jamb for most pre-hung units.
- Side-wall clearance for hidden and bookcase doors: A bookcase door that swings into a room also requires clearance for the bookcase depth (typically 10–12 inches from the wall face). The full arc of a loaded bookcase door is wider than that of a standard slab. Measure and mark the full swing path on the floor before ordering.
How do standard door sizes compare, and which is right for your opening?
Knowing standard sizes lets you check quickly whether your opening can accept a stock door or requires a custom order.
| Application | Standard Width | Standard Height | Standard Thickness |
|---|---|---|---|
| Interior bedroom / hallway | 28", 30", 32", 36" | 80" (6'8") | 1-3/8" |
| Interior bathroom | 24", 28", 30" | 80" (6'8") | 1-3/8" |
| Exterior entry | 32", 36" | 80" or 96" (8'0") | 1-3/4" |
| ADA-compliant passage | 36" minimum clear | 80" minimum | 1-3/4" |
| Murphy Door® hidden / bookcase door | Custom to rough opening | Custom to rough opening | Varies by unit |
If your rough opening falls between standard sizes, always order to the next standard size up and adjust the rough opening framing rather than trying to cut down a pre-hung unit. Hidden doors and bookcase doors from Murphy Door® are built to your specific rough opening dimensions, which is why precise measurement matters even more for those applications.
How do you measure for a Murphy Door® hidden door specifically?
Murphy Door® hidden doors require the same fundamental measurements as any door, but the tolerances are tighter and a few additional dimensions are needed. Every Murphy Door® hidden door is built to the exact rough opening you provide, so errors in measurement become errors in the finished unit.
- Measure rough opening width three times (top, middle, bottom) and record the smallest.
- Measure rough opening height twice (left side, right side from subfloor to header underside) and record the smallest.
- Measure wall thickness (jamb depth) as described above. Murphy Door® units are available in multiple jamb depths, and specifying the wrong one affects how the casing sits.
- Measure the floor-to-ceiling height in the room where the door will swing open. Tall ceilings affect trim alignment for full-height bookcase doors.
- Mark the swing path on the floor. Use a string tied to the hinge-side corner of the opening, pulled taut at the full door-plus-bookcase depth, and swept in an arc. Confirm that no baseboards, outlets, or furniture fall within that arc.
- Note wall construction on the hinge side. The Patented Ball Bearing Lower Pivot Point, part of a hinge and hardware system tested to 1,540 lbs, transfers load directly to the floor and the header. Confirm the framing on the hinge side is sound before installation.
- Photograph your measurements. Send photos of the rough opening alongside your written dimensions when you configure your door. This allows Murphy Door® to flag any discrepancies before production begins.
Every Murphy Door® hidden door is built on Vaultwood Core panels (a proprietary MDF engineered stronger than standard MDF) with real wood veneer skins, solid hardwood face frames, and solid-wood components. The Vaultwood Core substrate stays dimensionally stable across humidity cycles, which means the door you receive matches the dimensions you ordered regardless of seasonal changes.
Start Your Project
Ready to put your measurements to work? Call Murphy Door® at 888-458-5911 to speak with a door specialist who can confirm your dimensions and help you configure the right hidden door or bookcase door for your space. You can also visit murphydoor.com/pages/contact to send your measurements online.
About Murphy Door
Murphy Door®, headquartered in Ogden, Utah, has shipped over 200,000 products since 2012 from our Utah and Kentucky factories. As the original hidden door company, Murphy Door® designs and manufactures hidden doors, bookcase doors, hidden wall beds, and hidden furniture solutions for residential and commercial applications. Every unit is built on Vaultwood Core (a proprietary MDF engineered stronger than standard MDF) with real wood veneer skins, solid hardwood face frames, and solid-wood components, backed by a Limited Lifetime Warranty: lifetime warranty on labor, hardware, and hinges; a 5-year warranty on finishes and wood veneer; and a 1-year warranty on electronics. The Patented Ball Bearing Lower Pivot Point, part of a hinge and hardware system tested to 1,540 lbs, is manufactured in-house at our Utah and Kentucky factories.
Frequently Asked Questions
Q: Should I measure the door slab or the rough opening?
A: Measure the rough opening whenever possible. It is the most reliable reference point because it reflects what actually exists in the wall, independent of any modifications made to a previous door slab. Use existing door slab measurements only as a cross-check or when the rough opening is completely inaccessible.
Q: What is the difference between rough opening size and door size?
A: The rough opening is the framed hole in the wall before the jamb, shims, or casing are installed. The door slab size is the finished panel that swings in the frame. For a pre-hung unit, the door slab is approximately 2 inches narrower and 2 inches shorter than the rough opening, because the jamb and shimming space consume that margin. Always communicate which measurement you are providing when ordering a door.
Q: What if my rough opening is not square?
A: Out-of-square openings are common, especially in older homes. Measure the diagonal from corner to corner in both directions. If the two diagonal measurements differ by more than 1/4 inch, the opening is racked and should be reframed or shimmed back to square before ordering a door. Trying to fit a square door into a racked opening leads to binding, gaps at the corners, and latching problems.
Q: How much clearance should a door have at the bottom?
A: Standard undercut clearance between the bottom of a door slab and the finish floor surface is 1/2 inch to 3/4 inch for interior doors. This gap allows air circulation for HVAC return air and prevents the door from dragging on the floor as the building settles. Increase the undercut to 1 inch or more in bathrooms where moisture-related swelling is a concern.
Q: How do I measure for door handing (which way it swings)?
A: Stand outside the room the door swings into (or outside the building for exterior doors) and face the door. If the hinges are on your left, the door is left-hand; if the hinges are on your right, the door is right-hand. For pre-hung units, handing is a factory setting that cannot be changed after the unit is built, so confirm this before ordering.
Q: Do I need special measurements for a hidden bookcase door?
A: Yes. In addition to standard rough opening width, height, and wall thickness, a hidden bookcase door requires the swing-path arc measured across the floor, confirmation of ceiling height for full-height units, and verification that the hinge-side framing is structurally sound. Murphy Door® representatives can walk you through the full measurement checklist at 888-458-5911.