The GSTR-1 and GSTR-3B reconciliation for September 2026 is one of those checks that costs a few minutes now and a great deal of correspondence later if you skip it. Both returns fall due within days of each other — GSTR-1 for September on 11 October, GSTR-3B on 20 October — and they describe the same month from two different angles. GSTR-1 lists every outward supply you made. GSTR-3B is the summary return where you declare your liability for the month and actually pay it. When the tax on the first does not match the tax on the second, that difference does not quietly disappear.
What the GSTR-1 and GSTR-3B reconciliation for September 2026 actually checks
The reconciliation is narrow and specific: does the output GST on the supplies you have reported in GSTR-1 tie back to the output tax liability you have declared and discharged in GSTR-3B for the same tax period? In a clean month the two are the same number, arrived at two ways — invoice by invoice in GSTR-1, and as a single summary in GSTR-3B. The gap, when there is one, is usually not fraud. It is timing and housekeeping: an invoice raised late and pushed into next month’s GSTR-1 but already taxed in this month’s GSTR-3B, a credit note entered on one side only, an amendment that moved a figure in one return and not the other, or a manual GSTR-3B total that was rounded or keyed by hand.
September deserves an extra look because it closes the July–September quarter. Whatever slipped through the cracks in July and August is still sitting in the difference between your two returns, and the quarter is the natural point to square it — before the numbers harden into the figures your annual return and your customers’ records are built on.
The gap runs in two directions, and they carry different risks. If the tax on your GSTR-1 supplies is higher than what you paid in GSTR-3B, you have declared a liability you have not fully discharged — the shortfall is payable, and interest runs on it until it is paid. If GSTR-1 is lower than GSTR-3B, you may have paid on supplies you never reported to your customers, which means their input tax credit will not match what you actually charged them. Neither is a comfortable conversation to have after the fact, and both are visible in a two-minute comparison before you file.
Two returns, two due dates — 11 and 20 October
For a monthly filer, GSTR-1 for September 2026 is due on the 11th of the following month — 11 October 2026. That date is a Sunday this year, so treat the working day before as your real deadline and check the GST portal for any notified extension rather than assuming one. GSTR-3B for September follows on 20 October 2026 (a Tuesday). The nine-day gap is deliberate: GSTR-1 goes first so that the outward-supply detail is on record, and GSTR-3B follows as the return where the liability is summarised and the tax is paid.
The order matters for the reconciliation. Because GSTR-1 is filed first, it is the natural place to get the outward side right — every invoice, credit note and amendment for September in, and nothing carried in error. GSTR-3B then has to agree with it. Reconciling the two after both are filed is firefighting; reconciling before you file GSTR-3B is the whole point.
Why the gap now comes back to you
The GST system no longer waits for a scrutiny notice to notice a mismatch. Where the tax liability you have declared in GSTR-1 exceeds what you have paid in GSTR-3B by more than a set margin, the portal raises an automated intimation (Form DRC-01B) that asks you to do one of two things: pay the shortfall, or explain the difference. Until you respond, filing of your next GSTR-1 can be held up — so a gap you ignore this month can freeze next month’s compliance. The exact threshold and the current response process are set on the portal and worth confirming there before you rely on them, but the direction of travel is settled: the difference is now the department’s to see, automatically, and yours to resolve.
A mismatch between GSTR-1 and GSTR-3B is not, by itself, a wrongdoing. But an unexplained one is a question you will be asked — and it is far cheaper to answer it in your own books today than in a portal reply next month.
The reconciliation to run before you file
Keep it mechanical. Take the total taxable value and output tax (IGST, CGST, SGST separately) from your September GSTR-1, take the corresponding figures from your draft GSTR-3B, and put them side by side. Where they agree, you are done. Where they do not, trace the difference to a specific document — a late invoice, a credit note, an amendment — before you file, not after. Reverse-charge liability sits only in GSTR-3B, so exclude it from the comparison of outward supplies. And if you file quarterly under QRMP, the same discipline applies to the quarter: your outward supplies and the liability you pay still have to tell one story.
None of this is difficult when your books are current. It becomes difficult — and expensive — when the reconciliation is left until a mismatch shows up on the portal. If closing September cleanly across both returns is one more thing than your month has room for, that is exactly the kind of routine an outsourced finance team runs quietly in the background. Talk to our team and it stops being your problem to remember.