GST Invoice Format: Mandatory Fields and Rules 2026

What Is a GST Invoice?

A GST invoice, or tax invoice, is the document a registered supplier issues to a buyer for the supply of goods or services. Under Section 31 of the CGST Act, 2017, issuing a tax invoice is mandatory for every taxable supply made by a registered person. The invoice records the value of supply, the rate and amount of tax charged, and the details of both parties. Crucially, it is the primary document on which your buyer relies to claim Input Tax Credit under Section 16, so accuracy is non-negotiable.

When Must a Tax Invoice Be Issued?

The timing rules differ for goods and services:

• For goods: The invoice must be issued on or before the removal or delivery of goods to the recipient.
• For services: The invoice must be issued within 30 days from the date of supply of service. For banks, NBFCs, and insurers, the limit extends to 45 days.
• For continuous supply of goods or services: The invoice is tied to successive statements of accounts or payments.

Mandatory Fields in a GST Invoice

Rule 46 of the CGST Rules prescribes the particulars that every tax invoice must contain. Missing even one of these fields can render the invoice defective. The mandatory GST invoice fields are:

Supplier and Recipient Details

1. Name, address, and GSTIN of the supplier.
2. A consecutive serial number — unique for a financial year, not exceeding 16 characters, containing alphabets, numerals, special characters such as hyphen or slash, or any combination.
3. Date of issue of the invoice.
4. Name, address, and GSTIN or UIN of the recipient, if the recipient is registered.
5. For unregistered recipients where the taxable value is Rs 50,000 or more, the invoice must also show the name and address of the recipient, the address of delivery, and the name of the State along with its code.

Supply and Tax Details

6. HSN code for goods or SAC for services. The number of HSN digits required depends on turnover (discussed below).
7. Description of goods or services.
8. Quantity (for goods) and unit or Unique Quantity Code.
9. Total value of supply.
10. Taxable value of supply after adjusting any discount.
11. Rate of tax — separately for CGST, SGST/UTGST, IGST, and cess.
12. Amount of tax charged under each head.
13. Place of supply along with the State name, in the case of inter-State supply.
14. Address of delivery where it differs from the place of supply.
15. Whether tax is payable on a reverse charge basis.
16. Signature or digital signature of the supplier or an authorised representative.

HSN Code Reporting Requirements

The number of HSN digits to be mentioned depends on the aggregate annual turnover in the preceding financial year:

• Turnover up to Rs 5 crore: 4-digit HSN code (optional for B2C supplies in some cases).
• Turnover above Rs 5 crore: 6-digit HSN code.

Getting the HSN right matters because the GSTN portal increasingly validates HSN against tax rates during return filing.

Types of Invoices and Related Documents

Bill of Supply

A composition dealer under Section 10, or a supplier dealing exclusively in exempt goods or services, cannot charge GST and therefore issues a Bill of Supply instead of a tax invoice. A composition dealer must also display the words "composition taxable person, not eligible to collect tax on supplies" on the bill.

Receipt Voucher, Refund Voucher and Payment Voucher

When a supplier receives an advance, a receipt voucher must be issued. If the supply does not materialise and no invoice is issued, a refund voucher is given. A payment voucher is issued by a recipient who pays tax under reverse charge.

Debit Notes and Credit Notes

Under Section 34, a credit note is issued when the taxable value or tax charged in an invoice was excessive, or for sales returns. A debit note is issued when the value or tax was understated. These must reference the original invoice and are reported in GSTR-1.

E-Invoicing Rules in 2026

E-invoicing requires notified businesses to generate invoices on their own systems and report them to the Invoice Registration Portal (IRP), which returns an Invoice Reference Number (IRN) and a QR code. As of 2026, e-invoicing applies to businesses with aggregate turnover exceeding Rs 5 crore in any financial year since 2017-18. The QR code and IRN become part of the legally valid invoice.

A key procedural point: there is a time limit for reporting invoices to the IRP. Taxpayers above a specified turnover threshold must report invoices within a defined window (commonly 30 days) from the invoice date, failing which the IRP rejects the IRN generation. Always confirm the current threshold and reporting window applicable to your turnover bracket before relying on it.

E-Way Bill Linkage

For movement of goods exceeding Rs 50,000 in value, an e-way bill is also required. The invoice details feed directly into the e-way bill, so consistency between the two documents is important to avoid detention during transit.

Number of Invoice Copies

Rule 48 prescribes the number of copies:

• For supply of goods: three copies — original for the recipient, duplicate for the transporter, and triplicate for the supplier.
• For supply of services: two copies — original for the recipient and duplicate for the supplier.

With e-invoicing, the QR code requirement has largely replaced the need for physical triplicate handling, but the underlying rule still applies for non-e-invoice supplies.

Special Cases

Export Invoices

An export invoice must carry the endorsement "Supply meant for export on payment of integrated tax" or "Supply meant for export under bond or letter of undertaking without payment of integrated tax," as applicable. It should also mention details such as the recipient's country and the conversion rate where relevant.

Reverse Charge Supplies

Where GST is payable under reverse charge, the invoice must clearly state that the tax is payable on reverse charge basis, and the recipient discharges the tax through a self-invoice if buying from an unregistered supplier.

Common Mistakes to Avoid

Even experienced businesses slip up on invoicing. Watch out for these errors:

1. Non-sequential or duplicate serial numbers. The invoice number must be consecutive and unique for the financial year. Resetting the series mid-year without a clear logic, or repeating numbers, invites scrutiny.
2. Wrong place of supply. Mixing up CGST/SGST with IGST because of an incorrectly determined place of supply is one of the most expensive mistakes. It can lead to paying tax twice and a cumbersome refund process.
3. Incorrect or missing HSN/SAC codes. Using a code that does not match the applicable rate can trigger mismatches and ITC denial for buyers.
4. Charging GST on a Bill of Supply. Composition dealers and exempt-only suppliers cannot collect tax; doing so attracts penalties.
5. Issuing invoices beyond the time limit. Late invoices for services beyond 30 days can disturb the time of supply and create interest liability.
6. Missing the reverse charge mention. Omitting the reverse charge declaration confuses the recipient about who bears the tax.
7. Not generating an IRN where e-invoicing applies. An invoice without a valid IRN is not a legal document for notified taxpayers, and the buyer cannot claim ITC on it.
8. GSTIN errors. A single wrong digit in the recipient's GSTIN can block their credit. Always validate the GSTIN before issuing.
9. Rounding inconsistencies. Tax amounts should be computed accurately; arbitrary rounding can cause reconciliation mismatches in GSTR-2B.

Best Practices for Compliant Invoicing

Use GST-compliant billing software that auto-validates GSTINs, generates IRNs where applicable, and maintains a clean serial sequence. Reconcile your outward invoices in GSTR-1 against your books every month, and ensure your customers can see your invoices in their GSTR-2B so their ITC flows smoothly. Retain all invoice records for the prescribed period — generally six years from the due date of the annual return for the relevant year. A disciplined invoicing process protects both your compliance rating and your customer relationships.