What does the law require of an invoice number?

Rule 46 of the CGST Rules requires a consecutive serial number, unique within a financial year, no more than sixteen characters, made up of letters, numerals, hyphens and slashes or any combination of them. That is the whole constraint. Everything else about your format is your choice.

Can I restart numbering each financial year?

Yes, and most businesses do. Uniqueness is required within the financial year, so a series that restarts at 001 each April is fine as long as the year itself appears somewhere in the number. That is why the 2026-27 style prefix is so common.

What happens if I skip a number?

Gaps invite questions at assessment, because the assumption is that a suppressed invoice sits in the gap. If you cancel an invoice, keep the number and mark it cancelled rather than reusing or skipping it. A cancelled invoice you can produce is easier to explain than a number you cannot account for.

Can I run more than one series at once?

Yes. Separate series per branch, per business vertical or per document type are allowed, provided each series is itself consecutive and unique within the year. Many businesses run one series for tax invoices and another for credit notes.

Is sixteen characters really the limit?

Yes, and it is easy to exceed by accident. A format like INVOICE/2026-2027/000001 is 24 characters. Shorten the prefix, use a two-digit year form such as 26-27, or use fewer padding zeros. The preview here counts the characters for you.

Invoice number generator

Design your invoice numbering series and see the next numbers before you commit to it. Checks the sixteen-character limit and the characters Rule 46 permits.

Choosing a format you will not regret

An invoice number does two jobs. It satisfies Rule 46, and it lets you find one invoice among four hundred without opening files. Formats that fail the second job are the ones people abandon halfway through a year, which then creates the gap problem.

A few things worth deciding once. Put the financial year in the number, not the calendar year, because your books run April to March and your series restarts with them. Pad the sequence to at least three digits so numbers sort correctly in a spreadsheet, where 10 otherwise sorts before 9. And keep the prefix short, because the sixteen-character ceiling arrives faster than you expect once the year is in there.

Do not encode the customer or the month in the number. It feels helpful in month one and becomes a problem in month seven, when a customer changes name or an invoice slips across a month boundary and the number now says something untrue about the document.

The rule, in full

A tax invoice needs a consecutive serial number, not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters such as hyphen or dash and slash, and any combination thereof, unique for a financial year.

Paraphrased from Rule 46(b) of the CGST Rules 2017. Note what is not there: no requirement for a prefix, no required format, no requirement that the number mean anything. The constraints are length, character set, consecutiveness and annual uniqueness.

Common questions

What does the law require of an invoice number?: Rule 46 of the CGST Rules requires a consecutive serial number, unique within a financial year, no more than sixteen characters, made up of letters, numerals, hyphens and slashes or any combination of them. That is the whole constraint. Everything else about your format is your choice.
Can I restart numbering each financial year?: Yes, and most businesses do. Uniqueness is required within the financial year, so a series that restarts at 001 each April is fine as long as the year itself appears somewhere in the number. That is why the 2026-27 style prefix is so common.
What happens if I skip a number?: Gaps invite questions at assessment, because the assumption is that a suppressed invoice sits in the gap. If you cancel an invoice, keep the number and mark it cancelled rather than reusing or skipping it. A cancelled invoice you can produce is easier to explain than a number you cannot account for.
Can I run more than one series at once?: Yes. Separate series per branch, per business vertical or per document type are allowed, provided each series is itself consecutive and unique within the year. Many businesses run one series for tax invoices and another for credit notes.
Is sixteen characters really the limit?: Yes, and it is easy to exceed by accident. A format like INVOICE/2026-2027/000001 is 24 characters. Shorten the prefix, use a two-digit year form such as 26-27, or use fewer padding zeros. The preview here counts the characters for you.

Last reviewed 2026-09-10. Written and maintained by Kartikeya Mishra.