Open the window fully and remove any blinds, curtains, or screens
Measure the width inside the existing window frame at the top, middle, and bottom
Record the smallest width measurement
Measure the height inside the existing window frame at the left, center, and right
Record the smallest height measurement
Measure the depth of the window frame from the inside stop to the outside stop
Check that the frame is square by comparing diagonal measurements
Measure each window separately, even if they appear identical
Use a steel tape measure for accuracy
Write measurements in width x height format
Note the window style and any obstructions around the opening
Measure from the inside of the frame, not the trim
Confirm whether the replacement window will be insert-fit or full-frame
Double-check all measurements before ordering