How to Number Invoices Correctly in India (GST Rules and Examples)

Quick answer

Under Rule 46(b) of the CGST Rules, each tax invoice needs a consecutive serial number of up to 16 characters, in one or more series, unique for the financial year. You can use letters, numbers, a hyphen (-) and a slash (/). Many businesses add a prefix and the financial year, such as INV/26-27/0001, and start a new series every April.

Why invoice numbers matter

• They make every invoice traceable, which tax officers and auditors expect.
• Gaps or duplicates raise questions during GST scrutiny.
• GSTR-1 asks for a summary of documents issued (Table 13), including the series and any cancelled numbers, so a clean series makes returns easier.
• Clients use your invoice number to match payments.

The GST rules in short

• Serial number must be consecutive.
• Maximum 16 characters.
• Allowed characters: letters (A to Z), numbers, hyphen or dash, and slash.
• Must be unique for a financial year (April to March).
• You may run more than one series, for example separate series for each branch or document type, as long as each one is unique and consecutive.

Formats that work

• Simple: 0001, 0002, 0003 (fine for very small businesses, but harder to search)
• Prefix plus number: INV-0001
• Year-based: INV/26-27/0001 (14 characters)
• Branch-based: PUN/26-27/001 and MUM/26-27/001 for two locations
• Document-type based: INV/ for tax invoices, BOS/ for bills of supply, CN/ for credit notes

Count characters before you finalise a format. "PIKZIYSTUDIO/2026-27/0001" is 25 characters and too long. "PZS/26-27/0001" is 14 characters and within the limit.

Worked example

A Pune agency issues its last invoice of FY 2025-26 on 28 March 2026 as PZS/25-26/0148. On 2 April 2026 it issues PZS/26-27/0001. Both are valid because each financial year has its own unique series. Within FY 2026-27, the next invoices are 0002, 0003 and so on.
If an invoice is cancelled, do not reuse its number. Keep the cancelled invoice on record, marked cancelled, so the series has no unexplained gap.

Numbering your other documents

Invoices are not the only GST documents that need a number. Each of these should have its own consecutive series, unique for the financial year:

A separate prefix for each document type keeps them apart in your records and in GSTR-1, where the summary of documents issued lists each series with its first and last number and the count cancelled.

Year-end checklist

• Note the last number used in each series on 31 March.
• Mark any cancelled invoices clearly and keep them on file.
• Decide the new series for 1 April and set it up before the first invoice of the year.
• Check that your GSTR-1 document summary matches your actual series.

Common mistakes

• Using the same number for two clients.
• Restarting at 0001 in the middle of a year.
• Formats longer than 16 characters, often caused by long company prefixes.
• Special characters like #, _ or spaces.
• Deleting cancelled invoices instead of marking them cancelled.

Setting up your series

1. Choose a short prefix of 2 to 4 letters.
2. Decide whether to include the financial year.
3. Decide whether you need separate series for branches or document types.
4. Write the rule down so everyone on the team follows it.
5. Use software that numbers invoices automatically, so manual errors do not creep in.

Frequently asked questions

Next step

Write your numbering rule down today and apply it from your next invoice. PikZiy Books numbers invoices, quotes and credit notes automatically with your own prefixes, so a number is never repeated or skipped.

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