How to Measure a Door for Replacement (Slab & Prehung)
How to measure a door for replacement: slab vs prehung, width, height, rough opening, jamb depth, and swing—order the right size first.
- Difficulty
- Beginner-friendly
- Time
- 15 minutes to read
- Est. cost
- Free (measuring only)
- Season
- year round
By Keepstead Editorial Team · Published August 29, 2026 · Updated October 13, 2026 · 5 min read
Last reviewed October 13, 2026
Quick answer
To measure a door for replacement: decide slab vs prehung first. Slab = measure the door panel (width in 3 places, height on both sides, thickness). Prehung = measure the rough opening, jamb depth (wall thickness), thickness, and swing/handing. Always record the smallest width and shortest height. Use a tape measure you can read in fractions.
Ordering the wrong size for a door replacement delays projects and creates return hassles. Standard catalog sizes exist, but older homes and remodels often do not match the label—measure first, then buy.
How to Measure a Door for Replacement (Quick Checklist)
- Slab or prehung? — Panel only vs full unit in a frame (see table below).
- Width (multiple places) — Top, middle, and bottom of the slab—or jamb to jamb for rough opening.
- Height (multiple places) — Left and right of the slab—or floor to header for rough opening.
- Door/slab thickness — Usually 1-3/8" or 1-3/4"; must match hinge and latch alignment.
- Rough opening — For prehung units: inside width and height vs the manufacturer's required opening.
- Jamb depth — Wall thickness the frame must fit (e.g. 4-9/16", 6-9/16").
- Swing / handing — Hinge side and open direction when you stand on the hinge side.
| Measurement | Door slab | Prehung door | What to record |
|---|---|---|---|
| Width | Door face at top, middle, bottom | Rough opening jamb to jamb (3 places) | Smallest width |
| Height | Top to bottom edge (both sides) | Floor/subfloor to head jamb (2 places) | Shortest height |
| Thickness | Edge of door panel | Match existing if reusing frame | Exact fraction (e.g. 1-3/8") |
| Rough opening | N/A (unless reframing) | Required for unit size | Compare to spec sheet |
| Swing / handing | Same as existing if reusing hinges | Left- or right-hand on order form | Hinge side + swing |
| Jamb depth | N/A for slab-only swap | Wall thickness frame must span | Match wall depth |
Slab vs prehung: what to measure
| Type | What you measure | When to choose |
|---|---|---|
| Slab only | Existing door panel width, height, thickness (and hinge spacing if reusing hinges) | Frame and hinges are sound; only the panel is damaged |
| Prehung unit | Rough opening width and height, jamb depth, swing | Frame is rotten, out of square, or you want a new jamb and stops |
If you replace only a damaged slab and keep hinge locations, match the old slab dimensions and hinge spacing.
Measure door width (slab)
- Measure the door face—not the trim—unless your retailer specifies otherwise.
- Measure top, middle, and bottom width.
- Record the smallest width if readings differ.
Standard nominal widths (actual may be slightly under): 24", 28", 30", 32", 36". Confirm fractions with how to read a tape measure.
Measure door height (slab)
Measure left and right from top of door to bottom edge. Use the shorter reading if unequal.
Common height: 80" (6'-8"). Some entries use 84" or 96" heights.
Measure thickness
Typical interior slabs: 1-3/8" or 1-3/4". Exterior doors are often 1-3/4". Match for hinge mortises and latch alignment.
Rough opening for prehung doors
Measure inside the frame where the old unit sat. Use a tape you read confidently—see how to read a tape measure for inch and fraction basics before you write numbers down.
- Width: jamb to jamb in three places
- Height: floor (or subfloor) to head jamb in two places
- Jamb depth: jamb width must fit wall thickness (4-9/16", 6-9/16", etc.)
Manufacturers list a required rough opening—usually slightly larger than the unit. Compare your readings to the spec sheet, not a catalog “door size” alone. Confusing rough opening with slab size is a common ordering error.
Swing and handing
Stand on the hinge side when the door opens toward you:
- Hinges on left = left-hand
- Hinges on right = right-hand
Exterior security doors and some sliders use different conventions—confirm on the order form.
Before you buy checklist
Confirm all of the following before you order or leave the store:
- Slab vs prehung decision is clear
- Width recorded from three places (smallest)
- Height recorded from both sides (shortest)
- Thickness matches hinges/latch needs
- Rough opening matches prehung spec (if applicable)
- Jamb depth matches wall thickness (prehung)
- Swing/handing noted on the order
- New flooring height considered (door may drag if floor rises)
If new flooring will be higher than today, plan undercut or order height accordingly—coordinate with flooring measurement.
Common measuring mistakes
- Measuring only the old slab when buying a prehung unit
- Confusing rough opening with the labeled door size
- Ignoring jamb depth (unit too shallow/deep for the wall)
- Forgetting door thickness
- Forgetting swing/handing
- Measuring once instead of top/middle/bottom
After install: seal gaps
Even a well-sized door needs weatherstripping at exterior entries. See how to weatherstrip a door for jamb seals and sweeps, and how to caulk windows and doors for exterior trim and frame gaps.
DIY vs professional install
Homeowner-friendly: swapping a slab on existing hinges when sizes match; installing prehung units in standard openings with good shims and level.
Hire a carpenter for:
- Non-standard masonry openings
- Structural header changes
- Severely out-of-square frames
- Entry doors with sidelights and multipoint hardware
Tools
A sturdy tape measure from your beginner DIY tool kit is enough. A level helps check head jamb straightness before ordering custom sizes.
Frequently asked questions
- How do I measure a door for replacement?
- For a slab, measure width at top, middle, and bottom and height on both sides—record the smallest readings—plus slab thickness and hinge spacing if reusing the frame. For prehung, measure rough opening width and height in multiple places, jamb depth for wall thickness, and note swing/handing. Use a tape measure you can read accurately and compare to the manufacturer's rough-opening spec before ordering.
- How do I measure for a prehung door?
- Measure the rough opening width and height in several places (use the smallest readings), measure jamb depth to match wall thickness, and note swing/handing. Compare your rough opening to the manufacturer's required opening—not only the nominal door size on the box.
- Do I measure the door or the frame for replacement?
- For slab replacement, measure the existing door slab. For prehung units, measure the rough opening and existing jamb width. Retailers often list standard sizes—confirm with three width and height readings.
- What are standard interior door sizes?
- Common prehung heights are 80 inches (6 ft 8 in) with widths such as 24, 28, 30, 32, and 36 inches. Older homes may be non-standard—always measure.
- How much gap should be under an interior door?
- Many interior doors clear flooring by about ½ to ¾ inch for swing clearance; exterior doors often use a threshold and sweep instead. Follow product and flooring height when ordering.
- What is the difference between a door slab and a prehung door?
- A slab is the door panel only—you reuse the existing frame and hinges when sizes match. A prehung unit includes the door already hung in a frame and needs rough-opening and jamb-depth measurements to fit the wall.
Related guides
How to Read a Tape Measure for DIY Projects
Read inch and foot markings on a tape measure—fractions, hook slack, and inside vs outside measurements—for accurate home project layout.
Door Drafts: How to Weatherstrip a Door
Stop cold air and drafts around exterior doors with the right weatherstripping type, prep steps, and when to adjust the door instead.
How to Caulk Windows and Doors
Seal stationary gaps around window and door trim with the right interior or exterior caulk while preserving drainage paths.
How to Measure for Flooring Installation
Measure rooms for flooring orders—square footage, waste allowance, and L-shaped layouts—without overbuying or coming up short.
This guide is for general informational purposes. Conditions vary by home, local codes, and equipment. See our disclaimer.