A GSTIN carries its own proof of typing. The 15th character is a checksum over the other 14, so a transposed digit almost always breaks the arithmetic. This tool runs that checksum in your browser, along with three structural checks, and names the one that failed.
That covers the errors you hit in practice. A digit swapped on a supplier invoice, a state code that was never issued, a letter sitting where the PAN expects a number. What no offline check can tell you is whether the number belongs to a live registration. Only the GST portal knows that.
How the 15 characters break down
Characters 1 and 2 are the state or union territory code. Characters 3 to 12 are the PAN of the registered person, in the usual five letters, four digits, one letter shape. Character 13 is the entity number, which separates multiple registrations held on the same PAN within one state. Character 14 is always the letter Z. Character 15 is the check digit.
The tool reads each block out for you. If the PAN section parses cleanly, you also get the holder type encoded in the PAN's fourth letter: C for company, P for individual, F for a firm or LLP, and so on.
What the check digit catches
Each of the first 14 characters has a value from 0 to 35 (digits first, then A through Z). Values in odd positions are taken as they are. Values in even positions are doubled, then split across 36: the quotient and the remainder both get added to a running total. The check digit is whatever brings that total up to the next multiple of 36.
Because of how the doubling folds, every single wrong character changes the result. Change one letter anywhere in the first 14 and the 15th no longer matches. That makes it a reliable catch for typos and OCR slips. It says nothing about whether the number was ever issued: a made-up GSTIN with a correctly computed check digit passes this test happily.
Why the state code is worth reading
The leading two digits decide whether a sale is intra-state or inter-state, which in turn decides CGST plus SGST against IGST. A customer who gives you a GSTIN starting 27 is registered in Maharashtra. Compare that against your own place of supply before you pick the tax split.
Two codes surprise people. 97 is Other Territory, used for the offshore and continental shelf area. 99 is Centre Jurisdiction. Both are valid and both are rare.
What a pass does not prove
Structural validity is a typing check, nothing more. The registration could be cancelled, suspended, or belong to a different legal name than the one on the invoice in front of you. If you are claiming input tax credit against that invoice, search the number on the GST portal and confirm the trade name, the status, and the registration date.
Use this tool to catch the bad number before it reaches your books. Use the portal to confirm the good one.
Frequently asked questions
Does a valid GSTIN mean the business is registered?
No. Every check here runs offline on the characters themselves, so it proves the number is well formed and nothing else. A fabricated GSTIN with a correctly computed check digit will pass. Search the number on the GST portal to confirm the registration is real and active.
Why does the tool accept a GSTIN with spaces or in lower case?
Spaces, hyphens and case get stripped before any check runs, because GSTINs pasted out of PDFs and emails often carry them. The cleaned 15-character version is what gets validated, and that is the version shown in the result.
The check digit failed but everything else passed. What now?
You have a typo somewhere in the first 14 characters. Compare against the source document character by character, paying attention to pairs that look alike in print: 0 and O, 1 and I, 5 and S, 8 and B. One of those substitutions is the usual culprit.
Can one PAN have more than one GSTIN?
Yes. A business registers per state, so the same PAN appears inside a separate GSTIN for each state it operates in. Character 13, the entity number, distinguishes multiple registrations held on that PAN within a single state.
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