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 sides of the opening
Record the smallest height
Measure the depth of the window frame
Measure from inside stop to inside stop for width
Measure from sill to head jamb for height
Measure the diagonal from corner to corner to check for square
Measure each window separately
Measure in at least three places for each dimension
Round down to the nearest 1/8 inch if needed
Use the exact manufacturer’s measuring instructions if provided
Note whether the window is single-hung, double-hung, casement, slider, or fixed
Measure the rough opening if replacing the entire unit
Measure the visible glass area if only replacing the sash
Check for obstructions around the frame
Confirm whether the replacement window will be insert or full-frame
Write down all measurements in width by height format
Verify measurements before ordering
Use a metal tape measure for accuracy
Measure from the interior unless instructed otherwise