Measure the width at the top, middle, and bottom of the window opening
Record the smallest width
Measure the height at the left, middle, and right of the window opening
Record the smallest height
Measure the depth of the window frame or jamb
Measure from the inside edge of the trim or stop, not the outer casing
Check that the opening is square by comparing diagonal measurements
Measure each window opening separately
Note the window type and whether it is a replacement or new construction window
Verify the manufacturer’s required rough opening size before ordering
Use a steel tape measure for accuracy
Write all measurements in width by height format
Double-check every measurement before placing the order