GSTR-3B Explained: How to File Your Monthly GST Summary Return

If you are registered under GST in India, GSTR-3B is the return you will file most often — and the one where money actually changes hands. It is a monthly (or, for smaller taxpayers on the QRMP scheme, quarterly) summary return in which you declare your total output tax, claim your input tax credit, and pay the net GST you owe.

It is easy to think of GSTR-3B as "just a summary", but it is the return that fixes your tax liability for the period. Get the numbers wrong, or file late, and the consequences are real — interest, late fees, and blocked credit for your customers. This guide walks through what goes into GSTR-3B, how the tax nets off, and the deadlines that carry a cost.

What GSTR-3B actually is

GSTR-3B is a self-declared summary of your GST position for a tax period. Unlike GSTR-1 — which reports every outward invoice in detail — GSTR-3B reports totals: your total taxable supplies, the output tax on them, the input tax credit you are claiming, and the net tax payable after setting one off against the other.

You file GSTR-3B even in a month with no activity — a nil return is still required. And critically, GSTR-3B is where you pay: the return is not considered filed until the tax due is paid in full.

The due dates that matter

For monthly filers, GSTR-3B is due on the 20th of the following month. So the return for June is due by 20 July, July by 20 August, and so on. GSTR-1 for the same period is due earlier, by the 11th — so the natural rhythm each month is GSTR-1 by the 11th, then GSTR-3B by the 20th.

Businesses on the QRMP (Quarterly Return, Monthly Payment) scheme file GSTR-3B quarterly, but still pay tax monthly through a challan. If you are unsure which applies to you, check your GST profile or ask your accountant — the filing frequency is tied to your turnover and your election.

How output tax and ITC net off

The heart of GSTR-3B is the set-off between what you owe (output tax on your sales) and what you can reclaim (input tax credit on your purchases). You do not pay tax on your full sales — you pay the difference.

Say you collected ₹5,400 of output GST on your sales for the month, and you have ₹4,338 of eligible input tax credit from your purchases. Your net tax payable is ₹5,400 minus ₹4,338 = ₹1,062. That ₹1,062 is what you actually pay.

The set-off follows an order set by GST law. Input IGST credit is used first against IGST liability, then can spill over to CGST and SGST. CGST credit can only be used against CGST (and IGST), never against SGST — and SGST credit likewise cannot touch CGST. This "cross-utilisation ban" between CGST and SGST is a rule a lot of manual calculations get wrong.

The structure: what each table captures

GSTR-3B is organised into a handful of tables, and knowing what each one does removes most of the confusion. You do not need to memorise them, but understanding the shape helps you spot when a figure has landed in the wrong place.

• →Table 3.1(a): outward taxable supplies (your standard sales) and the output tax on them.
• →Table 3.1(b) and (c): zero-rated supplies (exports, SEZ) and nil-rated or exempt supplies.
• →Table 3.1(d): inward supplies liable to reverse charge — GST you owe as the recipient, not the supplier.
• →Table 3.2: of your inter-state supplies, the portion made to unregistered persons, composition dealers and UIN holders.
• →Table 4: eligible input tax credit — ITC available, ITC reversed (Rule 42/43 and others), and the net ITC you can claim.
• →Table 5: values of exempt, nil-rated and non-GST inward supplies.
• →Table 6.1: payment of tax, where the net liability is discharged from your cash and credit ledgers.

A full worked example

Numbers make this concrete. Imagine a trading business in Karnataka for the month of June, and walk the whole return through to the payment.

Sales: ₹3,00,000 of local (intra-state) sales at 18% generate ₹27,000 of output GST — split ₹13,500 CGST + ₹13,500 SGST. Plus ₹1,00,000 of inter-state sales at 18% generate ₹18,000 IGST. Total output tax for the month: ₹45,000.

Purchases: ₹2,00,000 of local purchases at 18% give ₹18,000 ITC (₹9,000 CGST + ₹9,000 SGST), and ₹80,000 of inter-state purchases at 18% give ₹14,400 IGST credit. Gross ITC is ₹32,400 — but assume ₹1,400 is blocked under Section 17(5) (a business-lunch bill), leaving ₹31,000 of eligible credit.

