Invoice Numbers Under GST: The 16-Character Limit, Branch Series and GSTR-1 Table 13

Invoice numbers look like an admin detail until the e-invoice portal rejects a duplicate, a GSTR-1 upload fails, or an auditor asks why invoice 0457 is missing. The rules are short, but most errors come from series set up years ago. This post covers what a number may contain, how to run series across branches, what to do on 1 April, and how Table 13 of GSTR-1 reports it all.

What the rule says

Rule 46(b) of the CGST Rules requires every tax invoice to carry:

• ✓a consecutive serial number;
• ✓of not more than 16 characters;
• ✓in one or multiple series;
• ✓using letters, numerals, hyphen (-) and slash (/) only, in any combination;
• ✓that is unique for a financial year.

Rule 49 applies the same rule to bills of supply, and the same idea applies to credit notes, debit notes, receipt vouchers and delivery challans, which each carry their own consecutive numbers.

Spaces, underscores, hashes and dots are not allowed.

Case does not make a number unique

GSTR-1 has always treated invoice numbers as case-insensitive. From 1 June 2025, the e-invoice portal (IRP) does the same: it converts the document number to upper case before generating the IRN (GSTN advisory of 4 April 2025). So "abc/101" and "ABC/101" are one number. If two branches used the same digits and relied on lower and upper case to tell them apart, the second e-invoice will now be rejected as a duplicate.

Series per branch, counter and GSTIN

Each GSTIN is a separate registration with its own GSTR-1, so numbering is unique per GSTIN. Inside one GSTIN you may run as many series as you need, provided each number is unique across all of them for the year. Practical set-ups:

• ✓a prefix per branch or warehouse: AHM/, SRT/, VAD/;
• ✓a separate series per document type: INV for tax invoices, BOS for bills of supply, CN for credit notes, DC for delivery challans;
• ✓a separate series for each POS counter or billing system, so two systems never issue the same number.

Keep each series strictly consecutive. Do not skip numbers or reuse a cancelled number. If an invoice is cancelled, the number stays used and is reported as cancelled.

Restarting on 1 April

Since uniqueness is per financial year, you can restart at 1 on 1 April. Most businesses put the year in the prefix (26-27), so April's invoice 1 cannot be confused with last April's. Before restarting, check three things:

1. The year tag fits within 16 characters with your longest number. "SRT/26-27/" is 10 characters, leaving 6 digits, up to 999,999 invoices.
2. Every billing system, including the e-commerce plug-in and the POS, switches on the same day.
3. Credit notes issued in the new year against old invoices take a new-year credit note number but quote the old invoice number and date.

Reporting series in Table 13 of GSTR-1

Table 13, "Documents issued", lists each document series used in the month. GSTN made it mandatory from the May 2025 tax period; the return now shows an error if B2B or B2C supplies are reported and Table 13 is blank. For each series you report:

Add one row per series. A business running three branch prefixes for invoices reports three invoice rows.

A worked example

A Surat textile trader has one GSTIN in Gujarat and two billing points: the Ring Road shop (prefix RR/26-27/) and the Sachin godown (prefix SG/26-27/). In September 2026:

1. Ring Road issued invoices RR/26-27/01241 to RR/26-27/01530. Total = 1,530 − 1,241 + 1 = 290.
2. Two of those were cancelled on the same day. Net issued = 290 − 2 = 288.
3. Sachin issued SG/26-27/00310 to SG/26-27/00402. Total = 402 − 310 + 1 = 93, none cancelled, net 93.
4. Credit notes CN/26-27/0041 to CN/26-27/0049. Total = 49 − 41 + 1 = 9.

Table 13 has two rows under "Invoices for outward supply" (290/2/288 and 93/0/93) and one row under "Credit notes" (9/0/9). Ring Road's 288 invoices carry a taxable value of ₹72,00,000 and Sachin's 93 invoices ₹18,60,000. Those values go in Tables 4 and 7, because Table 13 records only document counts, not amounts. The 381 net invoices should match the invoice count in your sales register for the month (B2B invoices appear one by one in Table 4; small B2C sales are consolidated in Table 7). If they do not, an invoice was missed in GSTR-1 or a number was skipped.

Frequently asked questions

Sources

• ✓CGST Rule 46, tax invoice (serial number up to 16 characters), CBIC
• ✓GSTN advisory on case sensitivity in IRN generation, TaxGuru
• ✓HSN dropdown and Table 13 mandatory in GSTR-1 from May 2025, Taxmann
• ✓CGST Rule 46, tax invoice, GSTZen

This is a summary, not legal advice. Confirm the current rule text on cbic-gst.gov.in and GSTR-1 advisories on gst.gov.in.