Open the door and measure the width of the door slab at the top, middle, and bottom
Use the smallest width measurement
Measure the height of the door slab on the left side, middle, and right side
Use the smallest height measurement
Measure the door thickness
Measure the width and height of the door frame opening if replacing the entire door unit
Measure the jamb depth from the interior trim to the exterior trim
Measure the hinge locations from the top of the door to each hinge
Measure the backset and bore hole locations if the door has a lockset
Check the swing direction by standing on the side where the door opens toward you
Record whether the door is left-hand or right-hand
Measure the threshold and sill dimensions if needed
Verify all measurements in inches and note them clearly