GSTR-1 reports your sales invoice by invoice. GSTR-3B reports the totals and pays the tax. In a perfect world the outward tax in both would be identical every month. In practice, a missed invoice, a credit note entered late or an amendment made in only one return is enough to create a gap — and the GST system now spots that gap automatically.

If you are new to the two returns, start with our explainer on the difference between GSTR-1 and GSTR-3B. This guide is for the next step: finding out why your numbers don't match, fixing them, and responding if the department has already noticed.

Why the Mismatch Matters More in 2026

Since 2023 the GST portal compares the liability in GSTR-1 (and IFF) with the tax paid in GSTR-3B for every taxpayer. When GSTR-1 shows more tax than GSTR-3B beyond a notified limit, the system issues an intimation in Form DRC-01B. And from mid-2025, the outward liability auto-populated into GSTR-3B from GSTR-1, GSTR-1A and IFF is locked, so the old habit of quietly editing GSTR-3B no longer works — corrections must go through GSTR-1A or a later GSTR-1.

A mismatch in the other direction (GSTR-3B higher than GSTR-1) usually means you overpaid or did not report invoices in GSTR-1, which hurts your buyers because their ITC depends on your GSTR-1.

The 8 Most Common Causes

Wrong tax heads are common with inter-state sales. If you are unsure whether a sale is IGST or CGST+SGST, check it with our place of supply tool and read CGST vs SGST vs IGST.

How to Reconcile GSTR-1 and GSTR-3B: Step by Step

1. Pull both summaries for the same period. Download the GSTR-1 summary and the filed GSTR-3B (or the auto-populated draft) for each month of the year.
2. Compare tax head by tax head. Put IGST, CGST, SGST and cess side by side for taxable value and tax. Compare Table 3.1(a) of GSTR-3B with B2B + B2CL + B2CS + exports + credit/debit notes in GSTR-1.
3. Compare cumulatively, not just monthly. A gap in July that reverses in August is a timing difference. A gap that keeps growing is a real error.
4. Trace each difference to invoices. Match your sales register against GSTR-1 invoice by invoice to find what is missing, duplicated or in the wrong month.
5. Record the explanation. Keep a short note for every difference; you will need it for a DRC-01B reply or at GSTR-9 time.

If the gap is on the purchase side instead — your ITC in GSTR-3B does not match GSTR-2B — use our GSTR-2B reconciliation tool and the ITC mismatch resolver.

How to Fix the Mismatch

Before filing GSTR-3B: use GSTR-1A

GSTR-1A lets you add missed invoices or correct details in a GSTR-1 you have already filed, as long as you do it before filing GSTR-3B for that period. The corrected figures then flow into GSTR-3B automatically. This is the cleanest fix, so make reviewing the GSTR-3B draft a monthly habit.

After filing: correct in the next period

Neither return can be revised once filed. If GSTR-3B under-reported tax, add the missing liability in the next GSTR-3B and pay interest at 18% per year from the original due date — calculate it with the GST interest calculator. If GSTR-1 missed invoices, report them in the next GSTR-1 so your buyers get their ITC. Amendments to earlier invoices go in the amendment tables of GSTR-1.

The year-end deadline

Corrections for a financial year can be made until 30 November of the following year or the date of filing the annual return, whichever is earlier. After that, differences can only be shown and paid through GSTR-9 and DRC-03. Our GSTR-9 helper walks you through the annual reconciliation.

Got a DRC-01B? How to Respond

The intimation shows the difference by tax head. You normally have 7 days to respond in Part B of DRC-01B on the portal, choosing one of these:

• Paid the difference — pay through DRC-03 and quote the reference number.
• Explain the difference — for example, a timing difference already corrected in a later return, or an amendment. Attach your reconciliation.
• A combination — pay part and explain the rest.

Ignoring the intimation can block filing of your next GSTR-1 or IFF until you reply, which then delays your buyers' ITC and can cost you customers. If the officer does not accept your explanation, they may issue a show cause notice, so keep the explanation factual and backed by invoice-level data.

A Monthly Checklist to Prevent Mismatches

• Close your sales register before preparing GSTR-1; don't file from a half-updated book.
• Reconcile e-invoices on the IRP with your books and GSTR-1.
• File GSTR-1 first, then review the auto-populated GSTR-3B against your sales register.
• Use GSTR-1A for any correction found at this stage.
• Record credit notes in the same period in both returns.
• Run a cumulative GSTR-1 vs GSTR-3B comparison every quarter, not just at year end.

Avoid the related errors covered in GSTR-3B common mistakes, and track every deadline on the GST return calendar.

Frequently Asked Questions

Why do GSTR-1 and GSTR-3B not match?

Usually because invoices, credit notes or amendments were reported in one return but not the other, or in different periods.

What happens if I ignore a DRC-01B notice?

Filing of your next GSTR-1 or IFF can be blocked until you pay or reply, and the officer may start recovery proceedings.

Can GSTR-3B be revised?

No. Corrections are made in the next period's GSTR-3B, with interest on any tax paid late.

Is a small mismatch a problem?

Small timing differences that reverse in the next month are normal. Keep a note of them so you can explain them in your annual return.

Free Tools for Reconciliation

• GSTR-2B Reconciliation — match purchases with GSTR-2B
• ITC Mismatch Resolver — fix ITC differences
• GST Interest Calculator — interest on late-paid tax
• GSTR-9 Helper — annual return reconciliation
• Place of Supply Tool — IGST or CGST+SGST

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