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How to Measure a Window for Replacement

Thursday October 20th, 2022
When replacing a window, it’s critical to order the correct size. To ensure you receive a window that will fit your window opening, you’ll need to measure the existing window you need to replace.

How to Measure a Window

  1. Gather Tools
  2. Ensure Windows are Square
  3. Measure Horizontally
  4. Measure Vertically
  5. Measure Depth of Opening
Ordering a replacement window is an exciting project, as you’re giving your home the upgrade it needs to improve its energy efficiency, aesthetics, value, and curb appeal. However, if you order a window that’s too big or too small for the opening, you’ll run into a host of problems that will cost you time and money.

Do You Measure a Window Inside or Outside?

It’s best to measure a window from both the interior and exterior because the dimensions of the window opening could be slightly different when measured from the inside or outside. Ideally, the measurements should be the same on both sides, or at least within an eighth or quarter inch of each other. Obtaining both measurements ensures accuracy and helps you get in touch with the space and material you’ll be working with.

There may be variances in dimensions due to a few different factors—such as the age of the home, thickness of the wall, and material. If you’re working in an older home, the frame and boards around the old window could have warped slightly, causing the window measurement to differ in certain areas. Also, the material around the window could be different from the interior/exterior, which could affect the measurement. For example, you may have a wood frame on the interior and aluminum or brick on the exterior.

Sometimes exterior frames are sloped, which could cause your exterior window measurement to be different than your interior measurement. If you’re working with a sloped window sill, you’ll want to measure the window height from the highest point of the slope, which will ensure your new window will fit into the opening.

When you’re outside measuring the exterior window opening, you’ll also want to inspect the space around the opening. This is necessary if you plan on installing a casement window or exterior shutters.

After you have recorded measurements from both the interior and exterior of the window, you’ll want to circle the smallest measurement of each. It’s okay if there’s a quarter inch difference in measurements, as the new window can be fitted properly with shims and caulking. If there’s a bigger difference in the measurements, though, it will need additional shim work on the window opening to ensure the new window fits into place.

The most important thing to remember is that you want your new window to be (slightly) smaller than the window opening. You most certainly don’t want it to be too big, because that would require more work, time, and money for window installation.  

How to Measure a Window

In order to avoid the complicated hassle of ordering the wrong-sized window, follow these steps to properly measure the existing window opening:  

1. Gather Tools

Before you measure the window, you’ll need to make sure you have all the necessary tools within reach. This will make the process easier and more efficient.

You’ll want to gather the following window measurement tools:
  • Measuring Tape
  • Level
  • Notepad (or something to write on)
  • Pen or Pencil
  • Ladder (if necessary)
Thankfully, measuring a window isn’t too complicated, nor does it require fancy tools. As long as you have a measuring tape and a way to record your dimensions, you’re set! A level isn’t necessarily required for the job, but it will show you if you’re window frame is sloped or unlevel in certain areas, which could affect your measurements.

You’ll want to record your measurements right away to ensure you write down the correct dimensions. This allows you to accurately compare each measurement, and then circle the smallest measurement at the end once you have all your measurements.
Before you begin, you’ll want to remove any blinds and structures that may get in the way of measuring the window.

2. Ensure Windows are Square

Now that you have tools handy, the first step is to check that your existing window frame is square. Square means that every corner of your window is at a 90° angle. When purchasing replacement windows, your new windows will be square, so you’ll want to ensure your window opening is also square for a proper fit.

As you might suspect, this is step is only for standard square windows. If you have an arched, round, or uniquely shaped window, finding its measurements will be slightly different. If that is the case, we recommend contacting a contractor to determine your window’s exact measurements.

To determine if your window frame is square, measure the window diagonally from the top corner to the bottom corner. To do this, hold the tape measure taut and place one end in the upper left corner of the window, then place the other end in the lower right corner. Record the measurement.

Doo this step again, but have the top end of the measuring tape in the upper right corner of the window this time and measure diagonally from there. Record the measurement and compare it to the first measurement. If the dimensions are within a quarter inch of each other, your window is square. If the difference is more than a quarter inch, you’ll want to speak with a contractor on how to best replace the window and see what additional work needs to be done.

