What is the correct GST invoice number format?

Any consecutive serial number of up to 16 characters made of letters, numbers, "-" and "/", unique within the financial year. INV/26-27/0001 is a common pattern.

Can I use more than one invoice series?

Yes. Rule 46 allows one or multiple series, for example one per branch or per type of sale. Each series must stay consecutive and unique for the financial year.

Can I reuse an invoice number after cancelling the invoice?

No. A cancelled number counts as used. Issue the next number in sequence and keep a record of the cancellation.

Do invoice numbers restart every financial year?

They must be unique within a financial year, so most businesses start a new series on 1 April, usually by including the year in the number.

GST Invoice Number Format: Rules, Series and Common Mistakes

The invoice number is the field sellers think about least and get wrong most. The rules are short, and breaking them is an easy way to make an otherwise correct invoice look careless.

The rule

Rule 46 of the CGST Rules requires a consecutive serial number that:

is no longer than 16 characters,
uses only letters, numbers, the hyphen "-" and the slash "/",
can run in one or several series, and
is unique for the financial year.

Formats that work

Format	Length	Use
INV/26-27/0001	14	Single series with the financial year
MUM/26-27/0001	14	One series per branch or warehouse
B2B-0001	8	Separate series for business buyers
EXP/26-27/001	13	Separate series for exports

Putting the financial year in the number makes uniqueness automatic: INV/26-27/0001 and INV/27-28/0001 can never clash.

Using more than one series

Multiple series are allowed and often useful — one per branch, warehouse or type of sale. Keep each series consecutive on its own, and keep credit notes and debit notes in their own separate series rather than mixing them into invoices.

Common mistakes

Going over 16 characters. "INVOICE/2026-2027/00001" is 23 characters. Shorten the prefix and the year.
Disallowed characters. Spaces, "#", "_" and "." are not in the allowed set. INV#0001 should be INV-0001.
Reusing a number after cancelling an invoice. A cancelled number stays used; issue the next one in sequence.
Restarting mid-year. Numbers must be unique within the financial year, so start a fresh series only from 1 April.
Gaps with no explanation. If a number is skipped or cancelled, keep a record of why. Your GST return reports the range of documents issued and how many were cancelled.

Starting a new financial year

From 1 April, begin a new series — most easily by changing the year inside the number, from INV/26-27/ to INV/27-28/. Don't carry the old count forward with no year marker, or numbers from two years can look identical.

Try it: Create an invoice — the generator checks the number for you — free, no signup, runs in your browser.

Sources

Checked on 1 October 2026. Tax rules change; confirm against current notifications or with your tax adviser before relying on them.