How to Measure for Window Replacement

What You're Actually Measuring

There are two different measurements that matter, and confusing them is the #1 mistake homeowners make:

Step-by-Step: Measuring the Rough Opening

Use this method when replacing the entire window unit, frame and all.

1. 1

   Remove interior trim (casing)

   Pull off the interior wood casing around the window to expose the rough framing. You need to see the actual framed opening, not the finished wall.

2. 2

   Measure width at three points

   Measure the width at the top, middle, and bottom of the opening. Record the smallest measurement — that's your width. Windows must fit the tightest point.

3. 3

   Measure height at three points

   Measure the height on the left side, center, and right side. Again, use the smallest measurement.

4. 4

   Subtract 1/2" from each dimension

   The replacement window should be 1/2" smaller than the rough opening on each dimension (1/4" gap per side). This allows for shimming and leveling.

5. 5

   Check for square

   Measure both diagonals. If they're within 1/4" of each other, the opening is square. If not, note the difference — your installer needs to know.

Step-by-Step: Measuring the Glass Unit (IGU)

Use this when the frame is in good shape and you only need to replace the glass — common for fogged or broken dual-pane units.

1. 1

   Remove the sash or panel

   Take the window sash out of the frame. For most double-hung windows, tilt the sash inward and lift out.

2. 2

   Measure the glass unit directly

   Measure the glass unit itself — width and height — at the outer edge of the spacer bar (the metal strip between the panes). This is the size you order.

3. 3

   Note the thickness

   Measure the total thickness of the glass unit. Standard dual-pane is 5/8" to 1". Triple-pane is typically 1" to 1-1/4". The replacement must match.

4. 4

   Check the spacer color

   Note whether the spacer bar is silver, black, or warm-edge. Matching it keeps the look consistent if you're only replacing one unit.

Common Mistakes to Avoid

• Measuring over the existing trim instead of the rough opening
• Using only one measurement point instead of three
• Forgetting to account for the 1/2" gap allowance
• Measuring in inches and fractions — always double-check your fractions
• Not checking for square — an out-of-square opening needs shimming
• Ordering the wrong glass type (single vs dual pane, tempered vs standard)

When to Just Call a Pro

Measuring is straightforward for standard rectangular windows. Call us instead if:

• The window is arched, angled, or custom-shaped
• The opening is significantly out of square (more than 1/2" diagonal difference)
• You're replacing a large picture window or floor-to-ceiling glass
• The frame shows signs of rot or water damage
• You're not confident in your measurements — a free in-home measure costs nothing