In the U.S. 99% of commercial doors are 1 ¾” thick. Most standard door hardware is manufactured to fit 1-3/8" - 2" thick doors. Some manufacturers have extension kits to fit a thickness over 2 inches. A typical residential interior door is 1-3/8" thick and an exterior door is 1-3/4" thick. Again, interior and exterior commercial doors are 1-3/4" thick.