The invoice number seems like a small detail, but GST law is surprisingly specific about it. Getting your numbering wrong is one of the easiest ways to attract questions during an audit — and one of the easiest things to get right once you know the rules.
What the law says
The requirements come from Rule 46(b) of the CGST Rules, 2017. Every tax invoice must 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.”
Breaking that down, an invoice number must be:
- Consecutive — issued in an unbroken running sequence.
- Unique for the financial year — no number repeats within the same FY.
- Maximum 16 characters — including the prefix and separators.
- Alphanumeric with only
-and/as special characters — no spaces,#,*, or other symbols.
The 16-character limit trips people up
Sixteen characters is not a lot once you add a prefix and a year. Consider RAISEBILL/2025-26/0001 — that is 21 characters and not valid. A compliant version might be RB/25-26/0001 (13 characters). Keep prefixes short so the whole string stays within the limit.
Financial-year-wise numbering
The series is unique per financial year, which in India runs from 1 April to 31 March. It is standard practice to reset the running counter to 1 at the start of each new financial year and to identify the year in the prefix so numbers never clash across years.
| Invoice date | Suggested number |
|---|---|
| 5 April 2025 | INV/25-26/0001 |
| 20 March 2026 | INV/25-26/0148 |
| 2 April 2026 | INV/26-27/0001 |
Multiple series are allowed
You are not limited to a single running series. GST permits one or multiple series in parallel — a common use is a separate series per branch or outlet, or distinct series for goods and services, or for exports. The only condition is that each series must itself be consecutive and unique within the financial year. For example:
DEL/25-26/0001…for the Delhi branchMUM/25-26/0001…for the Mumbai branch
Don’t leave gaps — handle cancellations properly
Because the series is meant to be continuous, missing numbers can raise a red flag in a GST audit. If you need to cancel an invoice, do not simply delete the number and move on. Instead:
- Mark the invoice as cancelled and retain the record, or
- Issue a credit note against it where a supply has already been reported.
That way the sequence stays intact and every number is accounted for.
A clean, compliant format
A good invoice number is short, readable and self-documenting. A reliable pattern is:
{PREFIX}/{FY}/{4-digit running number}For example INV/25-26/0042. The prefix identifies the business or branch, the FY prevents cross-year clashes, and the zero-padded running number keeps everything sorted and consecutive.
Common mistakes to avoid
- Exceeding 16 characters with a long prefix.
- Using disallowed symbols like spaces,
#or_. - Restarting mid-year or duplicating a number within the same FY.
- Deleting cancelled invoices and leaving a gap in the series.
- Running one series across two financial years without a year identifier.
The bottom line
Invoice numbering under GST is simple once you fix a short, alphanumeric, financial-year-wise format within 16 characters and keep the sequence unbroken. Decide your format at the start of the year and apply it consistently.
The free RaiseBill Bill Maker generates compliant, consecutive invoice numbers automatically, so you never have to track the sequence by hand.