Measure the interior width (from one side jamb to the other) and the interior height (from the top jamb to the sill) of the window frame.

- Especially for older homes, or if you see any separation between the moldings and the jamb, take several measurements (i.e. top, middle and bottom, or left, center, and right).
- Because a tape measure slants at the corners and throws the measurements off, use a card whose measurements you know (eg. one that is exactly 3″ long) to help you get an exact measurement. Add the card’s measurement to the number on the tape measure.
- If the window isn’t quite square, the 1/2″ foam tape around the winsert will make up for some of the discrepancies. If the measurements are within 1/4″ of each other, use the median measurement.
- If you have several windows that are close but not exactly the same size (i.e. within 1/4″ of each other), use the median measurements there, too, so that you can cut and make frames of the same size rather than fussing with very slightly different individual sizes.

Once you’ve measured the window, calculate the size that the frame needs to be by subtracting 3/4″ from each dimension. This will allow for 3/8″ of room for the slightly compressed foam tape that will be around the edge of the finished winsert.

eg. For a window with interior measurements of 24″ x 36″ window, the finished winsert frame should be 23 1/4″ x 35 1/4″. (Note that some people have made panel frames only 5/8” smaller than the window opening: 23 3/8″ x 35 3/8″. This gives a more snug fit, and is better for smaller straight-framed windows.)

Then calculate how long your frame pieces need to be.

- The two side pieces should be the full height of the frame (in the example above, 35 1/4″).
- The three cross pieces should be the width of the frame minus the widths of the two wide side pieces. So if you’re using 1″ wide strips of wood, the example above would be calculated as follows:

23 1/4″ minus 1″ for left side minus 1″ for right side = 21 1/4″ long cross pieces

If your strips of wood are 1 1/4″ wide, the calculation would be:

23 1/4″ minus 1 1/4″ for left side minus 1 1/4″ for right side = 20 3/4″ long cross pieces

NEXT – Step 2: Making the frame

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