Measure the width from the inside edge of the left jamb to the inside edge of the right jamb
Measure the height from the inside edge of the head jamb to the inside edge of the sill
Take width measurements at the top, middle, and bottom
Take height measurements at the left, center, and right
Use the smallest width measurement
Use the smallest height measurement
Measure the window opening in three places for both width and height
Record measurements to the nearest 1/8 inch
Measure from the inside of the frame, not the trim
Check that the opening is square by comparing diagonal measurements
Measure each window separately
Note whether the window opens inward, outward, or slides
Measure any depth needed for replacement or new installation
Verify measurements before ordering or cutting materials