We’ve all bought something online, found out that UPS will deliver our package, and then proceeded to obsessively check the delivery status of our order on the Web with a specific—and rather annoyingly long—tracking code. While you’d be forgiven for assuming that UPS tracking codes are generated randomly, those numbers (and letters) actually contain a lot of information to ensure that your package is delivered to the right doorstep. So what do all those numbers (and letters) mean?

The most common UPS tracking code format for standard domestic U.S. packages contains 18 alphanumeric characters: 1Z aaa aaa bb cccc ccc d. UPS has other tracking codes, which range from seven to 20 characters, but this format is the one you’ll see most commonly. So how does it work?

Let’s break it down:
• The first set of characters indicates the style or type of tracking code. All codes in this format begin with 1Z, which means that the package is a domestic shipment.
• The second and third sets of characters, aaa aaa, are the account number of the shipper, assigned by UPS for billing purposes. Both businesses and individuals can have an account with UPS.
• The service code, bb, is the shipping method of a package. A few codes:

  • 01 = UPS Next Day Air
  • 02 = UPS Second Day Air
  • 03 = UPS Ground
  • 12 = UPS Three-Day Select 

(A full list of codes and their meanings is available in the UPS Rates and Service Selection XML Tool [PDF].)

• The next seven characters, cccc ccc, are the package identifier. This is customizable. It’s a set of numbers and letters a shipper uses to recognize a package; many shippers use a truncated invoice number.
• The check digit, d, is used to detect errors and to verify that all of the characters in an identification number are entered correctly. Check digits are also found in UPC codes, ISBNs, credit cards and routing transit numbers on U.S. bank accounts. This number is generated through a proprietary algorithm. (More on how check digits work.)

Mystery solved.