Use this contractor-focused replacement entry door measurement guide to document frame size, rough opening, jamb depth, handing, sill conditions, trim, and potential installation risks.

Contractor Field-Measure Guide

How to Measure for a Replacement Entry Door Without Creating a Change Order

Accurate replacement-door measuring is not just about getting a unit into the opening. It is about defining the complete scope before the order is placed. For contractors working in Davenport, Bettendorf and throughout the Quad Cities, a disciplined field measure protects margin, prevents delays and gives the homeowner a cleaner, more predictable installation.

The short answer

Measure the door slab, exterior frame, available rough opening, jamb depth, sill condition, floor clearances, swing, handing, hardware locations, sidelites, transom and trim. Then photograph every condition that could affect installation. Never order a replacement entry door from a single width-and-height note.

The most expensive measurement mistake is often not a wrong number. It is a condition that was never recorded: a sloped sill, deep plaster wall, hidden rot, oversized casing, storm-door conflict or floor height that prevents the new slab from swinging.

Start by defining what is being replaced

Before reaching for the tape, identify the intended scope. A slab-only replacement and a complete prehung replacement require different information.

Slab-only replacement: the existing jamb, sill and most trim remain. Exact slab size, thickness, bevel, hinge locations, bore locations and weatherseal compatibility become critical.
Prehung replacement: the slab and frame are removed as a unit. Frame size, rough-opening capacity, jamb depth, sill support, flashing and trim transitions drive the order.
Reconfigured entry: sidelites, transom, swing, opening width or architectural proportions change. Structural, exterior-finish and interior-finish work must be identified separately.

If the existing frame is damaged or unreliable, do not force a slab-only scope to meet a lower initial price. Read our guide to prehung exterior doors and frame replacement before deciding what stays.

The measurements every replacement-door order needs

Field item	What to record	Why it matters
Existing slab	Width, height and thickness	Identifies the current door, but not necessarily the order size
Frame	Outside frame width and height, excluding trim	Defines the unit occupying the opening
Rough opening	Stud-to-stud width and subfloor-to-header height where visible	Confirms installation clearance and framing capacity
Jamb depth	Wall thickness from interior finish to exterior sheathing or jamb edge	Prevents recessed or projecting jambs and trim problems
Opening geometry	Plumb, level, plane and both diagonals	Exposes twist, slope and out-of-square conditions
Sill and floors	Support, slope, finished-floor heights and swing clearance	Controls threshold fit, drainage and operation
Configuration	Handing, swing, sidelites, transom and door count	Prevents an expensive configuration error

Step-by-step replacement entry door measurement

1. Photograph the opening before measuring

Take straight-on photos from the exterior and interior, plus close views of the sill, lower jambs, head, hinges, lock, trim transitions and any damage. Include a wider elevation photo showing porch columns, lighting, steps and nearby windows. These images help the salesperson, installer and homeowner discuss the same opening after the site visit.

2. Measure the existing slab

Record slab width at the top, middle and bottom. Record height at both edges, and measure thickness at a clean edge. Differences can expose a field-trimmed slab, swelling or a distorted frame. For a slab-only order, also document the door-edge bevel.

3. Measure the frame—not the brickmould

Record the outside width and height of the door frame itself. Brickmould and casing are trim, not frame dimensions. Therma-Tru’s installation instructions specifically direct installers to check frame width and height rather than brickmould when verifying a prehung unit.

4. Verify the rough opening where possible

The rough opening is measured between structural framing members and from the subfloor to the header. Removing a section of interior casing may be the only dependable way to see it. Do not represent a calculated opening as a verified opening; mark it clearly as estimated when framing is concealed.

Follow the current instructions for the exact product. Therma-Tru’s published prehung instructions describe checking for an opening approximately 1/2 inch taller and 1/2 to 3/4 inch wider than the frame. Product, configuration and local conditions still govern the final requirement.

5. Check plumb, level, square and plane

Use a long level on both jambs in both directions. Check the head and sill for level. Measure diagonally from corner to corner in both directions; equal diagonals indicate square. Also check whether the jambs occupy the same plane. A square opening can still be twisted.

Record the actual variance instead of writing “opening out of square.” Numbers let the installer plan shimming, determine whether framing correction is needed and decide whether standard trim coverage is realistic.

6. Measure jamb depth at several points

Measure total wall thickness at the head and both sides. Older Quad Cities homes may combine plaster, drywall overlays, furring, replacement siding, rigid insulation or masonry returns. One nominal wall description does not guarantee one consistent jamb depth.

7. Record the sill and floor relationship

Measure interior and exterior finished-floor heights relative to the sill. Open the door fully and check rugs, tile, wood flooring, porch surfaces and weather exposure. Photograph any slope, corrosion, softness or previous repair. Identify whether the existing sill is supported continuously and whether a sill pan or other water-management detail is visible.

8. Confirm swing and handing in writing

Do not rely on “same as existing.” Record whether the unit is inswing or outswing, which side carries the hinges and which viewpoint the supplier uses to define handing. Mark the drawing and photograph the open door from both sides. A handing error can make a correct-size unit unusable.

9. Measure sidelites and transoms as a system

For a door with one or two sidelites, record the overall frame dimensions, individual component widths, mull locations and glass alignment. For a transom, measure the entire assembly and note whether the head trim, siding or masonry limits height. Do not add nominal component sizes and assume they equal the actual unit.

