Tools Required:

1. Measurement tape
2. Square angle tool

STEP 1

 

Check the opening corners with a square angle. If they were perfectly squared, go to step #2.
If the opening was not perfectly squared, then measure the height as shown, then measure the width as shown. The correct width of the opening will be line AB.

STEP 2

CLEAR OPENING measurement:
1. Measure the width 3 times as shown. Deduct 1″ from the SHORTEST value. That will be your window WIDTH.
2. Measure the height 3 times as shown. Deduct 1″ from the SHORTEST value. That will be your window HEIGHT.

EXISTING FRAME measurement
(the frame will be removed later):
1. Measure the width 3 times as shown.
Add 1″ to the SHORTEST value. That will be your new window WIDTH.
2. Measure the height 3 times as shown.
Add 1″ to the SHORTEST value. That will be your new window HEIGHT.