How to Measure a Window for Replacement | Step-by-Step Guide

Replacing your windows doesn't have to be complicated, and it starts with one simple task: measuring your existing windows. With the right steps and a metal tape measure, most homeowners can do this confidently in under an hour. This guide walks you through exactly what to measure, where to measure it, and what to do with your numbers so you can order with confidence.

What You'll Need

All you need is a rigid metal tape measure, a pencil, and paper. A helper is optional but useful for larger windows. 

How to Measure Your Existing Windows

Whether your home has brick, stucco, wood siding, or vinyl cladding, the process is the same. You'll take measurements from both the exterior and interior to get everything Iris needs to build your replacement.

1. Measure the Exterior Width

From outside, measure the width of your window opening at three points, near the top, in the middle, and near the bottom. Measure from the inside edge of the brick (or exterior cladding) on one side to the inside edge on the other. Record all three and use the smallest number as your width.

Measure at three points because openings are rarely perfectly uniform. The smallest number protects you from ordering a window that won't fit.

2. Measure the Exterior Height

From outside, measure the height at three points: on the left side, at the center, and on the right side. Measure from the top of the brick opening (under the lintel) down to the bottom. Use the smallest of the three as your height. These two numbers — smallest width and smallest height — are your brick-to-brick measurements.

The sill is often the least level surface. Taking three readings catches any slope before it becomes a problem on install day.

3. Measure Your Window from Inside

From inside, measure the width and height of the existing window opening. Then add the jamb thickness, usually about ¾" per side, to each measurement to get the true interior size. This ensures proper space for shimming, insulation, and finishing trim once your new window is installed.

To find your jamb thickness: gently loosen a small section of interior trim if needed, and measure the thickness of the jamb directly.

4. Compare Exterior and Interior Measurements

Compare your exterior brick-to-brick measurements with your adjusted interior measurements. The difference between the two determines the brickmould size your new window needs, ensuring it fits the masonry opening and leaves room for shimming, insulation, and trim.

Enter your frame measurements into our product configurator. When you reach the brickmould selection, each option displays its exact brickmould size so you can match it to your opening directly.

5. Measure the Depth

From inside, place a flat bar against the most exterior part of your window, including the brickmould if applicable. Then measure from that point to the end of your interior jamb extension. This is your overall window depth.

The Iris window frame itself is 3¼" deep. If your window requires brickmould, add another 1½" to the total, bringing it to 4¾". Subtract that from your overall measured depth to find the jamb extension size you need.

For example: if your total depth measures 7⅜" and your window needs brickmould, you'll need a 2⅝" jamb extension (7⅜" − 4¾" = 2⅝").

At Iris, we offer jamb extensions in two materials:

• Primed Wood: 2⅜", 2⅝", 3⅜", 4⅝"
• PVC (Extruded Vinyl): 1⅜", 1⅞", 3⅜", 4⅝"

6. Record and Submit Your Numbers

Before ordering, write down your four measurements:

• Exterior width — smallest of your three brick-to-brick readings
• Exterior height — smallest of your three brick-to-brick readings
• Adjusted interior width and height — with jamb thickness added
• Overall depth — from the most exterior point to the end of the jamb extension

Enter these directly into the product configurator at iriswindows.com. Our windows are custom-built to your specs — there are no standard sizes to navigate around.

Suggested rough opening: Your rough opening should be 1" larger than your window size in both width and height (½" per side) to allow room for shimming, insulation, and levelling. A 12" × 16" window needs a 13" × 17" rough opening.

Dimension order: Window measurements are always listed Width × Height × Depth. Width goes first, always.

Common Mistakes to Avoid

• Only measuring once. Openings are rarely uniform. Take three measurements for both width and height and use the smallest each time.
• Forgetting to add the jamb thickness. The jamb (~¾" per side) must be added to your interior measurement to get the true interior size. Skipping this causes gaps and a messy finish.
• Skipping the exterior-to-interior comparison. This difference determines your brickmould size. Don't skip it — it affects both fit and finish.
• Rounding up. Always use the smallest measurement. When in doubt, go smaller, not larger.
• Measuring the glass instead of the full opening. You need the full brick-to-brick opening dimension, not just the visible glass area.

FAQ

Do I need to remove anything before measuring?

For exterior measurements, no — measure the brick-to-brick opening as-is. For the interior frame measurement, you may need to loosen a small section of trim to expose the jamb. Everything else stays in place.

What is brickmould, and how do I know which size I need?

Brickmould is the exterior trim profile that fills the gap between your window frame and the exterior wall surface. Enter your frame measurements into our configurator; each brickmould option shows its exact size so that you can match it to your opening directly.

Which brickmould type is right for my installation?

It depends on your installation type and exterior finish. New construction typically calls for a plank or nail fin brickmould, which fastens directly to the wall framing before siding or brick is applied. For retrofit replacements, a cap brickmould or sill nosing causes minimal disruption to existing exterior finishes. If you have vinyl siding, BMJ brickmould integrates directly into the siding channel. And if you have shaped or arched windows, all Iris vinyl brickmoulds can be bent to match.

All Iris brickmoulds are manufactured in-house and colour-matched to your window or door for a clean, seamless finish. Note that a nail fin generally indicates a new construction window. If you're doing a retrofit replacement, the existing fin from your old window will likely need to be cut or removed before the new window can be set in.