In the United States, ZIP codes are five digits long and go from 00000 to 99999: a total of just a hundred thousand options. Given that the US has a total land surface area of 9,158,022 square kilometers, this gives just under 100 square kilometers per potential ZIP code. They can never be more accurate than that (on average). This does help narrow down the delivery of mail, but the rest of the address is needed for finer resolution.
