First, gently remove all 3 sides of the millwork/trim from interior side of door. This will reveal where the wall surface meets the studs surrounding the door opening.

Depth of Opening: Place the end of your tape measure on the interior edge of the door jamb and measure to the exterior edge of the door jamb.

Height of Rough Opening: Place the end of your tape measure between the wall and stud above door opening and measure to the floor.

Width of Rough Opening: Place the end of your tape measure between the wall and stud. Then measure to the opposite side.