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  • Maggie Courtis

How Do I Make a Stained Glass Window Fit a Defined Space?

Updated: Sep 13, 2023




a stained glass window mounted into a wall

Making a stained glass window fit properly into a window opening can be a bit tricky. Follow these simple steps to achieve success with your project.


If your piece isn’t going into a predesignated defined space, you have flexibility in the finished size of your panel. But, if the panel is going to be installed in an opening, such as a window opening in a building, you have to be precise in the finished size of the window.



The size of the window opening

First – we need an overall dimension of the finished piece. Be careful to take extremely accurate measurements and ascertain the window opening is square.

  • Make sure you measure level and square, parallel to the perimeter, as shown in the horizontal lines. Measuring crooked as shown in the vertical arrows will result in inaccurate measurements.

Illus. #1

taking care to measure square
  • Take measurements across the window at both the top and the bottom, and take measurements vertically in several places too.

  • Check the corner angles to see if they are square, 90 degree corners. If the corners aren’t square then make a template of the opening with the accurate shape of the corners.

  • Be aware of the quality of the tape measure you’re using. Often the edge of the tape has a characteristic that makes exact measurement difficult. Consider that beginning point on your tape to decide if it is best to measure from a different starting point. You can start from the 1” mark on the tape measure and then subtract an inch from your measurement. You see in illustration #2, the window opening is measured and found to be 4” total. We started with the 1” on the tape and calculated 5’ – 1’ = 4’ to arrive at that total.

Illus. #2

measure from the 1 increment on the tape

Now you have a window opening dimension. This opening size we’ve just arrived at by using a tape measure and taking the measurement. It is called the Tight Size of the window, shown in a black line in illustration #3, below.


You would not try to install a piece of glass the exact size of the opening into that space. You need a little wiggle room to install the panel and to allow for wood swelling when there is moisture in the atmosphere. The allowance for the wiggle room space is 1/8” (or 2 mm) on each of the four sides. It can also be considered ¼” (4 mm) in each of the two directions. This is called the Fitting Size, shown in a green line in illustration #3.


The third size is your Sight Size, which is what you will see. That line is shown in an orange in illustration #3.


Illus. #3

showing the tight, fitting, and sight sizes relative to one another on the pattern

Let’s imagine this window opening in the side of a home is 26” tall and 40” wide, the Tight Size.

  • We’re taking off 1/8” on each side (1/4” in each direction)

  • The window will be built to 25 ¾” by 29 ¾”, the Fitting Size.

You will not see a window that is 25 ¾ X 29 ¾ though. That finished size includes a border surrounding all the glass pieces. You will only see the glass in the Sight Size of this panel. The difference between the fitted size and the sight size of the window is the size of the border used. Border materials are available in a variety of sizes, shapes, and materials. Please refer to our blog article, Borders for Stained Glass Windows, for that information.


Why is defining the sight size important? For the design to end up right, the border must be considered. Areas could end up “chopped” when some glass is lost to the viewer because it is stuck in the border.

Illus. #4

planning the design, considering losing a bit of the image around the edge

In illustration #4 we've sized a piece of paper to the finished size (the Fitting Size) of the window. We drew a design out to the very edge of the paper.

In the second image you see the border material. We hadn't thought about the portion of the window we will lose visually to border when we drew the image. Notice the tops of the petals and leaves are cut off of the design when the border is added? This illustrates what we mean when we say you will only see the glass in the Sight Size of this panel.


We are showing an image of 1/2" flat H came in illustration #5. Glass will fit into this border came as shown . If you use a different shape or size came, the principle illustrated still holds true: you will lose some glass visually to the border.


Illus. #5

glass in the outer border, showing the amount of material lost visually

Now you know how to make window fit into a designated space by:

  • carefully measuring and assessing the integrity of the square of the space

  • allocating "wiggle room" to install the window and allow for expansion of the casing

  • considering the loss of visual elements in the border.

Next up? Designing the Pattern for Your Stained Glass Window



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