How to Measure Windows for Replacement

Published August 11, 2020
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Lake Washington Windows

Getting correct measurements to replace your current windows is an important step for planning your window replacement project. Here are some tips if you want to DIY, or contact your local window replacement company to be connected to a window replacement specialist.

What is in this article:

  1. To know before you get started
  2. Check for square
  3. Measure width
  4. Round the numbers
measuring window sill

What to Know Before

Some things to know before getting started on your window replacement project.

  • Width first, height second. First thing to know is that when measuring or talking about window sizes, always mention width first.
  • Record measurements. Width x Height
  • Find the square. You’ll want your frame to be square.
  • Round numbers. Round measurements down to the nearest ⅛ of an inch.

Firstly, Is Your Frame Square?

Before any other measurements, you’ll want to check your window frame for square. Checking for square means to see if the angles are 90 degrees. If you find that your frame is out of square, then you will likely want to hire a window professional to handle the installation to ensure the opening is square and the window operates properly after installation.

How to Check for Square

Measure from top left corner to the bottom right corner.
You’re going to measure the diagonal length of the window frame. Go to the top left corner of the window, and measure from the inside of the trime to the inside of the on the bottom right.

Measure from the top right corner to the bottom left corner. 
Do the same as above but opposite.
An easy way to think of this is to make an X from the inside corners of the window frame. If the legs of the X are not equal, within 1/8 to 3/16 inch, then you have a frame that is out of square.

Taking Measurements for Your Replacement Windows

Measure Width.

To measure the width, you will need to measure in three places.

  1. One at the top
  2. One in the center
  3. One at the bottom 

Measuring between the side jams at the widest part you can access, move the sash out of the way to take accurate measurements. Circle the smallest measurement – this is your width measurement.

Measure Height.

Next, measure the height of the window. Here you will also take measurements in three different places.

  1. One at the far left
  2. One in the center 
  3. One on the far right

For the sake of accuracy, we’ll need to define the window sill. The sill is the horizontal board at the bottom of the window that the sash (part of the window that moves and contains the glass) sits on. The sill is not the bottom trim. The bottom trim is often called the sill, but it is actually known as the stool of the window, and the stool is not important here.

You will have to open the window to take measurements from the window sill to the head jamb. The head jamb is the top horizontal board above the window, like the window sill but uptop. 

Circle the smallest measurement – this is your height measurement.

Measure Depth.

The depth size is pretty standard for vinyl windows, and typically only varies in mobile homes and modular homes, both of which have narrower depths. However, you’ll want to measure it just in case.

You will only need one measurement for the window depth. The depth should be at least 2-⅞ inches.

To measure the depth, open the window and measure from the inside of the exterior and interior trims.

Round Your Measurements

Take all of your circled measurements – these should be the shortest measurements –  and round them down to the nearest ⅛ inch. So if you have any fraction where the denominator is not an 8, you’ll need to make it a fraction with an 8 as the denominator. 33-5/16 would be turned into 33-2/8 (33-¼ really but keep the 2/8).

Buy Your Replacement Window

Now that you have your measurements it’s time to buy your window. For purchasing options, contact your local window company.

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Written by Lake Washington Windows

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