How to Measure
For door kits and prehung doors, measure the rough opening width and height.
For slab only replacement, measure the slab itself.
Thickness: Standard entry doors are 1.75" or 1 3/4" thick while interior doors are typically 1 3/8" thick.
Width: Most steel and fiberlass exterior doors will measure 1/4" less than their common size (e.g., most 3/0 or 36" steel doors measure 35.75" wide) vs. most wood doors will measure nearly the full amount of their common size.
Height: The same is true for height, with most "6/8" or 6ft 8in steel and fiberglass slabs measuring 79.0" or 79.25" tall (not including the rubber sweep on the bottom). 6/8 wood doors are typically a full 80" tall. 8' or 96" tall doors can be special ordered.