Invoice Number Format Guide
What a GST-legal invoice series must look like.
Your invoice number
What the law requires
Rule 46 of the CGST Rules says an invoice number must be:
- Consecutive — no gaps, no repeats
- Unique within the financial year — the series resets each April
- 16 characters or fewer
- Made only of letters, digits, hyphens and slashes
The most common mistake
Deleting an invoice and reusing its number. A gap in the series is an audit red flag. If an invoice is wrong, cancel it or issue a credit note — never reuse the number.
BikriBook enforces all of this automatically, with a database lock so two staff members billing at the same time can never get the same number.