GSTR-1 is the monthly or quarterly return in which you report every sale you made — your outward supplies. Your customers' input tax credit depends on it (it flows into their GSTR-2B), and your GSTR-3B liability is cross-checked against it. A small mistake in GSTR-1 can mean an angry customer who can't claim credit, or a notice from the department asking you to explain a mismatch.
This guide walks through what goes into GSTR-1, when it is due, and the errors we see most often when helping businesses in Tenkasi and across Tamil Nadu set up their billing.
Who files GSTR-1 and when
Every regular GST-registered taxpayer files GSTR-1. Composition dealers do not (they file CMP-08 and GSTR-4 instead).
| Filing type | Who | Due date |
|---|---|---|
| Monthly | Turnover above ₹5 crore, or those who opt for monthly filing | 11th of the following month |
| Quarterly (QRMP scheme) | Turnover up to ₹5 crore who opt for QRMP | 13th of the month after the quarter |
| IFF (optional, QRMP only) | QRMP filers who want B2B invoices to reach customers monthly | 13th of the following month, for the 1st and 2nd months of the quarter |
If you are on QRMP and sell to other businesses, use the Invoice Furnishing Facility (IFF). Your B2B customers can then claim credit every month instead of waiting for your quarterly return, which keeps them happy and paying on time.
What goes into GSTR-1
- B2B invoices (Table 4) — sales to registered customers, invoice by invoice, with the customer's GSTIN.
- B2C Large (Table 5) — inter-state sales to unregistered customers where the invoice value is above ₹1 lakh (this limit was reduced from ₹2.5 lakh in 2024).
- B2C Small (Table 7) — all other sales to unregistered customers, summarised by state and tax rate.
- Exports and SEZ supplies (Table 6).
- Credit and debit notes (Table 9B) — for returns, discounts and price changes after the invoice.
- Nil-rated, exempt and non-GST supplies (Table 8).
- Advances received and adjusted (Table 11).
- HSN-wise summary (Table 12) — now reported separately for B2B and B2C supplies.
- Documents issued (Table 13) — the invoice and credit note number series used, including cancelled ones.
Step-by-step: filing GSTR-1
- Close your books for the period. Make sure every sale, return and credit note is entered in your billing software before you start.
- Run your GSTR-1 report from your billing software. Good software groups invoices into B2B, B2CL, B2CS, CDNR and HSN exactly as the portal expects.
- Review the B2B list. Check every GSTIN, invoice number, date, taxable value and place of supply.
- Upload the data. Either export the JSON file from your software and upload it with the GST offline tool, or enter the details directly on the portal. If you generate e-invoices, those B2B invoices are auto-populated — do not upload them twice.
- Fill the HSN summary and documents issued tables.
- Generate the summary and compare the totals with your sales register. They should match to the rupee.
- File with DSC or EVC (OTP to the registered mobile).
The 8 most common GSTR-1 errors
1. Wrong customer GSTIN
A single wrong character sends your customer's credit to someone else. Validate GSTINs when you create the customer, not at month-end. Our GST billing software checks the GSTIN format as you type it.
2. Wrong place of supply (IGST vs CGST + SGST)
Charging CGST + SGST on an inter-state sale, or IGST on an intra-state one, is one of the most common and painful errors, because the tax paid under the wrong head has to be refunded and re-paid correctly. Let the software decide the tax type from the customer's state.
3. Registered customers reported as B2C
If a customer gives their GSTIN after the bill is printed, and it isn't added, the sale lands in B2CS and the customer loses credit.
4. Missing credit notes
Sales returns and discounts given after invoicing must be reported as credit notes. Skip them and you pay tax on sales that never happened.
5. HSN summary not matching invoices
The HSN summary must add up to the same taxable value and tax as your invoices. Keep HSN codes in the product master so every bill uses the right one — businesses with turnover up to ₹5 crore need 4-digit HSN codes on B2B invoices, and 6 digits above that.
6. Duplicate e-invoices
If you are on e-invoicing, B2B invoices reach GSTR-1 automatically from the IRP. Uploading them again from your software creates duplicates.
7. Wrong tax rate
Rate changes and mixed-rate items (for example, textiles priced above and below a threshold) are easy to get wrong manually. Store rates per product and update them in one place when the rules change.
8. GSTR-1 and GSTR-3B don't match
Liability in GSTR-3B should match what you reported in GSTR-1. Large differences trigger scrutiny. If you spot a mistake after filing GSTR-1 but before GSTR-3B, you can correct it in GSTR-1A for the same period.
How billing software prevents these errors
Almost every error above starts at the time of billing, not at the time of filing. When your billing software validates GSTINs, picks IGST or CGST + SGST automatically, stores HSN codes and rates per product, and produces a portal-ready GSTR-1 report, month-end filing becomes a 15-minute review instead of a two-day reconciliation.
That is exactly what our GST Billing Software is built to do — with e-Invoice, e-Way Bill, multi-GSTIN support and WhatsApp invoice sharing included.
