"Ground floor" vs. "first floor"

It is ground floor in GB and first floor in the US.

The first three levels top to bottom are:

US:

Third Floor

Second Floor

First Floor

GB:

Second Floor

First Floor

Ground Floor

These are usually interchangeable, but I was recently visiting a relative at a major hospital on the shore of Lake Erie and when I came down the elevator I pressed "G" and found myself one floor below the one with the main entrance/exit. So, it's not entirely standard.

Normally there is a slight difference between the ground floor and the first floor depending on a commercial or residential building. Commercial structures refers to the entrance level as the first floor of the building and residential structures refers to the entrance level as the ground floor of the building and the first floor being above the entrance level.

I've heard it called the first floor in areas where basements are uncommon. "Ground Floor" is used to describe the floor on the ground because the first floor is actually the basement. I have seen this practice used at universities. University elevators have a first floor, but the ground floor (usually floor 2) is denoted with a star to indicate the "Ground Floor".