If you file monthly returns, your GSTR-3B for September 2026 is normally due on 20 October 2026. This guide gives you a practical pre-filing checklist so the numbers you pay and claim on the 20th are ones you have already checked. Always confirm on the GST portal whether any extension has been notified for your return.
What GSTR-3B is, in plain words
GSTR-3B is the summary return in which a regular taxpayer declares the month's outward supplies, claims input tax credit (ITC) and pays the net tax. Unlike GSTR-1, which lists invoices, GSTR-3B is where the money moves: whatever tax liability you show must be paid, in cash or through available credit, before you file.
Because it is a summary, errors are easy to make and not always easy to fix. Corrections generally have to be made in a later month's return, so a careful check before filing saves effort later.
Know your due date first
- Monthly filers: the statutory default for September is 20 October 2026.
- QRMP taxpayers: the quarterly GSTR-3B for July–September is normally due on the 22nd or 24th, depending on your state or Union Territory.
- Composition taxpayers: they file CMP-08 for the quarter instead; that is a different return with its own date.
The due date is a statutory default. Extensions are sometimes notified by CBIC for specific states or periods, so check the GST portal (gst.gov.in) before you assume the date.
Step 1: Close your sales for the month
Make sure every invoice, credit note and debit note dated in September is recorded in your books. Then compare your books with what you reported in GSTR-1 for the month. If the two differ, find out why before you open GSTR-3B. For a refresher on avoiding errors there, see our guide on how to file GSTR-1 without common mistakes.
Also check for supplies that are easy to miss:
- Advances received that attract GST.
- Exports and supplies to SEZ units, which are reported separately.
- Sales on which you pay tax under reverse charge for purchases from unregistered suppliers or specified services.
Step 2: Review your input tax credit
ITC is where most disputes and notices arise. Credit can generally be claimed only when the invoice is reflected in your auto-drafted GSTR-2B and the other conditions of Section 16 of the CGST Act are met. Compare your purchase register against GSTR-2B line by line. Our walkthrough on reconciling GSTR-2B with your purchase register shows how.
A simple triage works well:
- In both books and GSTR-2B: eligible, if the goods or services are used for business and are not blocked.
- In books but not in GSTR-2B: hold the credit and follow up with the supplier to confirm they have filed.
- In GSTR-2B but not in books: check whether the invoice belongs to you, and record it if it does.
- Blocked or ineligible items: keep them out of the claim. If you want to understand the basics, read GST input tax credit explained for small businesses.
Also remember that if you have not paid a supplier within 180 days of the invoice date, the credit claimed on it generally has to be reversed with interest. Keep an eye on ageing payables each month.
Step 3: Compute the tax and check the cash position
Set your output tax against the eligible ITC in the order the law prescribes: IGST credit is used first against IGST, then CGST, then SGST or UTGST, and so on. Whatever remains is payable in cash through the electronic cash ledger.
Check your electronic cash ledger balance a day or two before the due date. Challans can take a little time to reflect, and a shortfall on the last day is a common reason for late filing.
Step 4: Understand what late filing costs
ItemWhat to know Late feeCharged per day of delay, split between CGST and SGST, at a lower rate for nil returns. The amounts and caps depend on your turnover and current notifications, so check the portal for the figure that applies to you. InterestCharged on tax paid late under Section 50 of the CGST Act, at 18% per annum on the net cash liability. Blocked e-way bill generationPersistent non-filing of returns can restrict your ability to generate e-way bills.Filing a nil return on time is still required if you have no activity, as the late fee keeps running otherwise.
Step 5: File, then keep your evidence
After you submit and pay, save the filed GSTR-3B PDF, the payment challans and your working sheet showing how you arrived at each table. If a notice arrives months later, this small file is what lets you respond quickly. For a wider monthly routine, our GST filing checklist for small businesses is a good companion.
Key takeaways
- The statutory default for monthly GSTR-3B for September 2026 is 20 October 2026; verify any extension on the GST portal.
- Reconcile your books with GSTR-1 before you start GSTR-3B.
- Claim ITC only for invoices that appear in GSTR-2B and meet Section 16 conditions.
- Top up your cash ledger early so payment does not hold up filing.
- Late filing costs both a daily late fee and interest, so a small delay adds up.
- Keep the return, challans and working papers together, and confirm specifics with your chartered accountant.
To stay ahead of dates like this one, put each return on a shared calendar with a reminder a few days before the deadline. The AICOUNTLY Calendar is positioned as a compliance calendar for exactly this kind of recurring deadline, and you can read more about our approach to Smart Books if you are choosing accounting software.