You Want to Switch Invoicing Tools in August. Only One Thing Can Actually Go Wrong.

You can move to a different invoicing tool in the middle of a financial year. Nothing in GST requires you to wait for 1 April.

Rule 46(b) of the CGST Rules asks for one thing — a consecutive serial number, up to sixteen characters, unique for the financial year. It says nothing about which software produced it. The only real risk is that your new tool starts numbering at INV-001 while your old one already issued an INV-001 in April, and now two different invoices carry the same number in the same year. That is the failure. Everything else is housekeeping.

And one rule that overrides all of this: never renumber an invoice you have already sent. Once it's with the client, it's in their books and heading into their GSTR-2B. Changing it later creates a mismatch that lands on them, not you.

General information on invoice numbering under the CGST Rules, not advice on your specific facts. Confirm anything with consequences with a qualified Chartered Accountant.

What Rule 46(b) actually says

The clause is short. Every tax invoice must carry a serial number that is:

• consecutive — moving forward within its series, not jumping around
• not more than sixteen characters
• made of letters, numerals, or the special characters hyphen (-) and slash (/) only
• unique for a financial year
• in one or multiple series — plural is explicit in the rule

Read that list again for what isn't in it. There is no requirement to start at 001. There is no requirement to keep the same format all year. There is no requirement to tell the department you changed tools. And multiple series running at once are expressly permitted.

Two things in that list catch people out.

The character rule is narrower than you think. Only hyphen and slash. INV#0042 and INV_0042 and Ravi Design 42 are all outside the rule as written, and hash marks and underscores are exactly what a spreadsheet template hands you by default. If your Excel invoices used them, that is worth fixing on the switch — going forward, not retrospectively.

Sixteen characters includes everything. JEELSOFTWARE/2026-27/0042 is 25 characters and fails. JSC/26-27/0042 is 14 and passes.

If you're not sure what else has to be on the document, the GST invoice format guide covers the other fifteen fields Rule 46 asks for.

Your three options on switching day

Say your last invoice out of the old system was INV-0037, issued 5 August, and you're moving now.

Option 1 — carry on where you left off

Set the new tool's next number to INV-0038 and keep going. Cleanest choice if your existing format already complies. One continuous series for the year, one line in your GSTR-1 document summary, no explaining to do at audit.

The only work is making sure the starting number is right before you issue anything. Most tools let you set it; the mistake is issuing two invoices first and noticing after.

Option 2 — start a distinct second series

Keep INV- closed at 0037 and open something visibly different: JSC/26-27/001. Legitimate — the rule allows multiple series — and useful if your old format was non-compliant, or if you want a clean break you can point at later.

The condition is that the new series must be distinct, not a restart of the old one. Two live series is also two rows to report, and two counters to keep straight.

Option 3 — restart at INV-001 with the same prefix

Don't. You already issued INV-001 in April. Now two invoices in FY 2026-27 carry the identical number, and the uniqueness requirement is broken. This is the one genuinely non-compliant move available to you, and it is also the default behaviour of a fresh account in almost every invoicing tool. It happens by inaction, not by decision.

Pick one padding width and never change it

INV-1, INV-01, INV-001 and INV-0001 are four different strings. Mixing them inside one series is not a violation of Rule 46(b), but it breaks sorting everywhere downstream — in your own list view, in a CA's Excel, in the from/to range you report to the portal. INV-0009 and INV-0010 sort correctly. INV-9 and INV-10 do not.

Four digits is the sensible default for a solo practice. Decide before your first invoice in the new tool, because fixing it afterwards means either renumbering issued invoices — which you are not going to do — or living with a split series for the rest of the year.

Gaps: what to do about the numbers you skipped

If you jump from INV-0037 to JSC/26-27/001, you haven't created a gap — you've closed one series and opened another. That's clean.

A real gap is a missing number inside a series: a draft that was numbered and abandoned, or a cancelled invoice. The portal will not stop you filing. But a gap with no explanation is exactly the sort of thing a Section 65 audit asks about, and "the software did it" is not an answer anyone finds satisfying three years later.

Practical handling: keep a one-line note of every cancelled or voided number and why. A cancelled invoice is reported as cancelled in your GSTR-1 document summary, which is a documented gap rather than a missing one.

The part everyone forgets: GSTR-1 Table 13

Table 13 of GSTR-1 — the document summary — became mandatory from the May 2025 return period, under the GSTN's Phase-III changes to Tables 12 and 13. For each document type you report the opening serial number, the closing serial number, the total number of documents issued, and how many were cancelled. The portal then shows total documents, cancelled documents and net issued.

This is where a mid-year switch shows up, and it is why the series choice matters more than it looks:

None of this is difficult if you decided deliberately in August. All of it is painful if you're reconstructing it from two systems in October.

Credit notes and receipt vouchers carry their own numbering under their own rules, and they get their own rows here too — if you've taken advances, the receipt voucher series is separate from your invoice series.

The switching-day checklist

1. 1.Find your last issued number. The actual last one, including anything you raised manually outside the tool.
2. 2.Check your existing format against the rule. Sixteen characters, letters/numbers/hyphen/slash only. If it fails, use Option 2.
3. 3.Decide: continue or open a new series. Write the decision down with a date.
4. 4.Set the starting number in the new tool before issuing anything.
5. 5.Fix the padding width now. Four digits.
6. 6.Export everything from the old tool — invoices, clients, line items. Do it while you still have access. A tool that makes this hard is telling you something.
7. 7.Leave issued invoices alone. No renumbering, no reformatting, no reissuing.
8. 8.Note the switch date and the closing number somewhere your CA can find in January.
9. 9.Check the numbering on your first three invoices out of the new tool before sending them.

That's roughly twenty minutes of work, and it's the entire cost of switching. The reason it feels bigger is that nobody explains it, so it sits there as unspecified risk.

Switching in the middle of the year, without the reset

The failure mode is a fresh account that quietly starts at 001. Mavoin asks for your prefix and starting number in Settings when you begin — JS/26-27/ starting at 088, say — so a mid-year switch continues your series instead of resetting it, and stays consecutive across the rest of the financial year.

It handles the rest of the invoice too: CGST/SGST versus IGST by place of supply, SAC codes on the client and the service, and USD billing with INR books for foreign clients. GST and multi-currency invoicing from ₹199/month, after a 30-day trial. Self-serve CSV export is being built — until it ships, email hello@mavoin.com any time and we'll hand your clients, invoices and payments over. Your data is yours.

Start a 30-day trial — no card required.

Sources: Rule 46, CGST Rules 2017 (CBIC) · GSTR-1 filing user guide, GST portal

Mavoin is not a Chartered Accountant service. General information, current as of August 2026.