How to number invoices: the GST rules (16 characters, unique per financial year), practical formats with examples, restarting each April, and quotation and PO reference numbers.
Rules for GST invoice numbers
- Consecutive serial number.
- Unique within a financial year.
- Up to 16 characters.
- Only letters, numbers, hyphen (-) and slash (/).
Source: Rule 46. The same applies to credit notes, debit notes and delivery challans, each in their own series.
Invoice number format examples
| Format | Example | Characters | Good for |
|---|---|---|---|
| Prefix + year + number | INV/26-27/0001 | 14 | Most small businesses |
| Branch + year + number | AHM/2627/0001 | 13 | Multiple branches |
| Type + number | EXP-0045 | 8 | Separate export series |
| Year + month + number | 2610-0042 | 9 | High volume |
| Credit note | CN/26-27/007 | 12 | Credit notes |
Restarting each April
Start a fresh series on 1 April each year (INV/27-28/0001). Changing the year part keeps numbers unique across years. Never reuse a number, even for a cancelled invoice; mark the cancelled one and keep it.
Quotation and PO reference formats
Quotations and purchase orders are not tax documents, so there is no length rule, but the same approach helps: QT-2026-118, PO-2026-0215. Quote the other party reference on your document.
Numbering in the online generator
The invoice generator remembers your last draft, including the number. Change only the last digits for the next invoice. The GST checklist flags numbers longer than 16 characters or with characters that are not allowed.
Common numbering mistakes
Do
- Plan the format before your first invoice.
- Use leading zeros (0001) so files sort in order.
Avoid
- Do not use the date alone as a number.
- Do not share one series between two branches without a branch prefix.
Frequently asked questions
Can I use separate series for different products?
Yes, you can run more than one series as long as each is consecutive and unique.
What if I skip a number by mistake?
Keep a note of the skipped number and the reason. Do not fill it later with a back-dated invoice.
Checked and updated 14 Oct 2026. Independent template site; not tax or legal advice.