GSTR-1 and GSTR-3B Filing Guide for Small Businesses (2026)

A plain-English walkthrough of monthly GST returns. What to file, when to file, and how to avoid late fees.

GST Returns in India — the basics

If your business is GST registered, you must file returns monthly (or quarterly for small taxpayers). Missing the deadline costs ₹50/day per return in late fees.

Two main returns for most businesses

GSTR-1 — Sales return

Reports all outward supplies (sales) you made during the month.

**Monthly filers**: Due 11th of next month

**Quarterly filers** (QRMP): Due 13th of month after quarter

GSTR-3B — Summary return + payment

A summary of sales, purchases, and GST liability. You also pay any GST due here.

**Monthly filers**: Due 20th of next month

**Quarterly filers**: Due 22nd/24th depending on state

What goes in GSTR-1

You must report:

**B2B invoices** — all invoices issued to GST-registered buyers

**B2C invoices** — sales to unregistered buyers (consolidated if <₹2.5 lakh each)

**Export invoices** — with or without payment of IGST

**Credit notes/debit notes** — amendments to previous invoices

What goes in GSTR-3B

Total sales (taxable + exempt + zero-rated)

Total purchases (input tax credit claimed)

Net GST payable = Output GST − Input Tax Credit

Payment of net GST due

Input Tax Credit (ITC) — your biggest saving

ITC lets you claim back the GST you paid on your purchases. Example:

You bought raw materials for ₹1,00,000 + ₹18,000 GST

You sold finished goods for ₹1,50,000 + ₹27,000 GST

Net GST due = ₹27,000 − ₹18,000 = **₹9,000**

OwnStore automatically tracks your input GST from purchase orders and calculates your net liability.

Common mistakes to avoid

**Missing the 11th deadline for GSTR-1** — blocks your buyers from claiming ITC

**Wrong GSTIN on invoices** — invalid invoices get rejected

**Claiming ITC without GSTR-2B match** — ITC can only be claimed for purchases that appear in GSTR-2B

**Not filing nil returns** — even if you had no transactions, you must file

How OwnStore helps with GST

Auto-calculates CGST/SGST/IGST on every invoice

Generates GSTR-1 data ready for upload to GST portal

Tracks input tax credit from purchase orders

Sends reminders before filing deadlines

Try OwnStore GST features free →