Editing Code 128 (section)

===Check digit calculation===
The check digit is a weighted modulo-103 checksum. It is calculated by summing the start code 'value' to the products of each symbol's 'value' multiplied by its position's weight in the barcode string. The start symbol ''and'' first encoded symbol are in position 1. The sum of the products is then reduced modulo 103. The remainder is then converted back to one of the 103 non-delimiter symbols (following the instructions given [[#Conversion to char|below]]{{clarify|pre-text=invalid link, |post-text=(see {{diff||566495153||the difference}})|date=April 2025}}) and appended to the barcode, immediately before the stop symbol.

For example, in the following table, the code 128 variant A checksum value is calculated for the alphanumeric string PJJ123C:

{| class="wikitable" style="text-align:right;"
|-
! Code !! Value !! Weight !! Value × Weight
|-
| Start Code A || 103 || (1) || 103
|-
| P || 48 || 1 || 48
|-
| J || 42 || 2 || 84
|-
| J || 42 || 3 || 126
|-
| 1 || 17 || 4 || 68
|-
| 2 || 18 || 5 || 90
|-
| 3 || 19 || 6 || 114
|-
| C || 35 || 7 || 245
|-
|style="border-right:none"|
|colspan=2 style="border-left:none"| Sum || 878
|-
|style="border-right:none"|
|colspan=2 style="border-left:none"| Remainder mod 103 || 54
|}

So the value 54, which equals a V, is appended, then followed by the Stop code.

For the purpose of computing the check symbol, the shift and code-switch symbols are treated the same as any other symbol in the bar code.