GSTR-3B Due 20 September 2026: 3 Ways Interest Is Calculated

Last updated: 15 September 2026. GSTR-3B for the August 2026 tax period is due on Sunday, 20 September 2026 for monthly filers, and GSTR-5A shares the date. If you file late, the interest you owe is not one rule but three, and which applies turns on a detail most filers never look at: whether the liability was declared in the return for that period or surfaced somewhere else.

Applicability Note: This guide reflects GST provisions, notifications and GSTN advisories applicable as of 15 September 2026, and no extension has been notified for the August 2026 period as of this writing. Always verify the current position on gst.gov.in or with a GST professional before filing.

Who Should Care?

This guide applies to:

• Monthly GSTR-3B filers — anyone outside the QRMP scheme, whatever the turnover
• GSTR-5A filers — OIDAR and online money gaming suppliers reaching India from outside it
• QRMP taxpayers — no GSTR-3B for August, but your PMT-06 deposit falls on Friday, 25 September 2026
• Anyone reporting an older invoice in a current return, or paying an old liability through DRC-03

1. What Falls Due on Sunday 20 September 2026

The 20th is a payment deadline as much as a filing one: Section 39(7) requires the tax due as per the return to be paid "not later than the last date on which he is required to furnish such return".

Where the dates come from. GSTR-3B: Rule 61(1)(i), CGST Rules, as substituted with effect from 1 January 2021 by Notification No. 82/2020-Central Tax dated 10 November 2020 — "on or before the twentieth day of the month succeeding such month". GSTR-5A: Rule 64, as substituted by Notification No. 51/2023-Central Tax dated 29 September 2023, which widened the form beyond classic OIDAR to cover online money gaming from outside India and OIDAR supplied to a registered person who is not a non-taxable online recipient.

The Sunday does not move the date. Section 10 of the General Clauses Act, 1897 shifts an act to the next day only where the office is closed on that day or on the last day of the prescribed period, and the portal accepts returns on a Sunday. Our September 2026 filing calendar has the month's other dates.

2. The Default Rule: Interest Only on the Cash Part

Interest under Section 50(1) runs at 18% per annum, the rate notified at serial 1 of Notification No. 13/2017-Central Tax dated 28 June 2017. The sub-section's own ceiling is "not exceeding eighteen per cent", so 18% is both the notified rate and the maximum.

The proviso to Section 50(1) is the part that saves most filers real money. Where the supplies of a tax period are declared in the return for that period and that return is furnished after the due date, interest "shall be levied on that portion of the tax that is paid by debiting the electronic cash ledger" — credit used to discharge the same liability carries none. Rule 88B(1) does the arithmetic: the cash portion, for the period of delay beyond the due date, at the Section 50(1) rate.

Three conditions hold together: the supplies were declared in the return for that very period, the return was furnished late, and it was not furnished after proceedings under Section 73, 74 or 74A had commenced for that period.

How this became the law. The proviso was substituted with retrospective effect from 1 July 2017 by section 112 of the Finance Act, 2021, brought into force on 1 June 2021 by Notification No. 16/2021-Central Tax dated 1 June 2021; "or section 74A" was added by section 126 of the Finance (No. 2) Act, 2024. Rule 88B was inserted by Notification No. 14/2022-Central Tax dated 5 July 2022, also with effect from 1 July 2017. In Refex Industries Ltd v. Assistant Commissioner of CGST & Central Excise (Madras High Court, W.P. Nos. 23360 and 23361 of 2019, judgment dated 6 January 2020), the court read the proviso as clarificatory and applied the cash-only position to earlier periods.

Interest starts the day after the deadline. Section 50(2) calculates it "from the day succeeding the day on which such tax was due to be paid" — for August 2026, the clock starts on 21 September.

3. The Case That Costs More: Rule 88B(2)

Rule 88B(2) covers "all other cases" where interest is payable under Section 50(1), and it changes the base: interest runs on the amount of tax which remains unpaid, from the day after the tax fell due (Section 50(2)) until the date it is paid. There is no cash-only confinement, because the proviso that creates it never applied.

