Measuring suggestions for Retrofit Windows (for old aluminum or steel windows) . This is an example of retrofit windows. Before After Aluminum or Steel Windows Please note: Manufacturers suggest leaving the original frame in place and installing the new retrofit window inside the existing frame. 1. Remove screen and open the moving panel to identify the portion of the frame (jamb) that will remain when glass panels are removed. 2. Verify that the jambs are the same on both sides of the aluminum or steel window. 3. Width measurement: Using a tape measure, measure the distance between the sides (jambs), see detail. Measure at several locations. Keep the smallest or narrowest dimension. 4. Height measurement: Measure from the top (head) to the bottom (sill), see detail. Measure at several locations. Keep the smallest or shortest dimension. 5. Ordering Deductions: to insure the new window fits the opening size, deduct 1/4" from the width dimension and deduct 1/4" from the height. Note: Retrofit windows are custom made and may not be returned.