GST Invoice Number Format: The 16-Character Rule and Examples

The short answer

Under GST, a tax invoice must carry a consecutive serial number of no more than 16 characters, using letters, digits, hyphens or slashes, in one or more series, and it must be unique for the financial year. So INV/2026-27/0001 (exactly 16 characters) is valid, while INV-2026-27-00001 (17 characters) is too long. You can type any number you choose into the Invoice # field of the free Invoice Generator, so it is up to you to follow the format.

What the rule actually says

The requirement comes from Rule 46(b) of the CGST Rules, 2017, published on the CBIC tax information portal. It asks for:

• A consecutive serial number. Invoices follow in order: 0001, 0002, 0003. Do not skip numbers or reuse them.
• Not exceeding sixteen characters. Every character counts, including letters, slashes and hyphens.
• One or multiple series. You can run separate series, for example one per branch or one for exports.
• Allowed characters: alphabets, numerals, hyphen (-) and slash (/), in any combination.
• Unique for a financial year. The same number must not appear twice between 1 April and 31 March.

Valid and invalid examples

Character counts below were checked by code.

Invoice number	Characters	Valid?	Why
INV/2026-27/0001	16	Yes	At the limit
MUM/26-27/00001	15	Yes	Branch prefix, short year, five-digit counter
INV-2024-001	12	Yes, for that year	The Invoice Generator's sample format
INV-2026-27-00001	17	No	One character too long
INV#2026_001	12	No	# and _ are not among the allowed special characters
INV 2026 001	12	No	Spaces are not among the allowed characters

How to design a numbering scheme

A good invoice number does three things: it stays within 16 characters, it never repeats within the financial year, and it tells you something useful at a glance.

Step 1: Pick a short prefix

Two to four letters are enough: INV, EXP for exports, or a branch code such as DEL or BLR. The prefix is how you keep separate series apart.

Step 2: Add the financial year

Including the year makes numbers unique across years and avoids clashes when you restart the counter. 26-27 uses 5 characters; 2026-27 uses 7. If space is tight, use the short form.

Step 3: Choose the counter length

Count how many invoices you might issue in a year in your busiest series. A four-digit counter (0001 to 9999) is enough for most small businesses. Use leading zeros so numbers sort correctly in spreadsheets.

Step 4: Check the total length

Add it up. INV/2026-27/0001 is 3 + 1 + 7 + 1 + 4 = 16, exactly the limit. If you want a five-digit counter, shorten the year: INV/26-27/00001 is 15.

Step 5: Restart (or continue) each April

Because the number must be unique for the financial year, many businesses start a fresh series on 1 April with the new year in the prefix. Whatever you choose, apply it consistently across all your series.

Multiple series: when they help

You can use more than one series at the same time, as long as each number is unique for the year. Common reasons:

• Branches or warehouses: DEL/26-27/0001 and BLR/26-27/0001 can both exist because the prefixes differ.
• Different document types: keep credit notes, debit notes and export invoices in their own series.
• Online and offline sales: separate series make reconciliation easier.

Avoid creating so many series that tracking gaps becomes difficult. Each series should still run in order without missing numbers.

Using the Invoice Generator

Open the Invoice Generator.

Fill in your business name, address, GSTIN and email, then the client's name, address and GSTIN.

Replace the sample Invoice # (INV-2024-001) with the next number in your series. The tool does not check the length or keep a counter, so track the last number you used in a spreadsheet or notebook.

Set the date, add line items with quantity and rate, choose the GST rate and any discount percentage.

Check the totals: subtotal, discount, taxable amount, CGST and SGST (each half of the selected rate) and the final total.

Click Print / Download Invoice and choose Save as PDF in your browser's print dialog to keep a copy.

Know the tool's limits before relying on it for GST invoices:

• It always splits tax into CGST and SGST, which suits supplies within your own state. It has no IGST option for inter-state supplies.
• It has no separate fields for HSN or SAC codes, place of supply or reverse charge. Rule 46 requires these in many cases, so add them in the item description or the notes box, or use accounting software.
• Check that the GST rate options it shows match the current rate for your goods or services.

The guide to GST invoice fields lists the other details a tax invoice needs.

Common mistakes

• Going over 16 characters after adding a long branch code and full year.
• Reusing a number for a corrected invoice. Issue a credit or debit note, or follow your accountant's correction process, rather than editing and reissuing the same number for a different document.
• Skipping numbers when an invoice is cancelled. Keep a record of the cancelled number instead of silently jumping ahead.
• Different formats in different places, such as INV-0001 in one system and INV/0001 in another for the same series.
• Using spaces or symbols like #, _ or ., which are not in the allowed list.

Quick checklist

• 16 characters or fewer.
• Only letters, digits, - and /.
• Consecutive within each series.
• Unique for the financial year.
• Same format every time.

To work out the tax amount on an invoice, the GST calculator adds or removes GST from any amount.

This guide explains the general rules and is not tax advice.