If you are a GST-registered business in India, two forms stand between you and compliance every month: GSTR-1 and GSTR-3B. Miss them, and you face penalties. File them wrong, and you face scrutiny. But with the right billing setup, filing both forms can take under 30 minutes.
This guide explains what each form is, when it is due, what data you need, common mistakes, and how to file correctly in 2026.
What is GSTR-1?
GSTR-1 is a statement of your outward supplies — in plain language, a list of all the invoices you issued during the month. It is your "I sold these things to these buyers" form. Every B2B invoice you raised is reported here, along with total B2C sales aggregated by state.
Due date: 11th of the following month (for businesses with turnover above ₹5 crore). If your turnover is below ₹5 crore, you may file quarterly under the QRMP scheme, but you must still submit an Invoice Furnishing Facility (IFF) for B2B invoices in the first two months of the quarter.
What you need to file GSTR-1:
- All B2B invoices: invoice number, date, buyer's GSTIN, taxable value, GST rate, IGST/CGST/SGST amounts
- B2C sales: state-wise aggregated totals (individual invoices not required for B2C unless above ₹2.5 lakh interstate)
- Credit notes and debit notes issued
- Advances received (if applicable)
What is GSTR-3B?
GSTR-3B is a summary return — your monthly declaration of total output tax liability and total input tax credit (ITC). It is where you actually pay your GST. Think of it as saying: "I collected ₹X in GST from customers and I am claiming ₹Y in ITC from my purchases — here is the net ₹Z I owe."
Due date: 20th of the following month (for businesses above ₹5 crore); 22nd or 24th (state-wise) for others.
Step-by-Step: Filing GSTR-1
Step 1: Export your sales data from your billing software. In ZeroBillBook, go to Reports → GSTR-1. You will see a ready-to-file summary sorted by invoice type (B2B, B2C, Credit Notes, etc.).
Step 2: Log in to the GST portal (gstin.gov.in). Navigate to Returns → File Returns → GSTR-1.
Step 3: Add B2B invoices. Either upload the JSON file (exported from billing software) or enter manually. For most small businesses, the JSON upload is infinitely faster — it uploads hundreds of invoices in seconds.
Step 4: Add B2C details. Enter the state-wise breakup of your retail sales. ZeroBillBook's GSTR-1 report provides this automatically.
Step 5: Preview and submit. Review the summary screen. If everything looks correct, click Submit, then File with DSC or EVC (OTP-based verification).
Step-by-Step: Filing GSTR-3B
Step 1: Export your GSTR-3B summary from your billing software. This gives you: total outward taxable supplies, zero-rated supplies, ITC eligible from purchases, and net tax payable.
Step 2: On the GST portal, navigate to Returns → GSTR-3B.
Step 3: Fill in Table 3.1 (outward taxable supplies) and Table 4 (eligible ITC). These numbers should match your billing software report.
Step 4: The system auto-computes tax payable. Verify it against your billing software calculation. If there is a significant mismatch, check for un-exported invoices or purchase entries.
Step 5: Pay any tax due (if your ITC does not fully cover your output tax) and file.
Common Mistakes (and How to Avoid Them)
1. Mismatched GSTIN on invoices. Always verify buyer GSTINs before raising B2B invoices. An invalid GSTIN means the buyer cannot claim ITC — and they will call you angrily.
2. Wrong place of supply. If you sell goods/services across state lines, the place of supply determines IGST vs CGST+SGST. Getting this wrong is a common audit trigger.
3. Not reporting credit notes. Issued a credit note for a return or discount? It must be reported in GSTR-1. Missing credit notes cause GSTR-2A mismatches for your buyers.
4. Filing GSTR-3B without filing GSTR-1 first. While legally you can file them independently, it is best practice to file GSTR-1 before GSTR-3B so your ITC reconciliation is based on accurate data.
5. Using 2024 GST rates. As of September 2025, the rates changed. If you are still billing at 12% or 28%, your returns will show incorrect data and invite scrutiny. Update to 0%, 5%, 18%, or 40%.
The 30-Minute Monthly Ritual
With good billing software and a disciplined invoicing habit, filing GSTR-1 and GSTR-3B should take about 30 minutes each month:
- 5 minutes: Export GSTR-1 JSON from billing software
- 10 minutes: Upload and verify on GST portal, file GSTR-1
- 5 minutes: Export GSTR-3B summary
- 10 minutes: Fill GSTR-3B, verify ITC, pay and file
The alternative — scrambling through hand-written records on the 19th of every month — costs you a full afternoon and a lot of stress. The choice is yours.