How to Measure for Replacement Windows in 9 Easy Steps

Widen your view of window measurements

Alison Kasch
Written by Alison Kasch
Updated May 12, 2022
mother and toddler looking out the window
Photo: MoMo Productions/ Getty Images


You've got this!

Time to complete

1 hour

<1 hour, depending on the number of windows

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What you'll need:


  • Measuring Tape (non-retractable strips work best)
  • Pencil
  • Notepad

Whether due to a rogue ball or wear and tear, there sometimes comes a time when you know it’s time to replace your windows. For accurate quotes and to order the right size window replacement, precise measurements are key. Here’s how to measure for replacement windows clearly and accurately.

9 Steps to Measure For Replacement Windows

  1. Ensure Windows Are Square

    Before measuring for a replacement window, it’s important to ensure windows are square, meaning all four corners are at 90-degree angles. New windows will be square, so if the current glass isn’t, this could prevent the windows from opening properly.

    Here’s how to check for square:

    • From the inside of your home, start at the top right corner of the glass, right where the vertical and horizontal trims meet.

    • Hold the tape measure taut and measure diagonally to the bottom left corner where the two trims meet.

    • Record the measurement.

    • Take a second measurement starting from the top left corner and down diagonally, just as you did for the other side.

    • Record and compare the two measurements.

    If both measurements match with 1/4 of an inch, then you can safely order replacement windows for a correct fit. If not, you will need to replace the window frame beforehand or order a new-construction window instead of a replacement window.

  2. Record Proper Site Measures as Width by Height

    When you measure a window, always record your measurements as width x height. For example, if your windows are 60 inches tall and 34 inches wide, your measurement should read 34 inches x 60 inches.

  3. Measure the Width of the Window Opening From the Top, Middle, and Bottom

    woman measuring width of window
    Photo: Gerald Carter/Creative Flame/ Adobe Stock

    Measure the width of the window opening starting from the inside of the left jamb to the inside of the right jamb. Thejamb is the vertical part of the window where the window sash slides up and down. Record measurements from the top, middle, and bottom.

    You shouldn’t include the trim in these measurements.

  4. Note the Shortest Width Measurement

    After you’ve recorded the width from the three different spots, compare these measurements. The shortest measurement will be your true window opening width.

  5. Measure the Height of the Window Opening From the Left, Center, and Right

    Starting from the left side, measure vertically from the windowsill to the top of the window opening (excluding the window frame and trim). Record measurements for the left, center, and right sides.

  6. Note the Shortest Height Measurement

    Comparing your three measurements, circle the smallest number. This will be the true height of your replacement windows.

  7. Use the Following Method to Test Window Depth

    Window depth shouldn’t be an issue unless it’s too narrow to hold a replacement window. To test it, open the window and measure the sill from trim to trim. It should be a minimum of 3 1/4 inches. If your window depth is narrower than that, then you will need to order a new-construction window instead of a replacement window.

  8. Measure Each Window Separately

    Even if you’re sure that all the windows in your home are the same size, it's possible that they have slightly different measurements. To stay on the safe side, measure each window separately before you order new windows.

  9. Order Your Replacement Windows

    professionals installing new window in home
    Photo: yuriygolub/ Adobe Stock

    After you’ve recorded the smallest width and height measurements (width x height), you’re all set to get your replacement windows ordered. Replacements should be within 1/8 inch of your measurements. You should also decide which type of replacement windows will work best for your home and budget, whether it’s vinyl windows, composite, or another style.

    You may find the size you need at your local home improvement store, but it’s typically best to order custom windows for the perfect fit. Order online or contact a window replacement service near you to finish the project.

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