If you run a business in India and are registered under GST, every invoice you issue must follow a specific format defined by the government. Get it wrong, and your customers cannot claim input tax credit (ITC). Get it right, and you stay compliant, build trust, and avoid notices from the tax department.
This guide covers everything you need to know about GST invoice format requirements in 2026 — from the mandatory fields on every invoice to the rules around CGST, SGST, IGST, HSN codes, and e-invoicing.
What Is a GST Invoice?
A GST invoice (officially called a Tax Invoice) is a document issued by a registered supplier when selling goods or services. It serves three purposes:
- Legal proof of sale between the supplier and buyer
- Basis for tax collection — it shows the exact GST amount charged
- Enables ITC claims — the buyer uses this invoice to claim input tax credit on their GST returns
Without a valid GST invoice, the buyer cannot claim ITC, which makes the transaction more expensive for them. This is why getting the format right matters.
Who Needs to Issue a GST Invoice?
Every business registered under GST must issue a tax invoice for:
- Sale of goods worth more than Rs. 200
- All supply of services (no minimum threshold)
- Inter-state sales of any amount
- Sales to registered persons (B2B)
Exception: If your aggregate turnover is below Rs. 40 lakhs (Rs. 20 lakhs for services and special category states) and you are not registered, you issue a regular bill instead of a tax invoice.
Bill of Supply vs. Tax Invoice
Not every document is a tax invoice. There are two types:
| Document | When to Use | Tax Shown? |
|---|---|---|
| Tax Invoice | GST-registered, taxable supply | Yes (CGST/SGST or IGST) |
| Bill of Supply | Composition scheme dealers, exempt supplies | No tax breakup |
Hisaabo automatically selects the correct document type based on your GST registration status and the nature of the supply.
Mandatory Fields in a GST Invoice
Under Rule 46 of the CGST Rules, every GST tax invoice must contain these fields:
Supplier Details
- Name, address, and GSTIN of the supplier (your business)
- State name and state code of the supplier
- Invoice number — unique, sequential, not exceeding 16 characters, may contain alphabets, numbers, hyphens, and slashes
Invoice Details
- Date of issue
- Invoice number (unique serial number for the financial year)
- Due date (optional but recommended)
Recipient Details
- Name, address, and GSTIN of the recipient (if registered)
- State name and state code of the recipient
- For B2C invoices above Rs. 2.5 lakhs: name, address, and state code of the recipient
Item Details (Per Line Item)
- Description of goods or services
- HSN code (for goods) or SAC code (for services)
- Quantity and unit
- Rate per unit (excluding tax)
- Discount (if any)
- Taxable value (after discount)
Tax Details
- Tax rate (CGST + SGST rate or IGST rate)
- Tax amount (CGST + SGST amount or IGST amount)
- Total invoice value (taxable value + tax)
Other
- Place of supply
- Whether tax is payable on reverse charge basis
- Signature or digital signature of the supplier (or authorized representative)
CGST/SGST vs. IGST: Which Tax to Charge?
This is one of the most common areas of confusion. The rule is straightforward:
Intra-State Supply (Same State)
When your business and the buyer are in the same state, charge CGST + SGST.
Example: Your business is in Maharashtra, buyer is in Maharashtra.
- Item value: Rs. 10,000
- GST rate: 18%
- CGST (9%): Rs. 900
- SGST (9%): Rs. 900
- Total: Rs. 11,800
Inter-State Supply (Different States)
When your business and the buyer are in different states, charge IGST.
Example: Your business is in Maharashtra, buyer is in Karnataka.
- Item value: Rs. 10,000
- GST rate: 18%
- IGST (18%): Rs. 1,800
- Total: Rs. 11,800
How Hisaabo Handles This
Hisaabo compares your business’s state (from your profile) with the party’s state (from their profile). The correct tax type is applied automatically on every invoice. You never need to make this decision manually.
HSN Code Requirements
HSN (Harmonized System of Nomenclature) codes classify goods for GST purposes. SAC (Services Accounting Code) codes do the same for services.
When Are HSN Codes Mandatory?
| Annual Turnover | HSN Code Digits Required |
|---|---|
| Up to Rs. 5 crore | 4-digit HSN code |
| Above Rs. 5 crore | 6-digit HSN code |
As of 2026, all invoices must include HSN/SAC codes regardless of turnover. The number of digits required depends on your turnover.
