GST Invoice Numbering: Series Rules, Resets and the Gaps That Trigger Notices

The rule itself

Rule 46(b) requires a consecutive serial number, not exceeding sixteen characters, in one or multiple series, containing alphabets or numerals or special characters hyphen and slash and any combination thereof, unique for a financial year.

Every element of that sentence matters: the length cap, the permitted character set, the consecutiveness, and the financial-year scope.

What breaks it

1. Characters outside the permitted set: hash, ampersand, space, underscore and hash symbols are not allowed, though they appear routinely in invoice numbers copied from older accounting systems.

2. Exceeding sixteen characters: long prefixes encoding branch, year, vertical and sequence add up faster than expected.

3. Not resetting at the financial year: the series is unique for a financial year, so FY 2026-27 starts fresh.

4. Duplicates across parallel series: running separate series per branch is fine, but two branches issuing the same number in the same series is not.

Multiple series are allowed, and often sensible

The rule expressly permits one or multiple series. Businesses commonly maintain separate series for different branches, business verticals, export versus domestic supplies, or document types such as credit notes and self-invoices.

Each series must be internally consecutive and identifiable. A sensible convention encodes the place of business and financial year as a fixed prefix and leaves the sequence numeric, for instance BLR/26-27/0001, which is fourteen characters and leaves room to grow.

Cancelled invoices and gaps

If an invoice is cancelled, the number is not reused and the gap must be accounted for. Keep the cancelled document, marked as cancelled, and report it where required. A gap with no corresponding record is the pattern that attracts attention, because the obvious inference is a suppressed sale.

Where e-invoicing applies, a cancelled IRN must be cancelled on the portal within twenty-four hours, and the invoice number cannot be reused for a fresh IRN.

Why this surfaces in audits

The department does not need to inspect your books to see a numbering problem. GSTR-1 carries every invoice number you reported, so gaps, duplicates and non-conforming formats are visible in the filed data itself.

That makes numbering one of the cheapest things to get right and one of the most expensive to leave to manual entry.

Frequently asked questions

### Can I restart invoice numbering from 1 every financial year?

Yes, and you should. The serial number must be unique for a financial year, so the series resets on 1 April. Including the financial year in the prefix — for instance 26-27/0001 — keeps the numbers unique across years while satisfying the reset.

### Can I use a slash or hyphen in an invoice number?

Yes. Rule 46(b) permits alphabets, numerals, hyphen and slash in any combination. What it does not permit is other special characters such as hash, ampersand, underscore or spaces, which are common in legacy numbering schemes and need to be cleaned up before e-invoicing.

### What if I accidentally skip an invoice number?

Document the reason and keep the record. The number itself cannot be reused. An explained gap — a cancelled invoice, a spoiled document — is defensible; an unexplained one invites the inference of an unreported supply, so the explanation needs to exist at the time rather than being reconstructed later.

### Can two branches use the same invoice number?

Not within the same series. Use separate series with distinguishing prefixes, which the rule expressly permits. Each series must be internally consecutive, and the combination of series and number must be unique across the GSTIN for the financial year.