Monthly GST filing checklist: a day-by-day routine from invoices to GSTR-3B

The simplest way to stay compliant is a fixed monthly routine: close sales and purchase records in the first week, file GSTR-1 (or IFF) by the 11th or 13th, act on IMS and check GSTR-2B after the 14th, pay and file GSTR-3B by the 20th (or the quarterly date under QRMP), then save copies of everything. Most small businesses that get into trouble with GST do not lack software; they lack a routine and a person responsible for each step. This guide sets out the routine day by day.

It is part of the GST calculation worked examples section, and the monthly GST filing checklist tool gives you a tickable version.

Who does what

Even a two-person business should assign roles:

Role	Responsibility
Preparer (staff or bookkeeper)	Enters invoices, collects supplier bills, runs reports
Reviewer (owner or CA)	Checks reconciliation and the draft returns
Filer (owner, or professional with owner's OTP)	Files with EVC/DSC, makes payments
Keeper	Saves returns, challans, reconciliations and backups

One person can hold several roles, but the person who files should look at the reconciliation before filing.

Days 1–7: close the previous month

Enter all sales invoices, credit notes and debit notes. For each B2B invoice check the customer's GSTIN, place of supply (decides CGST + SGST or IGST) and HSN/SAC.
For e-invoicing businesses, check that every B2B invoice has an IRN and that none is approaching the 30-day limit (for AATO of ₹10 crore and above).
Record advances received for services, which can create a liability before the invoice.
Enter all purchase bills, including small expenses like courier and telecom that carry ITC.
Record reverse charge items: goods transport agency, advocate's services, imports of services, notified purchases from unregistered persons.
Match bank receipts with sales to catch unbilled sales.

Documents to have ready: sales register, purchase register, credit/debit note register, e-way bill list, bank statement.

Days 8–11 (or 13): file GSTR-1 or IFF

Run the GSTR-1 report from your software and review: total B2B vs B2C, inter-state B2C above the B2CL threshold, exports, nil/exempt supplies, the HSN summary (Table 12) and documents issued (Table 13).
Compare the GSTR-1 total with the sales register total for the month.
Monthly filers: file GSTR-1 by the 11th.
QRMP filers: upload B2B invoices in IFF by the 13th for months 1 and 2; file quarterly GSTR-1 by the 13th after the quarter.
If you spot an error after filing, plan a GSTR-1A amendment now. Since 2025 the liability in GSTR-3B is locked to GSTR-1/1A, so GSTR-1A must come before GSTR-3B.

Days 12–16: IMS and GSTR-2B

Open IMS and review inward invoices as suppliers upload them. Reject invoices that are not yours (wrong GSTIN, never received), keep pending those you are still checking, and accept the rest.
After GSTR-2B is generated around the 14th, download it and reconcile with your purchase register, as in ITC reconciliation with GSTR-2B.
Remove blocked credits under section 17(5) and reverse ITC on supplier bills unpaid for over 180 days.
Email suppliers whose invoices are missing.

Days 17–20 (or 22/24): pay and file GSTR-3B

Review the GSTR-3B draft: liability auto-populated from GSTR-1/1A, ITC from GSTR-2B, reverse charge and any reversals.
Check the set-off order and the cash needed; see GST calculation worked examples.
Create a PMT-06 challan and pay by net banking, UPI, card or NEFT/RTGS a day early, so that payment reflects in the cash ledger before filing.
File GSTR-3B by the 20th (monthly) or the 22nd/24th after the quarter (QRMP). File with EVC (OTP) or DSC.
QRMP months 1 and 2: pay PMT-06 by the 25th (fixed sum or self-assessment).

Days 21–31: records and review

Save PDF copies of filed GSTR-1, GSTR-3B, challans, GSTR-2B and your reconciliation outside the software.
Back up your accounting data, and for companies confirm the audit trail is enabled.
Log any late fee or interest paid and why, so the cause can be fixed.
Note open items: suppliers not filed, customers' disputes, ITC to reclaim.
Once a quarter, check the portal notices tab and email for intimations (DRC-01B, DRC-01C, ASMT-10). See GST notices and how to reply.

Calendar view for one month (monthly filer, illustrative)

Day	Task	Owner
1–7	Close sales, purchases, RCM, advances	Preparer
8–10	Review and file GSTR-1	Reviewer → Filer
11	GSTR-1 due date	—
12–16	IMS actions, GSTR-2B reconciliation, supplier follow-up	Preparer → Reviewer
17–18	GSTR-3B draft review, challan	Reviewer
19	Pay and file GSTR-3B	Filer
20	GSTR-3B due date	—
21–31	Save records, backup, open items	Keeper

Year-end additions

In the last months of the financial year add: checking that all credit notes for the year are issued (and reported), confirming ITC still to be claimed before 30 November, preparing for GSTR-9 if turnover exceeds ₹2 crore (see the GSTR-9 guide), opting in or out of QRMP or composition for the next year, and renewing software before it lapses near a due date.

Frequently asked questions

Can I file GSTR-3B before GSTR-1?

For monthly filers, file GSTR-1 first: GSTR-3B's liability is auto-populated from it and locked. Filing GSTR-1 late also delays your buyers' credit.

What if the portal is down on the due date?

Do not wait for the last day. If there is a widespread outage, the government sometimes extends the date by notification, but you cannot assume it.

How long should I keep GST records?

Section 36 of the CGST Act requires accounts and records to be kept for 72 months from the due date of the annual return for the year, and longer if proceedings are pending.

Should I file a nil return if the business was closed for the month?

Yes, as long as the registration is active. If the business has closed permanently, apply for cancellation instead.

Sources

GST portal — returns dashboard, IMS, PMT-06, nil filing by SMS
CBIC GST — CGST Act sections 16, 36, 37 and 39; CGST Rules on returns