Invoice Numbering: Sixteen Characters and Two Symbols

Invoice Numbering explained: this guide covers what it means, who it applies to, the step-by-step process, documents required, fees, due dates and penalties in India — so you can stay compliant with confidence and avoid costly mistakes.

Of all the Rule 46 particulars, the serial number is the one systems break most often — usually because the numbering convention predates GST and nobody revisited it.

Rule 46(b): 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 "/" — and any combination thereof, unique for a financial year. The same formulation appears in Rule 49 (bill of supply), Rule 50 (receipt voucher), Rule 51 (refund voucher), Rule 52 (payment voucher), Rule 53 (revised invoice and credit or debit note), Rule 54(1) (ISD invoice) and Rule 55 (delivery challan).

The four constraints

1. Sixteen characters maximum. Including the separators. "INV/2026-27/MUM/0001234" is twenty-three characters and fails.

2. Only alphabets, numerals, hyphen and slash. No space. No hash. No ampersand. No underscore. No colon. No dot. "INV 001" fails on the space; "INV#001" fails on the hash; "INV.001" fails on the dot.

3. Consecutive. Within each series, no gaps and no reuse. A cancelled invoice number is not reused; it is reported as cancelled.

4. Unique for a financial year. The series restarts on 1 April, and no number is repeated within the year across the same series.

Multiple series are expressly permitted

"in one or multiple series" — so a business may run separate sequences for:

• each place of business — branch, depot, factory;
• each document type — invoice, credit note, debit note, delivery challan;
• each channel — B2B, B2C, e-commerce, exports;
• each business division.

Each series must be independently consecutive and unique within the financial year.

The uniqueness requirement is per GSTIN. Two different GSTINs of the same PAN may use the same number, because the IRN is generated on supplier GSTIN + document type + document number + financial year — the GSTIN disambiguates them.

But two locations sharing one GSTIN cannot use the same number. That is the most common IRN duplicate-rejection cause: two branches under one registration, each running a sequence starting at 1.

What the e-invoice portal enforces

The IRP validates the document number against the rule and rejects:

• more than sixteen characters;
• disallowed characters;
• a number starting with 0 in some validation states;
• a duplicate of an IRN already generated for the same GSTIN, document type, number and financial year.

The last one is the operational trap. Once an IRN is generated, the number is consumed for that GSTIN and financial year. Cancelling the IRN within 24 hours does not free the number for reuse — a fresh document with a new number is required.

Cancellations and gaps

Rule 48(3): the serial number of invoices issued during a tax period shall be furnished electronically through the common portal in FORM GSTR-1 or in FORM GSTR-1A.

That reporting is what makes gaps visible. A series running 001 to 100 with 047 missing invites the question of what happened to 047.

The correct handling:

• cancelled before issue — record the number as cancelled, retain the voided document, report it as cancelled;
• cancelled after issue — a credit note under s.34 is the instrument, not a deletion;
• e-invoice cancelled within 24 hours — the IRN is cancelled and the number is not reused;
• never leave an unexplained gap.

Practical notes

• Design the series to fit sixteen characters including separators. "26-27/MUM/000123" is sixteen exactly.
• Encode the financial year in the series where multiple years' documents coexist in the system.
• Give each location under one GSTIN a distinct prefix or block, so numbers cannot collide.
• Do not embed the customer code or the PO number in the document number — it consumes characters and creates collisions.
• Test the series against the IRP before go-live for a new location or a new document type.
• Apply the same discipline to credit notes, debit notes and delivery challans — the rule is identical for each, and challan numbering is the one most often left free-format.

Key takeaways

• Sixteen characters maximum, including separators.
• Only alphabets, numerals, hyphen and slash — no spaces, hashes, dots or underscores.
• Consecutive within each series and unique for a financial year.
• Multiple series are permitted, per location, document type or channel.
• Uniqueness is per GSTIN — two locations sharing a GSTIN must not collide.
• A cancelled IRN does not free the number for reuse.

Read next

• Rule 46: Every Particular a Tax Invoice Must Carry
• Rule 48(5): An Invoice Without an IRN Is Not an Invoice
• E-Invoice Cancellation and Amendment Rules
• Credit and Debit Note Particulars: Rule 53(1A)

Disclaimer: Positions stated as on 5 September 2026, based on the CGST Rules as amended to 31 March 2026 (ICAI Bare Law, 12th edition) and the ICAI Handbook on Invoicing under GST (2025).