Measure the width at the top, middle, and bottom of the window opening
Measure the height at the left, middle, and right sides of the window opening
Measure the diagonal from top-left to bottom-right
Measure the diagonal from top-right to bottom-left
Use the smallest width measurement
Use the smallest height measurement
Measure from jamb to jamb, not from trim to trim
Measure the window opening in at least three places for both width and height
Record measurements to the nearest 1/8 inch
Check that the opening is square by comparing diagonal measurements
Measure the depth of the window frame if required by the manufacturer
Remove interior trim only if necessary to expose the rough opening
Measure the rough opening if ordering a full-frame replacement window
Measure the existing window sash if ordering an insert replacement window
Verify the window type and opening style before ordering
Double-check all measurements before placing the order