How to Measure Your Old Window for a Perfect Replacement Fit


Replacing a window is a significant home improvement project, and getting the measurements right is absolutely crucial for a successful installation.1 A window that's too big won't fit, and one that's too small will leave gaps that are difficult to seal. This guide will walk you through the simple steps to accurately measure your old window, ensuring your new one is a perfect fit.

Why Accurate Measurements Matter

Before diving into the "how," let's quickly touch on the "why."

  • Ease of Installation: A precisely measured window will slide into its opening with minimal fuss, making the installation process smoother and less time-consuming.2

  • Energy Efficiency: Gaps around a window lead to drafts, heat loss in winter, and heat gain in summer.3 Accurate measurements mean a tighter seal, improving your home's energy efficiency and lowering utility bills.4 🌬️

  • Aesthetics: A well-fitting window looks professional and enhances your home's curb appeal.

  • Cost Savings: Avoiding costly mistakes from incorrect measurements saves you money on reordering or custom fitting.5

Tools You'll Need

Gather these simple tools before you start:

  • Tape Measure: A sturdy, metal tape measure is essential.

  • Pen and Paper (or a digital device): To record your measurements.6

  • Safety Glasses: Always protect your eyes, especially when working around glass. 👓

  • Gloves: For a better grip and to protect your hands.7

  • Screwdriver or Pry Bar (optional): You might need these if you need to remove trim for more accurate measurements.

Measuring Your Window: A Step-by-Step Guide

When measuring for a replacement window, you'll generally need to take three key measurements: width, height, and depth.8 It's always best to measure in at least three different places for each dimension and use the smallest measurement.

1. Measuring the Width

This is typically the horizontal measurement.

  • Inside the Existing Frame: Open your window so you can access the frame. Measure the width from the inside edge of the jamb (the vertical sides of the window frame) on the left to the inside edge of the jamb on the right.9

  • Measure at the Top, Middle, and Bottom: Take three measurements: one at the top of the opening, one in the middle, and one at the bottom.10

  • Record the Smallest Measurement: Note down the smallest of these three width measurements. This is your rough opening width.

2. Measuring the Height

This is the vertical measurement.

  • Inside the Existing Frame: Measure the height from the inside edge of the sill (the bottom horizontal piece of the window frame) to the inside edge of the head jamb (the top horizontal piece of the window frame).11

  • Measure on the Left, Middle, and Right: Take three measurements: one on the left side, one in the middle, and one on the right side of the opening.

  • Record the Smallest Measurement: Note down the smallest of these three height measurements. This is your rough opening height.

3. Measuring the Depth (or Thickness)

This measurement ensures the new window will be deep enough to fit into your wall opening and allow for proper installation of interior trim and exterior sealing.

  • Locate the Deepest Point: Look at the side of your window frame where it meets the wall. You want to measure the depth from the exterior face of the window frame to the interior face of the window frame.

  • Measure Across the Frame: Take a measurement across the entire depth of the window unit, including any brick mold or exterior trim that will be part of the replacement.

  • If Necessary, Measure Wall Cavity Depth: In some cases, especially with older homes, you might need to remove a small piece of interior trim to see the depth of the wall cavity. Measure from the exterior edge of the window frame to the back of the wall cavity. Your new window should be at least as deep as this measurement to ensure proper sealing.

Important Considerations

  • Measure Twice, Order Once: This old adage is especially true for windows! Double-check all your measurements before finalizing your order.

  • Check for Squareness: Use a carpenter's square or measure diagonally from corner to corner. The two diagonal measurements should be very close (within 1/8 inch). If they are significantly different, your opening is not square, and you may need to consult a professional or adjust your ordering.

  • Account for Rough Opening vs. Actual Window Size: Most window manufacturers will ask for your "rough opening" measurements. The actual window unit will be slightly smaller than your rough opening to allow for shimming, leveling, and sealing. Always confirm with your window supplier what size they need from you.

  • Exterior Trim (Brick Mold): If your existing window has exterior trim (like brick mold) that you plan to keep or replace, measure its width as well. This can affect how the new window sits.

  • Record Everything: Keep a clear record of all your measurements, including which was width, height, and depth, and where you took them.

By following these steps, you can confidently measure your old window and set yourself up for a successful replacement project!

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