You land here whenever the liability was not declared in the return for its own period — an August invoice reported in a later month's GSTR-3B, a liability paid through DRC-03 instead of through the period's return, or an omitted reverse-charge liability picked up in a later reconciliation.

The portal now makes this visible. Under the GSTN advisory dated 30 January 2026, GSTR-3B auto-populates a Tax Liability Breakup Table from the document dates reported in GSTR-1, GSTR-1A or IFF, separating out supplies of earlier periods discharged in the current return. A second advisory dated 16 March 2026 added a step: from the February 2026 tax period, after offsetting liability you must open the "Tax Liability Breakup, As Applicable" tab and confirm it with SAVE — or edit it — before the return will file. GSTN has recorded that the confirmation is currently being demanded in every case, and that this is under resolution.

4. Money Already in Your Cash Ledger Does Not Attract Interest

A proviso inserted into Rule 88B(1) by Notification No. 12/2024-Central Tax dated 10 July 2024 takes an amount out of the computation entirely. Where money was credited to the electronic cash ledger under Section 49(1) on or before the due date, is debited for payment of tax while filing the return after the due date, and has been lying in the ledger from the due date until the date of that debit, it is left out of the interest calculation. All three limbs matter — a deposit made after the due date, or one drawn down and topped up in between, does not qualify.

The legal benefit has existed since July 2024. The portal only began computing it from the January 2026 tax period: under the 30 January 2026 advisory, Table 5.1 interest is now system-computed with the benefit of the minimum cash balance in the ledger from the due date to the date of offset, which GSTN describes as being "in line with the proviso to Rule 88B(1)". Three things follow:

• the auto-populated interest cannot be amended downward as it stands — where the portal computed it without the cash-balance benefit, the RE-COMPUTE INTEREST button under Table 5.1 is the route to a lower figure, per the GSTN advisory dated 16 April 2026;
• it is expressly "only the minimum interest" — you are expected to self-assess and revise it upward where your own computation is higher, which is what a Rule 88B(2) liability requires;
• interest on a late return is auto-populated in the following period's GSTR-3B — file August late and the figure surfaces in your September return.

Treat the portal's figure as a floor, never a settled amount.

5. The Arithmetic, Three Scenarios

Take a monthly filer with an August 2026 liability of ₹4,00,000, of which ITC covers ₹3,20,000, leaving a cash portion of ₹80,000. Aggregate turnover last year: ₹3 crore. The return is filed on 29 September 2026 — nine days late, counting 21 to 29 September.

Scenario C counts 91 days: 21 September to 20 December 2026. The gap between B and C is not a penalty for lateness — it is the difference between declaring a supply in its own period and declaring it later.

QRMP filers get there by a different route. No GSTR-3B for August; the PMT-06 deposit is due 25 September, and Rule 61(3)'s third proviso lets a Month 2 depositor count the cash-ledger balance excluding the tax due for the first month of the quarter.

6. Late Fee Is a Separate Meter

Interest compensates for tax paid late; the late fee is charged for the return being late. Both run at once. Every figure below is the total across CGST and SGST/UTGST — the per-Act figure is half.

Sources for the fee. Section 47(1) levies ₹100 per day per Act, capped at ₹5,000 per Act. For GSTR-3B, Notification No. 76/2018-Central Tax dated 31 December 2018 waives everything above ₹25 per day per Act, and above ₹10 per day per Act where the central tax payable in the return is nil. The turnover-linked caps for tax periods from June 2021 onwards come from Notification No. 19/2021-Central Tax dated 1 June 2021. Taxpayers with aggregate turnover above ₹5 crore do not appear in that notification's table at all, so nothing reduces their fee below the Section 47(1) ceiling. GSTR-5A gets no reduction: the one granted by Notification No. 6/2018-Central Tax dated 23 January 2018 was rescinded by Notification No. 13/2018-Central Tax dated 7 March 2018.

