Invoicing
GST invoice numbering: one series per entity, each financial year
A CA firm checks its clients' invoice series every audit season, and its own bills are held to exactly the same rule. Rule 46 of the CGST Rules says what an invoice number may contain and how it must run — and a firm billing from an audit LLP and a separate tax practice has two series to keep, not one.
Here is the rule, the practice the GST Network advises, and what changes for a firm with more than one entity.
What Rule 46 actually requires
Rule 46(b) of the CGST Rules, 2017 asks for a consecutive serial number of not more than sixteen characters, in one or multiple series, using letters, numerals, the hyphen and the slash, and unique for a financial year.
Sixteen characters means the whole number — prefix, year and separators included. DKA/26-27/0042 is fourteen. A prefix that looks tidy at the start of the year can push the number over the limit once the count reaches four or five digits.
A fresh series each 1 April
The rule asks for numbers unique within a financial year. The GST Network's advisory of 4 April 2019 asked taxpayers to start a new series each financial year, and putting the year into the number — 26-27 — is the usual way to do it: the count restarts on 1 April and no number can repeat across years.
Since June 2025 the e-invoice portal treats invoice numbers as case-insensitive, so ABC101 and abc101 are the same number. A series that relied on upper and lower case to tell two numbers apart no longer does.
One series per GSTIN — and per entity
Each GST registration is a separate supplier, so each keeps its own series. A practice that bills audit work from an LLP and tax work from a proprietorship or a separate firm has two registrations and two series, and the numbers of one say nothing about the other.
Multiple series within one registration are allowed too — one for tax invoices and one for credit notes, for example — as long as each runs consecutively.
Where a gap shows up
GSTR-1 Table 13, mandatory from the May 2025 return period, lists every series with the first and last number issued, the total, and how many were cancelled. A cancelled number is reported, not hidden; a number that never appears anywhere is the gap an officer asks about.
Above ₹5 crore of turnover, e-invoicing adds the portal's own check: it refuses a document number already used for the same GSTIN, document type and financial year. And a taxpayer with turnover of ₹10 crore or more must report an e-invoice within 30 days of its date.
How T4Suite numbers a firm's bills
Each entity the firm bills from has its own series and prefix — letters, numerals, slash or hyphen, uppercase — with the financial year in the number if you choose, in which case the count restarts each 1 April. Setup refuses a prefix that would make the number longer than sixteen characters, a prefix another entity already uses, and a starting number that is already on a bill, so a firm moving over mid-year carries on from its last number.
The number is fixed only when a bill is issued, and two bills issued at the same moment can never take the same number. T4Suite does not file GSTR-1 or generate e-invoices; it keeps the series those filings report.