Every business registered under GST holds a 15-character GSTIN (Goods and Services Tax Identification Number) for each state it operates in. It looks like a random string, but every position means something - which is why you can catch a lot of fake or mistyped numbers before you even look them up.
Take 29AAACI1234F1ZY (a made-up number with a valid check digit) as an example.
The five parts
| Positions | Example | What it is |
|---|---|---|
| 1-2 | 29 | State code - 29 is Karnataka |
| 3-12 | AAACI1234F | The business's PAN |
| 13 | 1 | Registration count under that PAN in the state |
| 14 | Z | Always the letter Z |
| 15 | Y | Check digit |
State code (1-2)
The first two digits are the GST state code: 01 is Jammu & Kashmir, 27 Maharashtra, 29 Karnataka, 33 Tamil Nadu, up to 38 for Ladakh. 97 is "Other Territory" and 99 is the Centre.
Registration is per state, so a company with branches in five states holds five GSTINs on the same PAN. When you verify a supplier, check the GSTIN for the state the invoice is raised from.
PAN (3-12)
Characters 3 to 12 are the PAN of the business. Its fourth letter tells you the type of entity:
- C - company
- P - individual (a proprietorship)
- F - firm or LLP
- H - Hindu Undivided Family
- T - trust
If an invoice says "Pvt Ltd" but the PAN's fourth letter is P, the GSTIN belongs to an individual, not the company named on the invoice.
Registration count (13)
Position 13 counts registrations under that PAN in that state: 1 to 9, then A to Z. Most businesses have a 1 here.
The fixed Z (14)
Position 14 is always Z. Anything else is a typo or a made-up number.
Check digit (15)
The last character is computed from the first fourteen with a mod-36 checksum. Change any one character and the check digit stops matching - so a number that "looks right" but was typed from memory usually fails this test.
GST VerificationVerify business details from a GSTIN.Format is not status
A GSTIN can be perfectly formed and still useless to you. The registry also records whether the registration is Active, Suspended or Cancelled, the date it was granted, the legal and trade names, and every place of business. Input tax credit depends on the supplier being registered and filing, so a cancelled GSTIN on an invoice is a problem for your books, not just theirs.
That is why the check has two steps:
- Read the format - state, PAN type, the Z, the check digit.
- Look the number up in the GST registry and confirm the status and the legal name match the invoice.
Going further
The PAN inside the GSTIN connects to other registers. A company (fourth letter C) also has a CIN on the MCA register - a company lookup shows its directors and status. Small businesses often hold a Udyam registration too, which an MSME lookup can confirm.