10. Document hardware and accessories

Record bore diameter, backset, center-to-center spacing, strike location, hinge dimensions, hinge count and any multipoint lock. Also note alarm contacts, doorbell wiring, smart locks, mail slots, pet doors, kick plates, closers and storm doors. These details affect preparation, compatibility and labor.

Aesthetic measurements contractors often miss

Technical fit gets the unit installed; visual fit makes the replacement look like it belongs on the house. Measure and photograph the relationships surrounding the door:

Exterior brickmould width and projection.
Interior casing width, thickness, profile and corner treatment.
Distance from trim to switches, outlets, lights and adjacent walls.
Glass height relative to nearby windows and porch features.
Sidelite symmetry and available wall space.
Threshold color and height against interior flooring and porch material.
Door swing clearance from railings, closets, furniture and stairs.

A new entry can be dimensionally correct yet visually awkward when casing crowds a light fixture or the glass pattern fights the home’s window grid. Solve those conflicts while options are still open.

How older Davenport and Bettendorf homes complicate the measure

Local replacement work frequently involves layered renovations. Aluminum wrap may cover original wood trim. New siding may have been installed around an old frame. Interior casing may hide plaster returns, and several flooring layers may have raised the finished floor above the original threshold.

Look closely for these scope triggers:

Soft lower jambs or staining beneath the sill.
Caulk joints wide enough to hide an undersized past replacement.
Out-of-plane walls that will twist a new frame.
Masonry or stone that limits exterior trim changes.
Porch slabs that pitch toward the doorway.
Raised flooring that reduces inswing clearance.
Painted components that may require lead-safe work practices when disturbed.

The photo set that protects the order

Use the same photo sequence on every project so important views are not forgotten:

Full exterior elevation.
Straight-on exterior view with the door closed.
Straight-on interior view with the door closed.
Door open, showing swing and nearby obstructions.
Close-ups of every hinge and the lock edge.
Head, both lower jamb corners and the full sill.
Interior and exterior trim at all four corners.
Visible rough opening after casing removal, when permitted.
Tape measure in place for critical dimensions.
Damage, prior repairs, wiring and accessory conflicts.

Label the photos with the project address and date. Do not rely on a camera roll filled with unidentified white doors.

Build a measurement sheet another person can verify

A good field sheet allows a second person to understand the opening without guessing what the estimator meant. Include:

Project address, opening designation, date and measurer.
A simple plan view showing interior, exterior, hinge side and swing.
Every measurement with units and the exact endpoints used.
Separate labels for slab, frame, trim and rough opening.
Observed conditions, assumptions and items that remain concealed.
Customer selections that affect configuration.
Required follow-up before release to production.

Use “verify” as a stop sign, not as a note someone can ignore. An order should not be released while a critical size, handing definition or configuration remains unresolved.

Change-order prevention checklist

Separate verified dimensions from estimates.
Have a second person review high-value or unusual configurations.
Match the field drawing to the supplier’s handing convention.
Confirm the exact manufacturer unit dimensions before ordering.
Price expected trim, paint, siding and flooring work.
State what happens if concealed rot or framing damage appears.
Review glass, color, sill, hardware and swing with the homeowner.
Attach the approved configuration and field photos to the order.
Check the delivered unit against the order before removing the old door.

Common measuring mistakes

Calling the slab size the rough-opening size. These dimensions describe different parts of the assembly.
Measuring brickmould instead of the frame. Trim can vary independently of the door unit.
Taking only one width and one height. Multiple points expose taper and distortion.
Ignoring wall thickness. The wrong jamb depth creates interior or exterior finish problems.
Assuming nominal sizes are actual sizes. Always verify current product documents and the specific configuration.
Skipping floor-clearance measurements. New flooring or rugs can block an inswing door.
Failing to photograph concealed-risk areas. The team cannot plan around a condition it never sees.

The bottom line

The goal of a replacement-door field measure is not to produce the fewest numbers. It is to remove uncertainty before uncertainty becomes cost. Measure the assembly, diagnose the opening and document the finish work with the same care used to select the new door.

The ruthless rule is simple: if the supplier, installer and homeowner could interpret a note differently, the note is not complete. Clarify it before the order is released.

Frequently asked questions

Do I measure the door slab or the frame for a replacement door?

Measure both, then verify the rough opening where possible. The slab describes the existing door, while frame and rough-opening dimensions help determine which complete replacement unit will fit.

What is the rough opening for an exterior door?

The rough opening is the framed opening between studs and from the subfloor to the header. Required clearance depends on the exact door system, so contractors should follow current manufacturer instructions.

How do I measure jamb depth?

Measure the wall thickness that the jamb must cover, generally from the interior wall surface to the exterior sheathing or appropriate frame edge. Check several locations because older walls may vary.

How can I tell whether a door opening is square?

Measure diagonally from corner to corner in both directions. Equal diagonals indicate square, but the jambs should also be checked for plumb, the sill and head for level, and the opening for twist.

What photos should contractors take before ordering a replacement door?

Photograph the full exterior, both sides of the closed door, the open swing, hinges, lock edge, sill, lower jambs, trim, visible framing, measurement points and every damaged or unusual condition.

Verify the opening before releasing the order

Suburban Wholesale & Supply helps Quad Cities contractors work through Therma-Tru entry door configurations, frame details, glass, finish and ordering questions. Visit 3602 North Harrison Street in Davenport or send the field information for review.

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How to Measure a Replacement Entry Door | Iowa