Open the window and remove any blinds, curtains, or trim that block access
Measure the width at the top, middle, and bottom of the window opening
Record the smallest width measurement
Measure the height at the left, middle, and right sides of the window opening
Record the smallest height measurement
Measure the depth of the window frame from the inside stop to the outside stop
Check that the opening is square by comparing diagonal measurements corner to corner
Record all measurements in inches
Measure each window separately, even if they appear identical
Measure from the inside of the existing frame, not the exterior trim
Use a tape measure and keep it straight and level
Double-check all measurements before ordering replacement windows