Every GST number in India is exactly 15 characters long and follows one fixed pattern. Once you can read it, you can spot a malformed GSTIN on sight — often without any lookup at all.
The format at a glance
Pattern: SS PPPPPPPPPP E Z C — state code, PAN, entity code, the letter Z, check digit. Applied to the example 27AAPFU0939F1ZV:
| # | Character(s) | Rule | In the example |
|---|---|---|---|
| 1–2 | Digits | State code of the registration, 01–38 plus special codes. Must be a real code — “00” or “45” is instantly invalid. | 27 = Maharashtra |
| 3–7 | Letters | First five characters of the PAN. The 5th of these (position 4 of the PAN) encodes the entity type: C company, P individual, F firm/LLP, H HUF, T trust, A AOP, and so on. | AAPFU — the F marks a partnership firm |
| 8–11 | Digits | The four numeric characters of the PAN. | 0939 |
| 12 | Letter | The PAN’s own check character. | F |
| 13 | Digit or letter | Entity code: 1–9 for the PAN’s first nine registrations in the state, then A–Z for further ones. | 1 — first registration in Maharashtra |
| 14 | Letter | Default character, currently always Z, reserved for future use. |
Z |
| 15 | Digit or letter | Check digit computed from characters 1–14. | V |
How the check digit works
The 15th character is not chosen — it is calculated. Each of the first 14 characters maps to a value (0–9 as themselves, A=10 through Z=35). Moving along the string, values are alternately multiplied by 1 and 2; each product is reduced by adding its base-36 “digits” together; the results are summed; and the check character is whatever value rounds that sum up to the next multiple of 36. Change any single character and the sum changes, so the stored check digit no longer matches.
You do not need to compute this by hand — every tool on this site runs the checksum automatically before touching the database. The point to remember is what a checksum failure means: the string cannot be a GSTIN. There is nothing to look up and no ambiguity. A GST number check that fails at this stage has already given you a definitive answer.
Common format mistakes (and how to catch them)
- Letter/digit swaps from scans and photos: 0↔O, 1↔I, 8↔B, 5↔S, 2↔Z. These dominate real-world errors. The character-type rules above catch many instantly — positions 8–11 must be digits, so an “O” there is impossible.
- Wrong length: 14 or 16 characters usually means a character was dropped or duplicated during copy-paste. A GSTIN is always exactly 15.
- Spaces and hyphens: “27 AAPFU0939F 1ZV” on a letterhead is a formatting choice, not part of the number. Strip separators before validating.
- Impossible state codes: the first two digits must appear in the official list — verify against the GST state code list.
- PAN mismatch: if you hold the vendor’s PAN, characters 3–12 must match it exactly. A difference means either a typo or a number that does not belong to that vendor — a fraud pattern explained under GST verification.
- Lowercase entry: GSTINs are conventionally uppercase; validators normalise case, but mixed-case numbers in your records are a hint of manual retyping worth double-checking.
Format-valid is not the same as genuine
Passing the format and checksum test means the string is a possible GSTIN — not that it is registered, active, or belongs to the business on the invoice. For that you need the registration record: search the number on the GST Search homepage, confirm the status on GST number check, and match the legal name. If you only have a PAN or a business name, start from search by PAN or search by name instead. For the broader picture of what the identifier is and who needs one, see what is GSTIN.
GST number format FAQs
How many digits are there in a GST number?
Fifteen characters in total — a mix of digits and letters, not digits alone: 2-digit state code + 10-character PAN + entity code + Z + check digit.
Why is the 14th character always Z?
It is a placeholder kept in reserve by design. Current registrations use Z by default; the position exists so the scheme can be extended later without changing the number’s length.
Can two businesses have GSTINs that differ by one character?
Two valid GSTINs can certainly differ at one position — but a single-character typo in a valid GSTIN almost always produces an invalid string, because the check digit stops matching. That protection is exactly why the check digit exists.
Is the GST number format the same in every state?
Yes. The 15-character structure is identical nationwide; only the two-digit state code at the front differs. A Kerala GSTIN starts with 32, a Delhi one with 07, and everything after position 2 follows the same rules.