GST Invoice Number Rules: Format, 16-Character Limit and Series

What the GST rule says

Rule 46 of the CGST Rules lists what a tax invoice must contain. Clause (b) covers the number: a consecutive serial number, not more than sixteen characters, in one or more series, using letters, numerals, hyphens and slashes in any combination, and unique for a financial year.

Four things follow from that sentence:

1. Consecutive: numbers go up in order without gaps you cannot explain.
2. 16 characters at most, counting letters, digits, hyphens and slashes.
3. Unique for the financial year (April to March): the same number must not appear twice in that year for your GSTIN.
4. One or more series are allowed, as long as each is consecutive and no number repeats.

Valid and invalid invoice numbers

"16 digit invoice number": what people get wrong

Many people search for a "16 digit invoice number" and assume the number must be 16 digits long. It does not. Sixteen is the maximum, and it counts all characters, not only digits. A 9-character number like A/26-27/1 is perfectly valid.

The same 16-character limit applies when your invoice numbers are reported in your GST return, so a longer number causes trouble at filing time. Keep yours comfortably short.

Choosing a format that lasts

A good invoice number has three parts: a short prefix, a year and a running number.

• Prefix: 2 or 3 letters, such as INV, or your shop's initials. Use a different prefix for each series (for example, a second branch).
• Year: 26-27 or 2026. It keeps numbers unique when a new year starts.
• Running number: 4 digits (0001 to 9999) is enough for most small businesses. Use 5 if you issue more than 9,999 invoices a year.

Count before you print. INV/26-27/0001 is 14 characters, which leaves room for one more digit later.

Worked example: a shop with two counters

Patel Hardware in Surat has a main shop and a small second outlet, both under one GSTIN. They want separate series so each counter's bills are easy to track.

• Main shop: PH/26-27/0001, PH/26-27/0002 and so on (13 characters).
• Second outlet: PH2/26-27/0001 and onwards (14 characters).

On 1 April 2027 both move to 27-28 and start again from 0001. Every number is unique in its financial year, each series is consecutive, and both fit well within 16 characters. In their GST return, they report both series, with the first and last number of each.

Common numbering mistakes

• Reusing a cancelled number. If you cancel an invoice, keep the number as cancelled and move on. Your return reports how many invoices were cancelled in each series.
• Two bill books with the same series. If two people write bills, give each a different prefix.
• Changing the number after sending the bill. The customer has the old number in their records. Do not change it.
• Restarting at 0001 without changing the year part. If you restart on 1 April, the year in the number must change too, or numbers repeat.
• Adding decoration like #, spaces or underscores. Stick to letters, digits, hyphens and slashes.

How BillBabu numbers your invoices

In BillBabu you never type an invoice number. When you save an invoice, BillBabu gives it the next number in the series for your business, in the format INV-2026-0001: 13 characters, using only letters, digits and hyphens. Estimates get their own series (EST-2026-0001) and purchase bills theirs (PUR-2026-0001), so the three never mix.

One thing to know: the year in a BillBabu number is the calendar year, so a new series (INV-2027-0001) starts in January rather than on 1 April. Every number is still unique within each financial year, which is what the rule asks for. If your CA prefers a series that restarts each April, tell them how BillBabu numbers so they can note it.

Good numbering also makes payments easy to match in your party ledger. If you make a daily summary bill for small cash sales, see our guide to the consolidated invoice under Rule 46A. Service providers can see the full layout in the freelancer invoice format. More on GST billing is on our GST billing software page.

GST invoice rules checked on 4 October 2026, under the GST 2.0 rates in force since 22 September 2025. Rates can depend on the exact item, its price or its packing, so confirm your item in the CBIC rate notification or with your CA.

Frequently asked questions

Can a GST invoice number have letters?

Yes. Letters, numbers, the hyphen and the slash are all allowed, in any combination, up to 16 characters.

Do I have to restart invoice numbers every April?

No. The rule is that a number must be unique within the financial year. Many businesses restart each April with the new year in the prefix, but a series that simply keeps going is also fine as long as no number repeats.

Can I have separate invoice series for two shops?

Yes. GST allows one or more series. Give each shop its own prefix so no number repeats, and report each series in your return.

What if I skipped an invoice number by mistake?

Do not fill the gap with a back-dated bill. Note the skipped number and the reason, and tell your CA so it is reported correctly.

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Get every invoice numbered right, automatically. BillBabu is a GST billing app for Android, with a web app at app.billbabu.com. Make GST invoices and estimates, share them as PDFs and see who still owes you money. Every account starts with a 14-day free trial, and you do not need a card. Get BillBabu on Google Play or see plans and prices.