Measure the width at the top, middle, and bottom of the window opening
Measure the height at the left, center, and right of the window opening
Record the smallest width measurement
Record the smallest height measurement
Measure the depth of the window frame or jamb
Measure the diagonal dimensions to check if the opening is square
Measure from inside stop to inside stop if replacing an insert window
Measure the full rough opening if replacing the entire window unit
Check the sill for level and the jambs for plumb
Measure each window separately, even if they appear identical
Note the type of window and opening style
Verify whether measurements are for the existing frame or the rough opening
Use a tape measure and record all measurements in inches
Double-check all measurements before ordering the replacement window