What the rule says
Rule 46(b) of the CGST Rules is unusually specific. A tax invoice must carry a consecutive serial number that is:
- No more than 16 characters.
- Unique for the financial year.
- Made only of letters, digits, hyphens and slashes — nothing else.
- In one series, or several, as long as each is consecutive.
“One or multiple series” is the permission most people miss. You may run a separate series per branch, per client type or per document type — you simply have to keep each one consecutive.
A format that works
The generator suggests INV-202608-001, and there are three reasons to keep it.
- It sorts correctly. Year before month before sequence means alphabetical order and chronological order are the same order, in every file browser and spreadsheet you will ever open it in.
- It is self-dating. Someone looking at the number alone knows which month it belongs to, which is what makes a folder of them navigable.
- It is padded. 001 rather than 1 means the tenth and the hundredth invoice still line up in a column.
Change the prefix per document type and you have your multiple series for free: QUO for quotations, PI for proforma, CN for credit notes. The generator does this automatically when you switch the document type.
What to avoid
| Format | Why it hurts |
|---|---|
| 1, 2, 3 | Tells your client you have issued three invoices ever. |
| 24/08/2026-1 | Slashes are legal, but day-first dates sort into nonsense. |
| INV#001 | The # is not an allowed character under Rule 46(b). |
| Random or hashed numbers | Legal, but not consecutive — which is the part that is required. |
| Restarting at 001 each month without a month in the number | Produces duplicates within the financial year. |
Gaps, restarts and mistakes
A gap in the series is the thing that gets asked about, because from the outside a missing number looks like a cancelled invoice somebody would rather not discuss. If you void one, keep the number and mark it cancelled rather than reusing it or letting it vanish.
Start a fresh series on 1 April, when the Indian financial year turns. Uniqueness is required *for the financial year*, so a new year is exactly when you are supposed to restart — and if your number carries the year, as the suggested format does, this happens on its own.
The one thing this tool cannot do is know about your other invoices. If some are raised in accounting software and some here, keep a single series across both. A duplicate number is the sort of thing that surfaces at exactly the wrong moment.