how to read the postal bar codes on letters

when you receive a letter you will see
a post office bar code printed near the
bottom of the letter under your name
and address.

the bar code is in two formats. the older
format is your 9 digit zip code plus one
digit for a check sum. the newer format
contains an 11 digit zip code plus one
digit for a check sum. im not sure what
the extra 2 digits are for. the first form
has 52 bars, and the 2nd form has 62 bars.

the tall bars are 1/8 inch tall (3 mm) and
the short bars are 3/32 inch tall (1 mm).
all the bar codes stacked together are
either 2 3/8 inches (61 mm) or 2 13/16 inches
(72 mm) long depending on the format.


to read the postal bar codes its
done like this.

the first  and last bars which are always
tall bars "|" are always ignored. they are 
used for the hardware to identify where 
the barcode starts and stops.

to read each digit of the zip code
we start grabbing bar codes in 
groups of 5 bars after the 1st bar
which is ignored.

for any of the bars if it is a
tall bar give it the following
values. if its a short bar give
it a value of zero. add the values
of the tall bars up to get the
value of that digit.

      1st bar  -   7
      2nd bar  -   4
      3rd bar  -   2
      4th bar  -   1
      5th bar  -   0   (yep thats zero)

add up the 5 lines and you have
the value for that digit. the only
odd ball case if if the 5 lines 
add up to 11 then use zero for that
digit

    0        ||...
    1        ...||
    2        ..|.|
    4        .|..|
    5        .|.|.
    6        .||..
    7        |...|
    8        |..|.
    9        |.|..

my tempe zip code is 

   85281-5603

so the bar code should say

  85281-5603-2

the extra digit at the end which
is a "2" is a check sum.

to caculate the check sum of a zip code
add up all the digits in your zip
code. take the last digit in the sum
and subtract it from 10 
to get the check sum.

for my tempe zip code of

    85281-5603

this is


    8
    5
    2
    8
    1
    5
    6
    0
    3
-----
   38

the sum is 38 for my zip code.

take the last digit which is 8
and subtract it from 10 to get
the check sum

   10-8=2

so my check sum for my zip code is 2.

the 11 digit bar codes also have a
check sum thats caculated the same way.
but im not sure which digit is the
check sum