Now the set-off. The ₹14,400 IGST credit is used first against the ₹18,000 IGST liability, leaving ₹3,600 IGST to pay in cash. The ₹9,000 CGST credit covers part of the ₹13,500 CGST liability, leaving ₹4,500 CGST. The ₹9,000 SGST credit covers part of the ₹13,500 SGST, leaving ₹4,500 SGST. Net cash payable this month is around ₹12,600, discharged in Table 6.1. The exact split shifts with the utilisation order, but the principle holds: you pay the gap, not the gross.

QRMP: the quarterly option for smaller businesses

If your aggregate turnover is up to ₹5 crore, you can opt for the QRMP scheme — Quarterly Return, Monthly Payment. Under QRMP you file GSTR-3B once a quarter instead of every month, which meaningfully cuts the filing workload for a small business.

The catch is in the name: even though the return is quarterly, you still pay tax for the first two months of each quarter through a challan (form PMT-06), using either a fixed-sum method (35% of the previous period’s tax) or a self-assessment of the actual liability. The quarterly GSTR-3B then trues everything up. QRMP is a genuine convenience, but it does not let you defer the cash — the tax still moves monthly.

Reverse charge and blocked credits

Two things commonly trip people up in GSTR-3B. The first is reverse charge (RCM): on certain notified supplies, you — the recipient — are liable to pay the GST rather than the supplier. In GSTR-3B this is declared as a liability in Table 3.1(d) and, where eligible, reclaimed as ITC in Table 4 — so for a fully-taxable business it nets to zero, but both halves must be shown.

The second is blocked credits under Section 17(5). Some input tax — on motor vehicles, personal-use items, and other specified categories — is simply not claimable, even if it appears in your GSTR-2B. Claiming blocked credit is a common error that surfaces in audits. Good software flags these automatically so they never make it into your eligible ITC.

Late fees and interest — the cost of missing the 20th

Filing GSTR-3B late attracts a late fee and interest. The late fee is ₹50 per day (₹25 CGST + ₹25 SGST), reduced to ₹20 per day for a nil return, subject to caps that vary with turnover. Interest runs at 18% per annum on the net tax paid after the due date — and can be higher where credit has been over-claimed.

There is a knock-on cost too: if you file GSTR-3B late, your customers may be unable to claim the corresponding input tax credit on time. In a B2B relationship, that makes late filing not just your problem but your customer’s — which is why timely, accurate 3B matters well beyond the penalty.

How interest is actually calculated

Interest at 18% per annum sounds abstract until you put numbers to it. Suppose your net tax payable for June was ₹50,000 and you paid it 15 days after the 20 July due date. Interest is ₹50,000 × 18% × (15 / 365) ≈ ₹370. Not ruinous on its own, but it compounds across months and across every business you run, and it is entirely avoidable.

A sharper rule applies to input tax credit: if you claim ITC you were not entitled to (for example, credit later found to be blocked or unmatched), interest can apply on that wrongly-availed credit from the date it was utilised. This is why over-claiming ITC is riskier than under-claiming — the interest clock runs against you.

Frequently asked questions

Can I revise GSTR-3B after filing? No — there is no revision facility. Errors are corrected in a subsequent period’s return, which is why getting it right the first time matters.

What if I file GSTR-3B but do not pay the tax? The return is not treated as filed until the tax is paid in full, so an unpaid GSTR-3B still attracts late fees and interest as if you had not filed.

Do I need to file if I only had exempt or nil-rated supplies? Yes — a return is still required; you report those supplies in the relevant tables and file a nil-liability return.

Can I claim ITC not appearing in GSTR-2B? No. Since provisional credit was removed, ITC is limited to what appears in your GSTR-2B. Credit in your books but not in 2B should be held back until the supplier files.

How Deskloc Flow files this for you

In Deskloc Flow, GSTR-3B is built from data you have already entered. Your sales invoices feed the output tax. Your purchase bills feed the input tax credit, automatically split into eligible and blocked (Section 17(5)). Reverse-charge purchases are handled net-neutral — self-charged in 3.1(d) and reclaimed as ITC. And the whole thing reconciles against your GSTR-2B so you can see, before you file, exactly which credit the portal has confirmed and which is still at risk.

The result is a return-ready GSTR-3B summary with the net payable already worked out — for you to review with your CA and file on the GST portal.