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What Is a GST Invoice and When Is It Required?
A GST invoice (also called a tax invoice) is a document that a GST-registered supplier must issue to the buyer whenever goods or services are sold. It is the primary legal document under the Goods and Services Tax framework that enables the buyer to claim Input Tax Credit (ITC) — meaning the buyer can deduct the GST paid on purchases from their own GST liability.
Every GST-registered business with turnover above the registration threshold must issue a GST tax invoice for every taxable supply. Getting the invoice format right is not just good practice — it is a legal requirement. Incorrect invoices can lead to ITC rejection for the buyer, notices from the GST department, and penalties for the supplier.
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Input Tax Credit
Buyers can claim credit for GST paid only with a valid tax invoice. Wrong format = ITC denial.
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Legal Compliance
GST invoices must be issued within 30 days of supply (45 days for banks). Late invoicing attracts penalties.
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GST Return Filing
All invoices are uploaded to GSTR-1. E-invoices above ₹5 crore threshold generate IRN automatically on the GST portal.
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Audit Trail
Sequential invoice numbers create a verifiable audit trail required during GST assessments and tax audits.
Who must issue a GST invoice?
Any GST-registered taxpayer making a taxable supply must issue a GST tax invoice. This includes sole proprietors, partnership firms, LLPs, private limited companies, and public companies. The threshold for mandatory GST registration is ₹40 lakh annual turnover for goods (₹20 lakh for services, ₹10 lakh for special category states).
Composition scheme dealers cannot issue a tax invoice — they must issue a Bill of Supply instead, as they cannot charge GST from their customers.
Types of GST Invoices — Tax Invoice vs Bill of Supply
| Document Type | Who Issues | GST Charged? | ITC Available? |
|---|---|---|---|
| Tax Invoice | Regular GST registered supplier | Yes | Yes (to registered buyer) |
| Bill of Supply | Composition dealer / exempt supply | No | No |
| Debit Note | Supplier (to increase invoice value) | Yes | Yes |
| Credit Note | Supplier (to reduce invoice value / returns) | Yes (reduction) | Yes (reversed) |
| Receipt Voucher | Supplier (advance received) | Yes on advance | Yes |
| Delivery Challan | Supplier (goods sent without sale) | No | No |
CGST vs SGST vs IGST — How to Apply Correctly
The most critical decision on a GST invoice is which type of tax to apply. Getting this wrong is one of the most common and costly GST mistakes — it directly affects the buyer's ability to claim ITC and can attract notices from the GST department.
| Scenario | Tax to Apply | Rate Example (18% GST) |
|---|---|---|
| Seller and buyer in same state | CGST + SGST | 9% CGST + 9% SGST |
| Seller and buyer in different states | IGST | 18% IGST |
| Export of goods/services | Zero-rated (IGST 0%) | 0% with bond/LUT |
| Sale to SEZ unit | Zero-rated (IGST 0%) | 0% with bond/LUT |
| Import of goods | IGST + Customs | Applied at port of entry |
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The ToolLoom GST Invoice Generator auto-detects this for you. Select the seller state and buyer state — if they match, CGST+SGST is applied; if different, IGST is applied. Each at the correct split rate.
Mandatory Fields in a GST Invoice (Rule 46, CGST Rules)
Under Rule 46 of the CGST Rules 2017, a GST tax invoice must contain all of the following fields. Missing even one mandatory field can make the invoice invalid for ITC purposes:
- Supplier name, address, and GSTIN
- Invoice number — consecutive and unique for the financial year
- Invoice date
- Recipient name and address — GSTIN if registered, else name and state
- Place of supply — determines CGST/SGST vs IGST
- HSN code (goods) or SAC code (services) — mandatory above certain turnovers
- Description, quantity, and unit of measure
- Total taxable value after discount
- GST rate and tax amount — CGST+SGST or IGST separately
- Total invoice value in words and figures
- Signature of supplier or authorised representative
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For B2C supplies (sales to unregistered consumers) above ₹2.5 lakh, the buyer's name, address, and state are mandatory. For B2C below ₹2.5 lakh, the buyer's GSTIN is not required but name and address are recommended.
5 Common GST Invoice Mistakes to Avoid
Mistake 1 — Applying CGST+SGST for inter-state supply
✗ Seller in Maharashtra, buyer in Karnataka — charging 9% CGST + 9% SGST
✓ Inter-state supply must use 18% IGST — not CGST+SGST
This is the most common and consequential GST invoice error. If you charge CGST+SGST on an inter-state supply, the buyer cannot claim ITC as the tax has been deposited with the wrong government. The seller must cancel the invoice, issue a credit note, and reissue with IGST. This triggers reverse charge and compliance complications for both parties.
