First, check for a TOP TRACK under the meeting rail of the window, where the two parts of the window come together. The TRACK with be a U-shaped channel that the screen will slide up into.
Next, find the LIP at the bottom of the window. This lip will be directly below the top TRACK. The bottom of the screen will fit behind this lip to hold it in place.
The aluminum window shown in the picture above has an inside lip and outside lip (inside for insect screen; outside for solar screen). If yours is similar to the picture sure and find the one closest to the window glass, the 'inside lip'.
Now that you've located the tracks, let's start measuring!!
Insert the tape measure into the TOP TRACK you found earlier. Make sure the tape measure fits all the way to the top of the track. Hold it downward as straight as possible.
Make sure the measuring tape is as far up into the TRACK as possible, see below.
Note: when measuring the bottom lip, make sure that you measure about 1/16'-1/8' ABOVE the top of the LIP, and provide the measurements to the nearest 1/16'.
This is the 'Height' measurement of your window screen.
Starting on the left and going right, place the measuring tape on the side of the window. Hold it as level and straight as possible.
As shown below, put the measuring tape on the left side of window, where the screen will be placed.
Then, measure to the right side of the window. Subtract 1/16' from this measurement to ensure the screen will not be too tight to fit in the window. Provide the measurements to the nearest 1/16'.
Remember, to ensure the screen is easy to install and remove, do not measure the screen too tight. Make sure to subtract about 1/16' from your measurement.
This is the 'Width' measurement of your window screen.