|Before you measure, you will need to remove the screen and open the window.
Width: Measure between the 2 jamb faces as shown in the image below for the width of your opening.
Your new window frame width is the measured width ('W') minus 1/2". This will be the width of the 'Net Frame Size' or NFS.
Always reference your measurements with the width dimension before the height (i.e. 48 1/4" (width) x 36 1/2" (height)).