GST invoice numbers: the 16-character rule and how to number a series

Rule 46(b) of the CGST Rules sets the format of the serial number on a tax invoice. Breaking it shows up when you upload GSTR-1.

The rules

• No more than 16 characters, counting every letter, digit, hyphen and slash.
• Only letters, digits, hyphen (-) and slash (/). Spaces, #, _, & and full stops are not allowed.
• Unique for your GSTIN in the financial year. The same number can't be used twice.
• Consecutive, in one or more series. Each series runs in order.

Designing a series that fits

1. Put the year in short form: INV/26-27/0001 is 14 characters. INV/2026-27/KA/00001 is 20 and fails.
2. Use separate series for separate purposes (a branch, credit notes) by a short prefix, and keep each one in order.
3. Pad with zeros if you like (0001), but then keep the same width for the whole year.
4. Start a fresh series each financial year. The rule doesn't say what number to begin with.

If you have a gap

A missing number is acceptable if the document was cancelled, but cancelled numbers are counted in the document summary of GSTR-1. The invoice number checker lists gaps, duplicates and numbers that break the rule.

Don't rewrite invoices you have already issued. Change the pattern from the next invoice.