Finding the Right HSN Code
Every product has a specific HSN code. For example:
- Rice: 1006
- Cotton fabrics: 5208
- Mobile phones: 8517
- Software development services: 998314
Store HSN codes at the item level in your billing software. This way, they appear automatically on every invoice without manual lookup each time.
In Hisaabo, you set the HSN/SAC code once when creating an item, and it flows through to every invoice and GST report.
GST Invoice Number Rules
Your invoice number must follow these rules:
- Unique for each financial year
- Sequential — no gaps in the series (gaps attract scrutiny)
- Maximum 16 characters including letters, numbers, hyphens (-), and slashes (/)
- One or more series per financial year (you can have separate series for different types)
Common Formats
INV-2627-0001(prefix + financial year + serial)SI/001(simple prefix + serial)MH-2627/0001(state code + financial year + serial)
Hisaabo generates invoice numbers atomically — using PostgreSQL transactions to ensure no two invoices ever get the same number, even under concurrent use. You configure the prefix and starting number; Hisaabo handles the rest.
E-Invoicing: Who Needs It?
E-invoicing is the process of generating invoices through the government’s Invoice Registration Portal (IRP) and obtaining an Invoice Reference Number (IRN).
Current Thresholds (2026)
| Aggregate Turnover | E-Invoicing Required? |
|---|---|
| Above Rs. 5 crore | Yes, mandatory |
| Below Rs. 5 crore | Not yet required |
The government has been progressively lowering this threshold. Businesses should prepare for e-invoicing even if they are currently below the limit.
What E-Invoicing Means for You
- Generate your invoice in a structured JSON format (as per the GST schema)
- Upload it to the IRP
- Receive an IRN and QR code
- Include the IRN and QR on your invoice PDF
Hisaabo structures all invoice data in a format compatible with e-invoicing requirements. When the threshold applies to your business, the transition is minimal.
Indian Financial Year Considerations
India follows an April-to-March financial year for GST purposes. This affects:
- Invoice numbering: Series should reset (or continue) at the start of each financial year (April 1)
- GSTR-1 filing: Monthly or quarterly returns within the financial year
- HSN summary: Annual HSN-wise summary in GSTR-9
Hisaabo respects the April-March financial year in all reports, numbering, and dashboards. You can also configure a custom financial year start if your business requires it.
GSTR-1 and Your Invoices
GSTR-1 is the return where you report all outward supplies (sales). Every invoice you issue must be reported here, categorized as:
- B2B invoices: Listed individually with GSTIN, invoice number, taxable value, and tax
- B2C large: Inter-state B2C invoices above Rs. 2.5 lakhs, listed with place of supply
- B2C small: All other B2C invoices, summarized state-wise
- HSN summary: Total value and tax grouped by HSN code and tax rate
If your invoices have all the mandatory fields, generating GSTR-1 is a matter of organizing the data. Hisaabo generates GSTR-1 and GSTR-3B reports directly from your invoices, already segmented into the correct categories.
Common GST Invoice Mistakes
Here are the errors that most frequently cause problems during audits or ITC claims:
- Wrong GSTIN: Typos in the buyer’s GSTIN make the invoice invalid for ITC
- Missing HSN codes: Invoices without HSN codes are technically non-compliant
- CGST/SGST on inter-state sales: Charging the wrong tax type is a common error
- Invoice number gaps: Unexplained gaps in invoice series attract scrutiny
- Missing place of supply: Required for determining the correct tax type
- Incorrect tax rate: Each HSN code maps to a specific GST rate — using the wrong rate affects the buyer’s ITC
- No reverse charge indication: If reverse charge applies, it must be stated on the invoice
Most of these errors happen because businesses manually manage their invoicing. Using software that enforces these rules eliminates the risk.
Simplify GST Invoicing with Hisaabo
Hisaabo is free, open-source billing software built specifically for Indian businesses. Here is how it handles everything covered in this guide:
- Automatic CGST/SGST vs. IGST based on party state
- HSN/SAC codes stored per item — appear on every invoice automatically
- Atomic invoice numbering — no gaps, no duplicates
- Tax Invoice or Bill of Supply selected automatically
- GSTR-1 and GSTR-3B reports generated from your invoices
- Indian financial year (April-March) built in
- E-invoicing ready data structure
You do not need a PhD in GST to issue compliant invoices. You need software that handles the complexity for you.