GST · 26 June 2026
What Is the GST Invoice Number Format Under Rule 46(b)?
The GST invoice number is governed by Rule 46(b) of the CGST Rules 2017. It must be a consecutive serial number not exceeding 16 characters, in one or more series, containing only letters, numerals and the special characters hyphen and slash, and it must be unique for the financial year. The rule exists so that every tax invoice can be tracked, matched to a return, and tied to the buyer input tax credit claim. There is no fixed prefix the law demands, so a business may design its own series such as INV/2026-27/001, as long as the numbering stays sequential, within 16 characters, and does not repeat within the year. For businesses under e-invoicing, the Invoice Registration Portal validates the document number against this format and rejects numbers that break it. Getting the series wrong can stall a buyer ITC claim and invite scrutiny, so the numbering is worth fixing once at the start of the year.
By Mrs. Kritika Joshi
Last reviewed
26 June 2026
In this section
Answers
- What Is the GST Invoice Number Format Under Rule 46(b)?
- What Is GSTR-1, the GST Return of Outward Supplies?
- What Is GSTR-3B, the Monthly GST Summary Return?
- What Is the Time of Supply Under GST, and When Does Tax Become Due?
- Advance Receipt Under GST: When Do You Issue a Receipt Voucher?
- Credit Note vs Debit Note Under GST: When Do You Issue Each?
- What Is GSTR-2B, and Why Does It Now Decide Your Input Tax Credit?
- What Is the Value of Supply Under Section 15 of the CGST Act?
- What Are the GST Rate Slabs in India After the GST 2.0 Reform?
- Exempt vs Nil-Rated vs Zero-Rated Supply: What Is the Difference?
You can number your GST invoices any way you like, even repeating last year's numbers.
Rule 46(b) of the CGST Rules requires a consecutive serial number, up to 16 characters, that is unique for each financial year.
What does Rule 46(b) say about the invoice number?
Short answer
It requires a consecutive serial number, not exceeding 16 characters, that is unique for the financial year, set out in Rule 46(b) of the CGST Rules.
The invoice number is one of the mandatory fields a tax invoice must carry under Section 31 and Rule 46. The number itself has its own sub-rule: it has to run in sequence, stay within 16 characters, and not repeat inside the same year. Those three constraints are what let a single invoice be traced from your books to your return to the buyer credit.
Which characters are allowed in a GST invoice number?
Short answer
Only letters, numerals, and the two special characters hyphen and slash; spaces and other symbols are not permitted.
- A series such as INV/2026-27/001 is valid, since it uses letters, numerals, the slash and the hyphen only.
- The whole string, prefix included, must stay within the 16-character ceiling, so long prefixes leave fewer digits for the running count.
- You may run more than one series at once, for example one per branch, as long as each stays unique within the year.
Why must the number reset and stay unique each financial year?
Short answer
Uniqueness within the year lets each invoice be matched to a return and to the buyer ITC claim; a repeated number breaks that match.
- Many businesses restart the series each financial year, embedding the year in the prefix so numbers never collide across years.
- A duplicate or non-sequential number is a common trigger for a mismatch in GSTR-1 and a query to both you and your buyer.
- This matters because a buyer denied a clean match can lose input tax credit, which sours the relationship over your paperwork.
How does e-invoicing check the invoice number?
Short answer
For taxpayers under e-invoicing, the Invoice Registration Portal validates the document number against the Rule 48 schema and rejects numbers that break the format.
- The portal applies its own validation, so a number with a space or an unsupported symbol fails before an IRN is issued.
- A rejected document number means no IRN, and without an IRN the e-invoice is not legally valid.
- A generator that enforces the 16-character, allowed-character rule up front keeps you from discovering the problem at the portal.
References & related
Primary sources
- Rule 46(b), Central Goods and Services Tax Rules 2017 — CBICThe 16-character consecutive serial number, allowed characters, unique per financial year.
- Section 31, Central Goods and Services Tax Act 2017 — CBICObligation to issue a tax invoice for a supply.
- Rule 48, Central Goods and Services Tax Rules 2017 — CBICE-invoicing and reporting the invoice to the Invoice Registration Portal.
- E-invoice document-number validation — GST e-invoice portalPortal validation of the document number format for e-invoices.
Last reviewed: 26 June 2026