Ground floor – First floor:
In British English, the floor of a building which is level with the ground is called the ground floor. The floor above it is called the first floor, the floor above that is the second floor, and so on.
In American English, the floor which is level with the ground is called the first floor, the floor above it is the second floor, and so on.
(Collins COBUILD English Usage)
Though there are exceptions to the above-mentioned usage,( and exceptions are not the issue here) in public buildings in the U.S., for instance, it’s also possible to call the street-level floor the ground floor, like in Britain, but how come that in the UK and Europe the ground level floor and the first floor are respectively referred to as the first floor and the second floor in the U.S. (and so on for higher floors). Was it a custom imported into the U.S. from a different culture?
Related:"Ground floor" vs. "first floor".