Invoice Number
Meaning, Format & GST Rules
5 min readEvery bill you raise needs one label that you, your buyer and the tax department can all point to.
What Is an Invoice Number?
An invoice number is the unique serial code printed on an invoice. It lets one sale be told apart from every other sale you make. Under Rule 46 of India’s CGST Rules it must be consecutive, no longer than 16 characters, and unique within a financial year. It is also one of the particulars a tax invoice cannot legally go without.
What Goes Into an Invoice Number
Anatomy of an invoice number RJT / 26-27 / 0417 Prefix Your outlet code Year code Changes every 1 April Running serial Never reused in the year All 14 characters, slashes included, count toward the 16-character limit, and the whole string must be unique for the financial year.The law is brief here. Rule 46(b) of the CGST Rules asks for a consecutive serial number that is:
- no more than 16 characters long
- made up of letters, numerals, a hyphen or a slash, in any mix
- run in one series or several
- unique for the financial year
The rest is your call, and a common Indian pattern builds the number from three parts.
| Part | Example | What it does for you |
|---|---|---|
| Prefix | RJT | Marks the outlet or counter that raised it |
| Year code | 26-27 | Shows the financial year, and most businesses restart the serial each April |
| Serial | 0417 | Climbs by one on every invoice |
The pattern reads Prefix/Year code/Serial, so RJT/26-27/0417 runs to 14 characters. If you still print from a book, this GST invoice template has the number field ready.
Invoice Number vs Invoice Reference Number (IRN)
Since e-invoicing arrived, the invoice number and the IRN get muddled. Both sit on the same B2B invoice once your turnover has crossed the e-invoice threshold in an earlier year, but they are not the same thing.
| Aspect | Invoice number | IRN |
|---|---|---|
| Who sets it | You, from your own series | The government portal |
| Length | Up to 16 characters | 64 characters, generated as a hash |
| Applies to | Every GST invoice | B2B invoices of businesses above the e-invoice limit |
| On the bill | Printed as the serial | Embedded in the QR code |
The portal builds the IRN from your GSTIN, the financial year, the document type and your own number, so a duplicate number means no IRN at all. And since 1 June 2025 the portal ignores letter case, which makes abc101 and ABC101 one number. Most restaurants never meet an IRN, as this piece on why most restaurant bills do not need an e-invoice explains.
Invoice Number Example
A tiles and sanitaryware showroom in Danavaipeta, Rajahmundry, runs a counter series (RJT) and a warehouse series (RJW). In October 2026 the counter opened at RJT/26-27/0391 and closed at RJT/26-27/0468, with two bills cancelled on the spot. The warehouse ran from RJW/26-27/0112 to RJW/26-27/0139 with none cancelled.
The showroom files monthly, so its GSTR-1 for October is due by 11 November. A quarterly filer under QRMP would report the same range by the 13th of the month after the quarter ends. In Table 13 (documents issued), each series shows as a range, with the cancelled numbers declared rather than hidden.
| Series | From | To | Issued | Cancelled | Net |
|---|---|---|---|---|---|
| Counter | RJT/26-27/0391 | RJT/26-27/0468 | 78 | 2 | 76 |
| Warehouse | RJW/26-27/0112 | RJW/26-27/0139 | 28 | 0 | 28 |
Note: this is an invented example for illustration only. The showroom, both series and every count are made up.
Why a Clean Series Saves You at Filing Time
GSTN’s advisory of 4 April 2019 put it plainly: a series that breaks Rule 46 can hold up an e-way bill, trip up a GSTR-1 and stall a refund claim. Where no specific penalty fits a Rule 46 breach, Section 125 of the CGST Act allows a general penalty of up to Rs.25,000, with a matching amount under the state or UT Act.
Duplicates are the usual culprit. Across the retail counters we work with, the duplicate that hurts almost always comes from two counters each running their own bill book under the same prefix, so the same number goes out twice.
Gaps need a reason, and since the May 2025 return, Table 13 is the mandatory place to give it. A cancelled bill is reported as cancelled there and its number is never issued again. A correction after the bill has gone out goes on a credit note or debit note, which runs its own series.
Rule 46 is not the whole story in three cases:
- Composition dealers, and anyone billing exempt supplies, issue a bill of supply, and Rule 49 applies the same 16-character, unique-per-year test to it
- Small walk-in sales under Rs.200, where the customer does not ask for a bill, can go on one consolidated invoice at the close of the day
- An unregistered business is outside Rule 46, though its own books still need a unique running number
See Every Invoice Number You Issued
A clean series means you can account for every number at month end without opening a bill book. For restaurants, Petpooja POSS punches every bill. Its day-end sales reports and its reports on cancelled orders mean you are not counting bills by hand. For retail businesses, Petpooja Invoice works out the GST on every invoice it generates and carries GSTR 1, 2 and 3B reports.
Picture ticking off every number you issued against a day-end sales report or a GSTR-1 report, and see what that looks like for your counters.
Frequently Asked Questions
Is an invoice number the same as a bill number?
In everyday use, yes, a bill number is the same thing. From a GST-registered business the bill is a tax invoice (or a bill of supply under composition), and its number must pass the same 16-character, no-repeats test. This guide to GST billing for restaurants shows what else each bill must carry.
Can I restart the series from 1 every April?
You can restart the serial at 1 each April. GSTN’s 2019 advisory asks for a fresh series each financial year, but the rule itself only demands uniqueness within a year, so a serial that keeps climbing across years is not a breach while the whole number stays within 16 characters.
How many characters can an invoice number have under GST?
A GST bill number can run to 16 characters at most. That count includes the prefix, the year code, every hyphen and every slash, not just the digits at the end.
Is a purchase order number the same as the invoice number?
No. The buyer issues the purchase order number before the sale, and you issue your own number when you bill. Quoting the PO number on the invoice helps the buyer match the two.