Invoice Numbering Under GST: Rules for Each Financial Year

Invoice numbering under GST follows a short rule: every tax invoice needs a consecutive serial number of up to 16 characters, using letters, numbers, hyphens or slashes, and that number must be unique within the financial year. You can run more than one series, for example one per branch, and most businesses start a fresh series every April with the year built into the number, such as INV/26-27/0001.

It sounds like a small detail, but a messy number series is one of the first things an auditor notices. Gaps, repeats or two bills with the same number in one year all lead to questions, and fixing them later takes far longer than setting them up right.

This is general guidance. Confirm your numbering method with your chartered accountant, especially if you have several branches or billing counters.

The GST rule on invoice numbering

Rule 46 of the CGST Rules asks for a serial number that is:

Requirement	What it means in practice
Consecutive	Numbers go up one by one, without skipping
Not more than 16 characters	Count every letter, digit, slash and hyphen
One or multiple series	You may run separate series, such as per branch or counter
Allowed characters	Letters, numbers, hyphen or dash (-) and slash (/)
Unique for a financial year	No two invoices in the same year share a number

The financial year in India runs from 1 April to 31 March. The number has to be unique within that year, which is why the year so often appears in the number itself.

Designing an invoice numbering format

A good format answers three questions at a glance: which business or branch, which year, and which bill.

Common formats

Format	Example	Characters	Notes
Year + running number	26-27/0001	10	Simple, unique per year
Prefix + year + number	INV/26-27/0001	14	Clear and still under 16
Branch + year + number	DL1/26-27/00001	15	One series per branch
Full year	INV/2026-27/0001	16	At the limit; no room to grow

Leave room for growth. If you expect more than 9,999 invoices a year, use five digits for the running number and keep the prefix short.

Characters to avoid

Spaces, underscores, hash signs and dots are not in the allowed list, so leave them out. Keep letters in capitals so "inv" and "INV" do not look like two series.

Running multiple series

Many wholesalers need more than one series:

• Separate branches or godowns, each with its own counter.
• Different document types, such as tax invoices and bills of supply for exempt goods.
• Counter sales and route sales, when two people bill at the same time.

Each series must stay consecutive on its own, and no number may repeat across series in the same year. Giving each series a distinct prefix handles both. If you are registered in two states, each GSTIN is a separate registration with its own invoices.

Starting a new financial year

1. Decide the new prefix before 1 April, such as changing 25-26 to 26-27.
2. Close the old series on 31 March. Note the last number used.
3. Start the new series at 1 for the first invoice dated 1 April or later.
4. Check the date on the first few bills. An invoice dated 31 March must carry an old-year number, and one dated 1 April a new-year number.

If you start a series in the middle of a year, for example when you register under GST or open a branch, start at 1 and keep going until 31 March.

Cancelled invoices and corrections

Mistakes happen. What you do next matters for the series.

• Do not delete and reuse a number. Keep the cancelled invoice in your records, marked cancelled, so the gap is explained.
• Corrections after the supply are generally handled with a credit note (when the value goes down, such as returns or a rate error) or a debit note (when it goes up), linked to the original invoice. These notes usually have their own number series.
• Report the numbers as your CA advises. Returns include a summary of documents issued, including the range of numbers and how many were cancelled.

Common invoice numbering mistakes

• Manual books on two counters that both start at 1, creating duplicates.
• Forgetting to change the year on 1 April, so April bills carry last year's prefix.
• Numbers longer than 16 characters after adding a long branch name.
• Reusing numbers from cancelled bills.
• Typing numbers by hand into spreadsheets, so one gets skipped or repeated.

Invoice numbering checklist

• One clear format, 16 characters or fewer, with only allowed characters.
• A distinct prefix for every series you run.
• A fresh series each financial year, starting at 1.
• Cancelled bills kept and marked, not deleted.
• Credit and debit notes in their own series, linked to the original invoice.
• A monthly check that there are no gaps or repeats.

For the rest of the fields a tax invoice must show, see our guide to the GST invoice format. Your number series also needs the right tax heads on each bill; our article on CGST SGST IGST explains how to decide.

How MaalMitra helps

MaalMitra takes invoice numbering out of human hands, which is where most gaps and repeats come from.

• Automatic numbering on delivery. When an order is delivered, or an instant counter sale is made, the GST tax invoice is created and numbered at once.
• Per wholesaler, per financial year. Each wholesaler has its own running number, which restarts in April with the financial year. Check with your CA that the number format suits your business.
• One sequence for all orders. Route deliveries and counter sales draw from the same series, so two staff members cannot issue the same number.
• The rest of the invoice follows. Delivered quantities only, HSN, taxable value, CGST + SGST or IGST by GSTIN state, round off, amount in words, print and PDF.

MaalMitra does not produce GST credit or debit note documents, file returns or generate e-invoices, so share your invoice register with your CA for those.

The bottom line

Good invoice numbering is boring by design: one format, consecutive numbers, unique within the financial year, a new series every April and no deleted bills. Set it up once with your CA, let software assign the numbers, and check monthly that the series has no gaps.

Want invoice numbers that assign themselves on every delivery? Get started free with MaalMitra.