Every GSTIN follows the same fixed pattern. Once you know the pattern you can read a GST number at a glance and spot many errors without running a lookup at all.
This guide uses the generic placeholder 22AAAAA0000A1Z5. It is a format illustration, not a real registration.
The pattern
A GSTIN is 15 characters: two digits, then ten PAN characters, then an entity character, then Z, then a check character.
Characters 1-2: state code
The first two digits identify the state or union territory that granted the registration. These codes come from the census state code list and are the same codes used across GST forms. A few common ones: 07 Delhi, 09 Uttar Pradesh, 19 West Bengal, 24 Gujarat, 27 Maharashtra, 29 Karnataka, 32 Kerala, 33 Tamil Nadu, 36 Telangana.
If the first two digits are not a code that exists, the number cannot be genuine.
Characters 3-12: PAN
The next ten characters are the PAN of the registered person, in PAN's own format: five letters, four digits, one letter. The fourth character of a PAN indicates the type of holder — for example P for an individual and C for a company — so this section quietly carries information about the entity type as well.
Because PAN sits inside GSTIN, all the GST registrations of one legal entity share these ten characters.
Character 13: entity number
This shows which registration it is for that PAN within that state. A business with a single registration in a state normally has 1 here. Additional registrations in the same state — separate business verticals, for instance — use 2, 3 and onwards, continuing into letters once the digits run out.
Character 14: Z
In the standard format this position is the fixed letter Z. It is a placeholder rather than a meaningful field.
Character 15: check digit
The last character is a checksum computed from the first fourteen using a weighted modulus calculation over the 36 allowed characters (0-9 and A-Z). Its job is to catch typing errors: change one character anywhere in the number and the checksum almost always stops matching.
Worked example
Take the placeholder 22AAAAA0000A1Z5 and read it left to right:
- 22 — the issuing state code
- AAAAA0000A — the PAN of the holder
- 1 — the first registration for that PAN in that state
- Z — fixed character
- 5 — the computed check character
Change the entity number from 1 to 2 and you are describing a different registration of the same legal entity in the same state — and the check character would change too.
Common format mistakes
- Wrong length. Anything other than 15 characters is not a GSTIN. Extra spaces pasted from a PDF are a frequent cause.
- Letter O typed for zero. The PAN block mixes letters and digits, so this is easy to do and always breaks the checksum.
- Lower case. GSTINs are conventionally written in upper case; most tools normalise this automatically.
- Confusing GSTIN with PAN. PAN is ten characters, GSTIN is fifteen and contains the PAN.
Format valid does not mean real
A number can satisfy every rule above and still belong to no one. The checksum proves arithmetic consistency, nothing more. To know that a registration exists, and what state it is in, you have to run a lookup.
Where to go next
- Test a number with the GSTIN validator.
- Run a real lookup with GST number search.
- See the full GST state code list.
- Read what is a GSTIN for the wider picture.