Decipher the Postal Bar Code

Could your mailbox be a crime scene? Bar codes are one way that the U.S. Postal Service streamlines mail delivery and keeps mail out of the wrong hands. A letter that comes without a bar code or cancelled stamp might not have been processed by the U.S. Postal Service, and it might indicate that something's fishy.

Can you decode a bar code the way the U.S. Postal Service can? Analyze and try out the methods that the U.S. Postal Service used to encode the mail.

Illustration: A hand reaching toward a mailbox


You Will Need

  • Several different pieces of mail

Here's How

1. Here's how to read the bar code on your letter.

  • Most U.S. letter bar codes have bars of two lengths: long and short.
  • Every bar code should start and end with long bars; ignore those.
  • Look at the rest of the bars in groups of five, starting with the second bar. Each group of five represents one digit.


2. Here's how to figure out what those digits are.

  • The five bars stand for one of these numbers: 7, 4, 2, 1, 0.
  • Some genius figured out that this range would allow you to come up with the digits 0 to 9. You do it by adding the numbers of the long bars together.

Say you have this bar code:

Image: A bar code

Here, the 4 and 2 places have long bars. Added together, they total 6, so this bar code stands for 6.

Say you have this bar code:

Image: A bar code

The 1 and 0 places have long bars. Added together, they total 1, so this bar code stands for 1.

Say you have this bar code:

Image: A bar code

The 7 and 4 pieces have long bars. Added together, they total 11, but there is no 11 in this system, so this bar code stands for 0.

3. Now let's look at the bar code on a letter.

Image: A bar code

 

Ignore the first and last bars of the whole bar code. Then look at the next 25 bars to find the zip code (in the example, the bar code is 06801). The next two sets of five bars give your mail route (in the example, the mail route is #1). The next two sets of five bars are your mail carrier’s number (the example mail carrier is #53). The rest of the bars in the bar code indicate the house or mailbox number. The house number is 32.


NOTE:
Many bar codes include an extra set of five bars at the end, after the house or box number. This is called a checksum. Add it to all of the other digits in the barcode, and the sum should be a multiple of 10. If it doesn’t add up, this shows something’s wrong or missing in the bar code. Additional numbers may be needed to indicate that the mail requires special services, such as a registered or certified piece of mail that needs a special signature.

 

Adapted with permission from the book Science Fair Winners: Crime Scene Science by Karen Romano Young. Available where all books are sold.

Illustration by David Goldin.