For most Indian manufacturers, GST compliance boils down to two monthly returns: GSTR-1 and GSTR-3B. Getting both right — on time and in sync with each other — is the foundation of GST compliance.

Here's exactly what each return covers, when it's due, and the common mistakes manufacturers make.

GSTR-1: Your Sales Register

GSTR-1 is a details return that reports all your outward supplies (sales) for the month or quarter. Think of it as your formal sales register submitted to the GST system.

What Goes Into GSTR-1?

GSTR-1 requires you to report:

Section	What It Covers
B2B invoices	All invoices to GST-registered buyers (with buyer GSTIN)
B2C inter-state invoices	Invoice-level or consolidated reporting per GST portal rules
B2C small invoices	Consolidated summary of small consumer sales
Exports	Sales to overseas buyers (with/without payment of tax)
Credit/debit notes	Adjustments to previously reported invoices
Advances received	Advances on which GST is payable
HSN summary	Sales summarised by HSN code

For e-invoice users (turnover above ₹5 crore): your B2B invoices auto-populate in GSTR-1 from the IRP. You still need to verify and file, but manual entry is eliminated for e-invoiced transactions.

When Is GSTR-1 Due?

Monthly filers (aggregate turnover above ₹5 crore): due by the 11th of the following month.

Quarterly filers on QRMP (aggregate turnover up to ₹5 crore): due by the 13th of the month after quarter end.

QRMP filers upload invoices monthly (IFF — Invoice Furnishing Facility) but file the full GSTR-1 quarterly.

Why GSTR-1 Matters for Your Buyers

When you file GSTR-1, your B2B invoice details appear in GSTR-2B — the auto-populated purchase summary for your buyers. Your buyers use GSTR-2B to claim Input Tax Credit (ITC).

If you don't file GSTR-1 or report an invoice incorrectly, your buyer's ITC is blocked. This is a relationship problem — buyers will stop doing business with suppliers who consistently cause ITC issues.

GSTR-3B: Your Tax Payment Return

GSTR-3B is a summary return where you declare your net tax liability and make the payment. Unlike GSTR-1 (which is invoice-level), GSTR-3B is a high-level summary.

What Goes Into GSTR-3B?

Table	What It Covers
3.1 — Outward supplies	Total taxable sales, exempt sales, zero-rated, and nil-rated
3.2 — Inter-state supplies	Sales to unregistered buyers in other states
4 — ITC (Eligible)	ITC claimed this period from purchases
5 — Exempt / non-GST inward supplies	Purchases that don't carry GST
6 — Tax payable	GST owed after offsetting ITC
6.1 — Tax paid	Payment breakup (IGST, CGST, SGST)

The key calculation in GSTR-3B: Output Tax (from your sales) minus Input Tax Credit (from your eligible purchases) equals Net Tax Payable.

You pay this net amount via your Electronic Cash Ledger on the GST portal.

When Is GSTR-3B Due?

Monthly filers (turnover above ₹5 crore): 20th of the following month. QRMP filers in Category I states: 22nd of the month after quarter end. QRMP filers in Category II states: 24th of the month after quarter end.

Interest at 18% per annum applies on unpaid tax from the original due date. This is not trivial — a ₹5 lakh unpaid tax for 3 months costs ₹22,500 in interest alone.

How GSTR-1 and GSTR-3B Are Connected

This is where many manufacturers get confused:

GSTR-1 reports what you invoiced. GSTR-3B reports what you owe.

They should match. The outward supply figures in GSTR-3B (table 3.1) should equal the total of invoices reported in GSTR-1.

If they don't match:

GST department notices the discrepancy
Your buyers' GSTR-2B may not fully reflect your GSTR-1 data
You may receive a scrutiny notice

Common mismatch scenario: A manufacturer files GSTR-3B and pays tax on ₹25 lakh of sales. But in GSTR-1, they only report ₹22 lakh because they forgot to upload 3 invoices. The ₹3 lakh gap will flag.

Monthly GST Compliance Calendar for Manufacturers

1st–10th: Compile all sales invoices for the previous month. Verify that e-invoice IRNs have been generated for all B2B invoices.

11th: File GSTR-1 (monthly filers). Ensure all B2B invoices are uploaded.

15th–18th: Calculate ITC from GSTR-2B (available by the 14th). Reconcile purchase invoices against GSTR-2B before claiming ITC.

20th: File GSTR-3B and pay net tax liability.

Year-end: File GSTR-9 (annual return) by December 31. File GSTR-9C as well if turnover exceeds ₹5 crore.

Common Mistakes Manufacturers Make

Mistake 1: Not Reconciling Invoices Before Filing GSTR-1

Many manufacturers file GSTR-1 with whatever invoices they have, then realise later they missed some. Amending GSTR-1 is possible but tedious. A monthly reconciliation — sales system vs. GSTR-1 data — prevents this.

Mistake 2: Claiming ITC Without Verifying GSTR-2B

You can only claim ITC that appears in your GSTR-2B. If a vendor hasn't filed their GSTR-1, their invoice won't appear in your GSTR-2B — and you can't claim that ITC. Vendors who don't file regularly cost you real money.

Mistake 3: Wrong HSN Codes

Manufacturers selling goods must report HSN code-wise summary in GSTR-1. Incorrect HSN codes cause classification disputes and can affect the tax rate applied. Confirm your HSN codes match your GST registration and the actual nature of goods.

Mistake 4: Forgetting Credit Notes

If you issued a credit note to a buyer (for returns, discounts, or corrections), it must be reported in GSTR-1. Missing credit notes overstate your outward supplies and cause GSTR-1 / GSTR-3B mismatches.

Mistake 5: Applying Advance Rules Without Checking the Supply Type

GST treatment differs between advances for goods and services. For most suppliers of goods, tax is generally paid at invoice or supply rather than on receipt of the advance, subject to applicable notifications and exceptions. Confirm unusual transactions with your GST practitioner.

How Manufacturing Software Simplifies GST Filing

The data needed for GSTR-1 and GSTR-3B comes from your invoices:

Invoice number, date, buyer GSTIN, item, HSN, value, tax rate

If your invoices are generated in a connected system (not manually in Excel or Word), the data flows automatically:

E-invoice generation pulls data from the sales order
GSTR-1 data is compiled automatically from generated invoices
GSTR-3B figures are derived from the same data

FactoStack's invoicing module generates GST invoices directly from dispatched orders, submits them to the IRP for e-invoicing, and provides a GSTR-1 ready report that your CA can upload to the GST portal with no manual re-entry.

GST Invoicing & E-Invoice

Generate GST invoices directly from dispatched orders, submit e-invoices to the IRP automatically, and get a GSTR-1 ready report for your CA.

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GSTR-1 vs GSTR-3B: What Manufacturers Need to File and When