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Construction worker measuring a door frame opening

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Measuring and Field Verification

Field measurement is where most quoting errors begin. Never rely on a single measurement, a customer’s memory, or a photo alone. Verify every critical dimension before building a quote.
Accurate measurement is the foundation of every clean order. This page covers every dimension you need to capture, how to capture it correctly, and what to do when field conditions are not ideal.

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Why Measuring Matters

What goes wrong with bad measurements:
  • Door unit too wide or narrow for the opening
  • Jamb depth mismatch — frame doesn’t sit flush
  • Slab hits the threshold on the swing arc
  • Double door active panel sized to full unit width
  • Handing determined from wrong vantage point
What good measuring prevents:
  • Remakes and restocking fees
  • Delayed installs and unhappy customers
  • Jamb extension costs
  • Margin erosion from field corrections
  • Repeat service calls

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The Required Measurements

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1. Rough Opening (RO)

1

Measure width at three points

Measure the rough opening width at the top, middle, and bottom — stud face to stud face. Record all three. Use the smallest number as your working width.
2

Measure height on both sides

Measure the rough opening height on both the hinge side and the lock side, from the subfloor to the underside of the header. Record both. Use the smallest number as your working height.
3

Check for square

Measure both diagonals of the opening. If they differ by more than 1/4”, the opening is out of square. Note this on the quote and discuss shimming requirements with the installer.
Standard rough opening allowance for a prehung door is approximately 2” wider and 2” taller than the door slab size. A 3068 slab (36” x 80”) typically requires a rough opening of approximately 38” x 82”.

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2. Slab Size

For slab-only replacements, measure the existing door panel directly:
If the slab is in good enough condition, you can verify size with the door open. Do not measure a warped door while it is in the frame — the frame may be holding it at a distorted dimension.

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3. Jamb Depth

Jamb depth must match the wall thickness from one face of the finished wall to the other.
Open the door and measure from the exterior face of the exterior trim (or siding) to the interior face of the interior trim. If trim is not yet installed, measure from one drywall face to the other and add the combined trim thickness.
  • 4-9/16” — Standard 2x4 framing + 1/2” drywall each side
  • 5-1/4” — Standard 2x6 framing + 1/2” drywall each side
  • Custom — Required for stucco, masonry, EIFS, and thick-wall construction
If the wall is thicker than standard, a jamb extension kit will be required. Note this clearly on the quote.
If the jamb is too narrow, it will not reach the interior drywall face, leaving a gap that requires an extension kit and additional labor. If the jamb is too wide, it projects past the drywall and must be cut down — which damages factory finishes.

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4. Door Thickness

Always confirm slab thickness for slab-only replacements. Common thicknesses:
  • 1-3/4” — Exterior doors (standard)
  • 1-3/8” — Interior doors (standard)
  • 2-1/4” or 2-1/2” — Heavy-duty or commercial applications
Hinge size, hinge prep, and lock bore prep are all spec’d to a specific slab thickness. If the replacement slab has a different thickness than the original, the hardware may not be compatible.

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5. Configuration Width for Multi-Panel Systems

For double doors or doors with sidelites, always capture the total system width — not just the active door slab.
A customer who says “my opening is 6 feet wide” may mean the rough opening, the total sidelite-to-sidelite trim width, or the slab pair width. Always clarify and measure each component separately.

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The Field Measurement Checklist

Run through this checklist on every job before leaving the site.
  • RO width at top, middle, bottom (smallest recorded)
  • RO height on hinge side and lock side (smallest recorded)
  • Diagonal measurements to check square
  • Jamb depth (wall thickness face to face)
  • Handing and swing direction confirmed
  • Threshold condition noted
  • Existing trim reuse or new casing needed
  • Sidelite widths (if applicable)
  • Floor level checked (flat or sloped at sill)
  • Any obstructions in swing arc noted

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Rough Opening to Door Size Reference

Always verify rough opening dimensions directly. Never assume a standard RO from the slab size alone — existing construction and remodels frequently have non-standard framing.