GST Invoice Numbering Rules: 16 Characters, Unique Per Year, and How to Design Your Series
The invoice number looks like the least important box on the page. Under GST it is one of the most closely watched. It ties your invoice to your GSTR-1, to your customer's input tax credit, to the e-way bill, and to the e-invoice IRN. A duplicated or badly formatted number can block all of them.
Here is what the law actually requires, and a practical way to design a numbering scheme that stays clean as your business grows.
What the rules say
Rule 46(b) of the CGST Rules requires every tax invoice to carry:
a consecutive serial number not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters, hyphen or dash and slash symbolised as "-" and "/" respectively, and any combination thereof, unique for a financial year.
Break that down and you get five rules:
- Maximum 16 characters, counting everything, including the slashes.
- Allowed characters: letters A–Z, numbers 0–9, hyphen (-) and slash (/). No spaces, no underscores, no hash signs.
- Consecutive: each number follows the previous one in its series.
- One or more series: you can run separate series, for example one per branch or one per document type.
- Unique for the financial year: a number cannot repeat between 1 April and 31 March.
The same structure applies to bills of supply, credit and debit notes, receipt vouchers and delivery challans.
Is it case-sensitive?
For e-invoicing and the GST portal, invoice numbers are treated as not case-sensitive. "inv/001" and "INV/001" count as the same number. Stick to capitals to avoid any confusion.
Do I have to restart at 1 every April?
No, you are not forced to restart. The rule is that a number must be unique within the financial year. Restarting from 001 each April is the most common choice, because it keeps numbers short. If you do restart, include the year in the number so that INV/001 of this year never looks like INV/001 of last year.
A numbering format that works
A simple and widely used pattern is:
PREFIX / YEAR / SEQUENCE, for example INV/26-27/0001
That is 14 characters, leaving room to spare. Here is why each part helps:
- Prefix tells you the document type at a glance: INV for tax invoices, BOS for bills of supply, CN for credit notes, DN or DBN for debit notes, RV for receipt vouchers, DC for delivery challans.
- Year makes the number unique across years and lets you restart the sequence each April.
- Sequence with leading zeros keeps numbers in order when sorted in a spreadsheet.
Multiple branches
If two branches under the same GSTIN both bill customers, give each its own series so they never clash: MUM/26-27/0001 and PUN/26-27/0001. Each series is consecutive on its own. Branches in different states have different GSTINs, so their series are already separate, but a branch code still helps your accounts team.
Formats to avoid
| Example | Problem |
|---|---|
| INV 001 | Space is not an allowed character |
| INV#2026_001 | # and _ are not allowed |
| OWNINVOICE/2026-2027/00001 | 26 characters, over the limit |
| 001 (every year) | Repeats across years; confusing in audits |
| Random numbers | Not consecutive |
What if I skip a number or cancel an invoice?
Mistakes happen. If you cancel an invoice before reporting it, do not reuse its number. Mark it as cancelled in your records and move on. A gap with a clear explanation is acceptable; a duplicate is not.
Table 13 of GSTR-1 asks you to report the document series you used during the period, with the first and last numbers, the total issued, and the number cancelled. If you keep a clean series, this table fills itself in.
For e-invoices, an IRN can be cancelled within 24 hours on the portal. After cancellation, the same invoice number cannot be used again for a new IRN. Use a fresh number.
Numbering for credit notes and debit notes
Credit and debit notes need their own series. Do not number them as part of your invoice series. Something like CN/26-27/001 and DBN/26-27/001 keeps them distinct, and the note itself must quote the number and date of the original invoice it adjusts.
Manual books vs software
If you write invoices by hand from a printed book, the book's pre-printed serial numbers usually satisfy the rule, as long as one book is not used in two places at once. When you move to software, carry the series forward or start a new one with a new prefix. Do not restart the same prefix from 001 in the middle of a year.
How OwnInvoice handles numbering
The OwnInvoice Invoice Studio suggests the next number for each document type automatically, using a type-specific prefix such as INV, CN, DBN or BOS. The number field is editable, so you can change it to fit your own scheme, such as INV/26-27/0001. Because each type has its own series, an invoice and a credit note never share a number.
Quick checklist
- 16 characters or fewer?
- Only letters, numbers, - and /?
- Next number in the series, no reuse?
- Year included, so it is unique for the financial year?
- Separate series for each document type?
Note: This article summarises the GST rules for general information. For business-specific questions, such as multiple registrations or migrating from old software, speak to your accountant.