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Not sure how to measure?

How to Measure Your Windows

Tools (5 things you need)

  • 25-ft tape measure (1/16" marks)
  • Notepad/phone (record as Width × Height)
  • Level (check plumb = vertical)
  • Framing square (check square = 90° corners)
  • Pencil/marker
    Nice to have: laser measurer, phone camera, small ladder.
Before You Start (quick prep)
  • Know your replacement type:
    • Full Frame: entire window/frame removed to studs
    • Retrofit: new window fits inside existing frame
  • Clear space: 2–3 ft around the window; lift blinds/curtains.
  • Always measure the frame/opening, not drywall/trim.
  • Be consistent: measure all from inside or outside—don’t mix.
💡 Pro Tips (accuracy matters)
  1. Measure every window separately—even “identical” ones vary.
  2. Record to the nearest 1/16" and always as W × H.
  3. Take 3 widths (top/middle/bottom) and 3 heights (left/center/right); use the smallest in each set.
  4. For large spans, get a helper—your tape stays straight, numbers stay true.
  5. Snap photos and note anything unusual (bowed frames, rot, obstructions).
Method A — Outside (Full-Frame / Brick-to-Brick or Siding)

Width: Left outside edge of brickmould/siding → right outside edge.
Take top/middle/bottom; use smallest = Final Width.

Height: Bottom sill/ledge (where it meets cladding) → top of opening.
Take left/center/right; use smallest = Final Height.

Note the exterior material: brick, siding, stucco, wood—this can affect installation details.

Method B — Inside (Jamb-to-Jamb / Retrofit or Quick Quote)

Open movable sashes if present (sliders/casements) to reach the true frame.

Width: Inside face of left jamb → inside face of right jamb.
Take top/middle/bottom; use smallest = Final Width.

Height: From interior sill stool/seat (where sash rests) → top jamb.
Take left/center/right; use smallest = Final Height.

(Optional) Jamb depth: Interior face → exterior face of jamb = Jamb Depth.

Square & Plumb Check (fast quality check)
  • Plumb: Place level on side jambs; bubbles centered = plumb.
  • Square: Measure diagonals: top-left → bottom-right and top-right → bottom-left.
    • If they differ by > 1/4", plan to shim or consider custom sizing.
Record & Next Steps
  • Final numbers (inches, to 1/16"): W × H
  • Method used (Outside Full-Frame or Inside Jamb-to-Jamb)
  • Window type (slider, casement, fixed) + any notes/photos

Ready to order? Enter your measurements on the Iris product page for pricing and checkout.

Want backup? Book a free measurement check with an Iris expert before you place the order.