A unique identifying number for all oil and gas wells drilled in the U.S. The system was developed by the American Petroleum Institute.

An API well number can have up to 14 digits. The first two digits are the state code, the next three digits are the county code, the next five digits represent the unique identification number for the well, and sometimes you will see an additional two digits that represent a sidetrack code and another two digits that represent a sequence of events.