3. Measure Horizontally

When ordering a replacement window, you’ll find that the dimensions are always recorded as width x height, no matter if you’re purchasing a traditional single hung window or custom-designed transom windows. Because of this, it’s best to record your width measurement first.

To find the width measurement, you’ll want to measure in three places: the bottom, middle, and top of the window. Starting with the bottom measurement, open the window, place the measuring tape in the center of the jamb and measure horizontally from the left jamb to the right jamb. Window jambs are the vertical part of the window where the sash (part of the window with glass) moves up and down in. For casement windows, this would be the part of the window that the sash sits in. Record your measurements.

If you can’t open the window, then measure from jamb edge to jamb edge, but don’t include the trim in this measurement. Repeat this step at the center of the window and again at the top. Record your measurements.

If you have a large picture window with indistinguishable jambs, measure from the inside of the window frame from one end to the other.
Once you’ve measured your existing window in three places horizontally, compare your dimensions and circle the smallest measurement. This is your rough opening width.

4. Measure Vertically

Now that you have your width measurement, you can find the height measurement for your replacement window. To determine the window height, you need to measure the window vertically in three different places, just like you did when finding the width measurement.

Measure from the head of the window frame down to the sill. The head is the top horizontal part of the window frame. The sill is the bottom horizontal part of the window frame where the sash sits when closed. Take a height measurement on the left, in the center, and on the right. Record your measurements.

This would be a good time to use a level to see if your window frame is level on the right, in the center, and on the left. This will tell you if there are any additional places you need to measure vertically to ensure you have the correct measurements.

Once you’ve measured your existing window vertically in three places, compare your findings and circle the shortest measurement. This is your rough opening height.

If you did steps two through four from inside the home, repeat these steps on the outside of the home or vice versa. Record your findings and compare your measurements. Circle the smallest measurement from your comparison.

Now that you have both the smallest measurements of the width and heights of your window, you can record it as width x height.

5. Measure Depth of Opening

While the width and height of your window opening are highly important, the window depth is also critical to ensure the new window fits properly. In most cases, window depth won’t be as much of an issue, but it’s important to ensure your window opening has enough depth for a replacement window. For instance, this could be more of an issue for mobile home windows or modular homes where the window depth is narrower.

To find the window opening depth, open the window and measure from the edge of the exterior frame to the edge of the interior window frame. You’ll want to measure in three different areas: on the left, in the center, and on the right. Record your findings and circle the narrowest measurement.

If your window has a window screen, it will need to be removed during this process so you can accurately measure the window depth.

If you can’t open your window, then you can find the window depth by measuring the depth on each side of the glass and adding them together. Then, add the thickness of the glass; one glass plane is usually three millimeters thick or an eighth of an inch.

You’ll want to make sure the window depth is at least three and a quarter inches for a new replacement window to fit.

Measuring A Replacement Window: Final Steps

Now that you have all the measurements recorded, you’re ready to purchase a replacement window. Before you do, however, it’s important to finalize your measurements.

Take a look at the width x height measurement you recorded (double-check it's the smallest of all the measurements). Now, round down those numbers to the nearest 3/8 of an inch. This will give you about 3/16 of an inch around the diameter of your replacement window. You want to shorten your measurement because you want your replacement window to be slightly smaller than your window opening.

You want to avoid a tight fit, as you want enough room for shims and caulking to fill the gaps properly. However, you don’t want too small of a window either, as it would increase the risk of drafts and leaks.

Tip: Measure all your home windows individually, even if they appear the same size! There may be variances in width and height, and even the slightest variance in window size can affect how well your replacement window fits. 

Purchase High-Quality, Custom Windows with Rustica Today!

If you are ready to upgrade your home with high-quality, energy efficient windows, browse our collection of custom windows today! Our windows are fully customizable, and can be made to fit any space you have in mind.

Once you have your measurements ready and a design in mind, all that’s left is for you to find a replacement window source you can trust. We believe we have the perfect window for you, and would be happy to assist in your window replacement project. Go ahead and contact Rustica today to find your dream replacement window!
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