Understanding the Indian GSTIN Modulo-36 Checksum Algorithm (Developer Reference)

Structure of a 15-Digit Indian GSTIN

Every GST identification number in India conforms to the following 15-character pattern:

[State Code: 2 Digits][PAN: 10 Chars][Entity Number: 1 Char][Default 'Z'][Checksum: 1 Char]
Example: 27 AABCS 1429B 1 Z 6

The Modulo-36 Calculation Logic

The 15th character is a mathematically calculated check-digit derived using a base-36 character set (0-9 followed by A-Z):

1. Assign code points 0 through 35 to characters 0-9A-Z.
2. Traverse the first 14 characters, alternating weight factors of 1 and 2.
3. Multiply the code point by the factor, add the quotient and remainder divided by 36.
4. Calculate the modulo-36 remainder and derive the expected 15th check character.

EntryFlow validates this locally in sub-millisecond time on every incoming invoice before running LLM extraction, eliminating invalid vendors before they enter your accounting system.