A UPS tracking number is an 18-digit code composed of letters and numbers that starts with "1Z" followed by a 6-character shipper number (a combination of numbers and letters), a 2-digit service level indicator, and finally, 8 digits identifying the package (the last digit being a check digit), for a total of 18 characters. It is important to note that UPS tracking numbers for domestic packages within the United States usually start with "1Z".