GST Invoice Numbering Rules: How to Set Up Your Invoice Series, Reset It Each Financial Year, and Avoid Gaps and Duplicates

A GST invoice number can be up to 16 characters long. It can use only letters, numbers, hyphens (-) and slashes (/), it must run in a consecutive series, and it must be unique within a financial year. Most businesses start a fresh series every 1 April with a format like INV/26-27/0001, keep separate series where it makes sense, and never reuse or delete a number. Cancelled invoices are reported as cancelled in GSTR-1.

That's the short answer. Below is how to set it up properly, so your CA isn't untangling it in March.

The basic rules for a GST invoice number#

The invoice rules under GST (Rule 46 of the CGST Rules) say your invoice must carry a serial number that is:

• Consecutive. Numbers go in order: 0001, 0002, 0003. No jumping around.
• Not more than 16 characters. Every character counts, including slashes and hyphens.
• Made of letters, numbers, hyphens or slashes only. No spaces, underscores, hashes (#), dots or brackets.
• Unique for the financial year. One number, one invoice, once per year.
• In one series or several. You're allowed more than one series, as long as each one follows the rules above.

The rules don't fix a format. INV/26-27/0001, 2627-0001 and A0001 are all fine. Pick one that's easy for your team and your CA to read, and stick with it.

Sample formats (and a few that break the rules)#

Here is a quick check of common formats. Count every character, including the separators.

A tip: decide how many invoices you might issue in a year before you choose the digits. A Surat textile trader issuing 40 bills a day needs five digits. A small design agency in Pune sending 15 invoices a month is fine with three or four.

Resetting the series every financial year#

The financial year runs from 1 April to 31 March. Because a number only has to be unique within a financial year, most businesses restart at 0001 on 1 April.

To restart safely, put the year in the number. Look at these two invoices:

• INV/25-26/0001, issued April 2025
• INV/26-27/0001, issued April 2026

Both end in 0001, but they can never be confused. If you restart without a year marker (plain 0001 every year), your own records, your client's books and any audit query become confusing very quickly.

You don't have to reset. Some businesses run one long series across years. That's allowed as long as no number repeats within a year. But the yearly reset with a year tag is the cleanest option for most small businesses.

On 31 March, note your last invoice number for each series. Your CA will need it for the documents summary in GSTR-1 (more on that below).

When separate series make sense#

You can run more than one series. Here's when it really helps:

• Branches or counters under the same GSTIN. A trader with shops in Andheri and Thane, both under one Maharashtra GSTIN, can use AND/26-27/0001 and THN/26-27/0001. Each shop bills on its own without waiting for the other, and you never get clashes.
• Different GSTINs. If you're registered in Delhi and Karnataka, each GSTIN is a separate registration with its own GSTR-1. Keep a separate series for each, for example DL/26-27/0001 and KA/26-27/0001.
• Export invoices. A software agency in Noida that bills Indian clients and also exports under LUT can keep EXP/26-27/0001 apart from domestic invoices. It makes export reporting and refund paperwork easier to follow.
• Credit notes and debit notes. These aren't tax invoices, so give them their own series, like CN/26-27/0001 and DN/26-27/0001. The same idea applies: consecutive, unique in the year, within the character limit.
• Other documents. Bills of supply, receipt vouchers for advances, refund vouchers and delivery challans usually get their own series too.

Don't create series for no reason, though. Five series for a ten-person business means five things to track and five lines your CA has to reconcile. Keep it to what your business actually needs.

Cancelled invoices, gaps and duplicates#

This is where most messy series come from.

Cancelled invoices. Say you raised INV/26-27/0045 to a Jaipur client for ₹1,18,000 and then the order was cancelled. Don't delete it, and don't reuse 0045 for the next client. Mark it as cancelled, keep a copy, and move on to 0046. It gets reported as cancelled in GSTR-1.

If the invoice was already reported in GSTR-1, or the goods or services were partly supplied, a credit note is usually the right fix rather than cancelling. Check with your CA.

E-invoicing. If your business is required to generate e-invoices (IRN), the portal won't accept the same document number twice in a financial year. An IRN can only be cancelled within a short window after it's generated, and even after cancelling you can't reuse that number. Check the current e-invoicing limit and cancellation window on the GST portal.

Gaps. Gaps happen: a billing software glitch, someone skipping a number, two staff using different books. Don't fill a gap by backdating a new invoice into it. Instead:

1. Find out why the gap happened.
2. Write a short note in your records (for example, "0102 to 0104 skipped due to software error on 12 Aug, never issued").
3. Tell your CA, so it's reported correctly as cancelled or not issued in the documents summary.

Duplicates. Two invoices with the same number in the same year is a real problem. It confuses your client's input tax credit and can trigger questions. If it has happened, talk to your CA about the right correction. Usually that means cancelling one (or issuing a credit note) and reissuing it under a fresh number. To prevent it, have one system or one person issue each series, not three people working from Excel copies.

How invoice numbers show up in GSTR-1#

GSTR-1 has a documents issued section (Table 13). For each type of document, it asks for:

• the series start number (from)
• the series end number (to)
• the total number of documents
• the number cancelled
• the net number issued

So if your sales series for April ran from INV/26-27/0001 to INV/26-27/0120 and two were cancelled, you'd report 120 total, 2 cancelled, 118 net issued. Credit notes, export invoices and each branch series go on their own lines.

This is why clean numbering matters. If the series has unexplained gaps or repeats, these numbers won't match your invoice-level data, and your CA spends filing week chasing it.

Clyrix keeps your GST-ready invoices, credit notes and export invoices (LUT/IGST) in one place, and gives your CA a GSTR-1 export they can work from. You can see how it works on the invoicing page.

A quick checklist before 1 April#

• Pick your format and count the characters (16 or fewer).
• Use only letters, numbers, hyphens and slashes.
• Add the financial year to the number, like 26-27.
• Decide your series: domestic, export, credit notes, branches or GSTINs.
• Note the last number of every series on 31 March.
• Never delete, reuse or backdate a number. Mark cancellations and record the reason.
• Make one person or one system responsible for issuing each series.

Get this right once and your invoice numbers stop being something you think about. Your GSTR-1 will thank you.

For more practical GST and invoicing guides, browse the Clyrix blog.