Mistake 2 — Non-sequential invoice numbering
✗ Invoice numbers: INV-001, INV-005, INV-003 (out of order)
✓ Invoice numbers must be strictly sequential: INV-001, INV-002, INV-003...
GST law requires invoice numbers to be consecutive and unique within a financial year. Gaps or non-sequential numbers raise red flags during GST audits and scrutiny. Start a fresh series each April 1. You can use alphanumeric formats (INV-2026-001) but they must be issued in strict order.
Mistake 3 — Wrong or missing HSN/SAC code
✗ Leaving HSN/SAC blank or using a generic code for all products
✓ Use the correct 4 or 6-digit HSN code specific to each product or service
HSN/SAC codes determine the correct GST rate and are mandatory for businesses above specified thresholds. Using wrong codes can lead to ITC denial, tax rate disputes, and penalties. Search for your product's HSN code on the GST Council's HSN code finder at gst.gov.in before issuing invoices.
Mistake 4 — Not issuing invoice within the time limit
✗ Issuing invoice 45 days after goods were delivered to the buyer
✓ Issue invoice at the time of supply or within 30 days (45 days for banks)
Late invoicing is a GST compliance violation. For goods, the invoice must be issued at or before delivery. For services, within 30 days of supply. Late invoices can lead to penalties under Section 122 of the CGST Act (₹10,000 or the amount of tax involved, whichever is higher) and ITC complications for the recipient.
Mistake 5 — Incorrect GSTIN on the invoice
✗ Typo in buyer's GSTIN or using old/cancelled GSTIN
✓ Always verify buyer's GSTIN on gst.gov.in before issuing invoice
An invoice with an incorrect buyer GSTIN means the buyer cannot claim ITC — their GSTIN won't match in GSTR-2B. Always verify the buyer's GSTIN is active using the GST portal's taxpayer search. A 15-character GSTIN has a specific format: 2-digit state code + 10-digit PAN + 1 entity code + 1 check digit — validate the format before issuing.
Frequently Asked Questions
A GST invoice is a document issued by a registered GST taxpayer to the buyer of goods or services. It must contain seller GSTIN, buyer details, HSN/SAC code, taxable value, GST rate, CGST+SGST (intra-state) or IGST (inter-state) amount, and total payable. It is the primary document for claiming Input Tax Credit (ITC).
CGST (Central GST) and SGST (State GST) apply to intra-state supply — when seller and buyer are in the same state. Each is half the total GST rate (18% GST = 9% CGST + 9% SGST). IGST (Integrated GST) applies to inter-state supply — when seller and buyer are in different states. IGST equals the full GST rate.
Any business with annual turnover above ₹40 lakh (₹20 lakh for services, ₹10 lakh for special category states) must register for GST and issue GST invoices. Below the threshold, businesses may issue a regular bill or invoice. Composition scheme dealers issue a Bill of Supply, not a tax invoice.
HSN (Harmonised System of Nomenclature) code classifies goods for GST. Businesses with turnover above ₹5 crore must use 6-digit HSN codes; ₹1.5–5 crore use 4-digit codes; below ₹1.5 crore are currently exempt. SAC (Services Accounting Code) is used for services instead of HSN. You can find HSN codes at gst.gov.in.
If you are not registered for GST, you cannot issue a GST tax invoice. You should issue a regular bill or invoice without mentioning GST. Charging GST without registration is illegal and punishable under the CGST Act. Register on the GST portal (gst.gov.in) if your turnover exceeds the threshold.
For goods: invoice must be issued at the time of removal or delivery. For services: within 30 days of supply. For banking and financial services: within 45 days. Late invoicing attracts penalties under Section 122 of the CGST Act — ₹10,000 or the amount of tax involved, whichever is higher.
GST invoice numbers must be sequential, unique, and not exceed 16 characters. They can include letters, numbers, hyphens, and slashes. The series resets each financial year (April 1). Example formats: INV-2026-001, 2026-27/001. Gaps in numbering are not allowed — if you skip a number, it must be explained during assessments.
E-invoicing is a system where B2B invoices are authenticated by the GST portal and assigned an Invoice Reference Number (IRN) and QR code. It is mandatory for businesses with annual turnover above ₹5 crore (as of 2024). E-invoices must be generated on the Invoice Registration Portal (IRP) at einvoice1.gst.gov.in before issuing to the buyer.
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📅 June 2026 · Written by the ToolLoom Team · Reviewed for accuracy June 2026
About ToolLoom: We build free tools for Indian students, professionals and creators. GST rules referenced from the CGST Act 2017 and GST Council notifications at gst.gov.in. Found an error? Email contact@toolloom.in
About ToolLoom: We build free tools for Indian students, professionals and creators. GST rules referenced from the CGST Act 2017 and GST Council notifications at gst.gov.in. Found an error? Email contact@toolloom.in