Open the door fully
Measure the width of the door slab from edge to edge
Measure the height of the door slab from top to bottom
Measure the thickness of the door slab from one face to the other
Measure the door frame opening width at the top, middle, and bottom
Measure the door frame opening height at the left, center, and right
Measure the frame depth from the inside edge to the outside edge
Record the smallest width and height measurements
Measure in inches or millimeters
Double-check all measurements before ordering or replacing the door