How the 15-Digit GST Number is Formatted in India
The Goods and Services Tax Identification Number (GSTIN) is a standardized 15-digit alphanumeric code assigned to every business registered under the Goods and Services Tax Act in India. Designed by the GST Network (GSTN), every character in the 15-digit string serves a precise legal and mathematical purpose.
Two digits representing the state or union territory where the business is registered according to the Indian Census 2011 (e.g., 07 for Delhi, 27 for Maharashtra, 29 for Karnataka).
The 10-digit Permanent Account Number (PAN) issued by the Income Tax Department. The 4th character denotes the legal constitution (C for Company, P for Person/Proprietor, F for Firm/LLP).
Represents the number of registrations the same PAN holder has within that specific state. It starts from 1 to 9, followed by alphabetical letters A through Z for further vertical business units.
The 14th character is currently defaulted to the letter 'Z'. The 15th character is a mathematically calculated check digit using the Luhn Mod-36 algorithm to detect typographical errors.
Entity Type Identification (4th Letter of PAN)
By inspecting the 4th letter of the PAN embedded inside any GSTIN (character #6 of the GSTIN), you can instantly determine the legal structure of your buyer or vendor:
- P — Individual / Sole Proprietorship: The most common business form for small retail shops, independent consultants, and freelancers.
- C — Company: Private Limited (Pvt Ltd) or Public Limited company registered under the Companies Act.
- F — Partnership Firm / LLP: Traditional partnership firms or modern Limited Liability Partnerships.
- H — Hindu Undivided Family (HUF): Joint family business structure governed under Hindu Law.
- T — Trust: Registered charitable or private trusts.
- A / B — Association of Persons (AOP) / Body of Individuals (BOI): Cooperative societies or unincorporated entities.
- G / L — Government Agency / Local Authority: Municipal corporations or state departments.
Understanding the Luhn Mod-36 Checksum Algorithm
The GST Network mandates the Luhn Modulo-36 algorithm to verify that an entered GST number is free of typographical mistakes, transpositions, or transcription errors before generating tax invoices. In base-36, numerals 0 through 9 are assigned values 0–9, and letters A through Z are assigned values 10–35. Alternating weights (1 and 2) are multiplied across the first 14 characters, and the resulting modulo-36 remainder produces the exact 15th check character.