In scenario B, the nine-day delay costs ₹355 interest plus ₹450 late fee — the fee is the larger of the two. Our earlier guide on GSTR-3B late fees and the checks worth running before you file works through the fee side in more detail.

7. A Third Meter: Wrongly Availed Credit

Interest under Section 50(3) is a different charge on a different base. It applies where input tax credit has been wrongly availed and utilised, and Rule 88B(3) runs it from the date of utilisation to the date the credit is reversed or the tax is paid. The Explanation to that sub-rule measures utilisation against the ledger: credit is treated as utilised when the electronic credit ledger balance falls below the wrongly availed amount, and the extent of utilisation is the amount by which it falls below.

The rate here is a standing trap. Section 50(3) permits a rate "not exceeding twenty-four per cent", and plenty of material still quotes 24%. The notified rate is 18% — serial 2 of Notification 13/2017-CT was substituted from 24 to 18, retrospectively from 1 July 2017, by section 116 read with the Sixth Schedule of the Finance Act, 2022. If you reversed and reclaimed credit in August, our walkthrough of GSTR-3B Table 4 reversals and reclaims covers where those entries belong.

Key Takeaways

• GSTR-3B for August 2026 and GSTR-5A are both due Sunday, 20 September 2026. The Sunday does not move the date, and no extension has been notified.
• The cash-only rule is conditional — the period's supplies declared in that period's return, the return late, and no Section 73/74/74A proceedings started.
• A liability declared in a later return costs far more. Rule 88B(2) charges interest on the whole unpaid tax from the original due date.
• Park the cash before the 20th if you know you will file late — money credited on or before the due date and left untouched until debit is excluded under the proviso to Rule 88B(1).
• The portal's interest figure is a floor — expressly the minimum, and not to be overwritten with a lower number; revise it upward where your own computation is higher.
• QRMP filers: no GSTR-3B for August, but the PMT-06 deposit is due Friday, 25 September 2026.

Frequently Asked Questions

Is GST interest charged on the full tax or only on the cash portion?

Only on the cash portion in one situation: where the supplies of a tax period are declared in the return for that period and that return is filed late, under the proviso to Section 50(1) read with Rule 88B(1). Every other case falls under Rule 88B(2), where interest runs on the whole tax that remained unpaid.

Does interest run from the due date or from the day after?

From the day after. Section 50(2) calculates interest "from the day succeeding the day on which such tax was due to be paid", so for the August 2026 return the clock starts on 21 September 2026.

I deposited the tax before 20 September but filed the return late. Do I still pay interest?

Not on the amount that was sitting there. The proviso to Rule 88B(1), inserted by Notification No. 12/2024-Central Tax dated 10 July 2024, excludes an amount credited to the electronic cash ledger on or before the due date that stays there until it is debited on filing. The late fee still applies for the days of delay.

Can I reduce the interest the portal has filled in?

Not by overwriting it. From the January 2026 tax period the Table 5.1 figure is system-computed and cannot be amended downward, per the GSTN advisory dated 30 January 2026. Where the system computed it without the cash-balance benefit, the GSTN advisory dated 16 April 2026 provides a RE-COMPUTE INTEREST button under Table 5.1 — after re-computing you may edit the figure down to the recomputed amount, but never below it. Upward revision is always open: the auto-populated figure is only the minimum.

Do QRMP taxpayers file GSTR-3B for August 2026?

No. QRMP taxpayers file GSTR-3B once a quarter. For August, Month 2 of the July–September quarter, the obligation is a deposit in FORM GST PMT-06 by 25 September 2026 under Rule 61(3); the quarterly GSTR-3B follows on 22 or 24 October 2026 depending on the state.

Disclaimer: This article is for informational purposes only and does not constitute professional tax advice. GST rules are subject to frequent changes through notifications and circulars. Please consult a qualified tax professional or verify the current provisions on the official GST portal (gst.gov.in) before making any compliance decisions.

Have a specific question about GSTR-3B interest or late fees? Our GST experts can help → gstconsultancy.com/dashboard/ask