An example of alphanumeric encoding in a single Code 128 bar code is:

The steps for calculating the check digit are as follows:

1. Take the value of the start character (103, 104, or 105) and make that the starting value of the running checksum.
2. Starting with the first data character following the start character, take the value of the character (between 0 and 102, inclusive) multiply it by its character position (1) and add that to the running checksum.
3. Take each additional character in the message, take its value, and multiply it by its character position, and add the total to the running checksum.
4. Divide the resulting running checksum by 103. The remainder becomes the checksum digit which is added to the end of the message.
5. The stop character is appended after the checksum digit.

Barcode	START-A	H	I	CODE-C	34	56	78
Character Value	103	40	41	99	34	56	78
Character Position	-	1	2	3	4	5	6
Calculation	103	40 * 1	41 * 2	99 * 3	34 * 4	56 * 5	78 * 6
Weighted Sum	103	40	82	297	136	280	468

Summing up the running checksum for each digit, we get 103 + 40 + 82 + 297 + 136 + 280 + 468 = 1406. This value divided by 103 is 1406 / 103 = 13 with a remainder of 67. Thus the checksum digit is the character which has a value of 67.

NOTE: Note that the checksum starts with the first Start Character, with a weight of 1, and that the first data character also has a weight of 1.

In the following text, we will discuss the encoding of the bar code by considering that the number "1" represents a "dark" or "bar" section of the bar code whereas a "0" represents a "light" or "space" section of the bar code. Thus the numbers 1101 represents a double-wide bar (11), followed by a single-wide space (0), followed by a single-wide bar (1). This would be printed in the bar code as:

The Code 128 specification defines three "character sets" or "character modes." The start code that is used determines which character set will be used. The character set may also be changed in the middle of the bar code. For example, in the bar code above the bar code starts in "Character set A" to encode the text "HI", and then switches to "Character set C" to more efficiently encode the numbers that follow.

To encode a value as a Code 128 bar code, the checksum digit must first be calculated (see procedure above) and the entire bar code, including check digit, may then be encoded as a sequence of bars and spaces.

A Code 128 bar code has the following physical structure:

1. Start code, which is the code 103, 104, or 105 from the encoding table below (either 11010000100 (Start-A), 11010010000 (Start-B), or 11010011100 (Start-C).
2. Each of the data bytes of the message, encoded with the encoding table below.
3. The checksum byte, calculated as described above and encoded using the table below.
4. Stop character of 11000111010.
5. Termination bar of 11.

We encode each digit using the encoding table above.

1. The START-A character: 11010000100.
2. The digit "H" encoded as: 11000101000.
3. The digit "I" encoded as: 11000100010.
4. The "CODE-C" character: 10111011110.
5. The digits "34" encoded as: 10001011000.
6. The digits "56" encoded as: 11100010110.
7. The digits "78" encoded as: 11000010100.
8. The checksum digit of 67 encoded as: 10000101100.
9. The STOP character: 11000111010.
10. The termination bar: 11.