GST Invoice Format 2026 — Rules, Templates & Free Generator

Quick answer: A valid GST invoice in India must include 16 mandatory fields under Rule 46 of the CGST Rules, use a unique sequential invoice number (max 16 characters), charge CGST+SGST for intra-state or IGST for inter-state supply, and — if your annual turnover exceeds ₹5 crore — be registered on the Invoice Registration Portal (e-invoicing) before sending to the buyer. Missing any field can cause Input Tax Credit rejection and audit penalties.

Every GST-registered business in India must issue invoices in a specific format defined by the Central Board of Indirect Taxes and Customs (CBIC). Getting the format wrong can lead to rejected Input Tax Credit (ITC) claims, penalties during audits, and unnecessary disputes with suppliers and buyers.

This guide covers the complete GST invoice format for 2026 — mandatory fields, format rules, templates, and how billing software can automate compliance so you never worry about invoice errors again.

What is a GST Invoice?

A GST invoice is a legal document issued by a GST-registered supplier to a buyer, recording the details of a taxable supply of goods or services. It is mandatory for every business registered under GST.

A GST invoice serves three critical purposes:

• Tax compliance — proof of tax collected and paid to the government
• Input Tax Credit — the buyer uses your invoice to claim ITC on their GST returns
• Legal record — serves as evidence in case of disputes, audits, or assessments

What Are the Mandatory Fields in a GST Invoice?

A GST invoice must contain 16 mandatory fields as per Rule 46 of the CGST Rules: supplier name, GSTIN, and address; unique sequential invoice number and date; buyer name, GSTIN, and address; place of supply; HSN/SAC code; description of goods or services; quantity and unit; taxable value; tax rate; tax amount (CGST/SGST/IGST/Cess); total invoice value; reverse charge status; and the supplier's signature.

Per Rule 46 of the CGST Rules, every GST invoice must contain these 16 fields, grouped below:

Supplier Details

1. Supplier name — as registered on GST portal
2. Supplier GSTIN — 15-digit GST Identification Number
3. Supplier address — registered place of business

Invoice Details

1. Invoice number — unique, sequential, max 16 characters (letters, numbers, hyphens, slashes only)
2. Invoice date — date of issue (DD/MM/YYYY)

Buyer Details

1. Buyer name
2. Buyer GSTIN (if registered) — for B2B transactions, this is mandatory
3. Buyer address — including state code and state name
4. Place of supply — determines whether CGST+SGST or IGST applies

Item Details

1. HSN/SAC code — Harmonized System of Nomenclature for goods / Services Accounting Code for services
2. Description of goods/services
3. Quantity and unit (for goods)
4. Taxable value — total value before tax
5. Tax rate — CGST, SGST, or IGST rate applicable
6. Tax amount — broken down into CGST, SGST, IGST, and Cess (if any)
7. Total invoice value — taxable value + all taxes

Additional Required Information

• Reverse charge — mention if tax is payable under reverse charge mechanism
• Signature — digital or physical signature of the supplier (or authorized representative)

CGST + SGST vs IGST — When to Use Which?

Charge CGST + SGST (typically 9% + 9% = 18%) when the supplier and buyer are in the same state. Charge IGST (18%) when they are in different states. Exports are zero-rated or IGST with refund under a Letter of Undertaking. The Place of Supply field — not the shipping address — determines which tax applies.

• Intra-state supply (seller and buyer in same state) → charge CGST + SGST (e.g., 9% CGST + 9% SGST = 18% total)
• Inter-state supply (seller and buyer in different states) → charge IGST (e.g., 18% IGST)
• Exports → IGST at applicable rate (refundable) or zero-rated with Letter of Undertaking

For services, place of supply rules vary by service type (refer to Section 12 and 13 of the IGST Act).

What Is the Correct GST Invoice Number Format?

A valid GST invoice number must be unique per financial year, sequential with no gaps, maximum 16 characters, and contain only letters (A–Z, a–z), numbers (0–9), hyphens (-), and slashes (/). Example formats: INV/2026-27/001 or RS-2627-0001.

The invoice number must be:

• Unique for each financial year
• Sequential (no gaps allowed)
• Maximum 16 characters
• Can contain only letters (A–Z, a–z), numbers (0–9), hyphens (-), and slashes (/)

Common formats: INV/2026-27/001 or RS-2627-0001

Multi-branch businesses: Each branch can maintain its own invoice series — e.g., KAR-001 for Karur, CHN-001 for Chennai — provided each series remains sequential and unique within the financial year.

What Are the Most Common GST Invoice Mistakes?

The six most common GST invoice mistakes that trigger Input Tax Credit rejection are: wrong buyer GSTIN, missing HSN/SAC codes, incorrect place of supply, non-sequential invoice numbers, missing reverse charge notation, and rounding tax on the total instead of per line item. Any one of these errors can invalidate the buyer's ITC claim and flag your account for audit.

• Wrong GSTIN — even one digit wrong means the buyer cannot claim ITC
• Missing HSN/SAC codes — mandatory for businesses with turnover above ₹5 crore (4-digit) or ₹1.5 crore (2-digit)
• Incorrect place of supply — leads to wrong tax type (CGST+SGST vs IGST)
• Non-sequential invoice numbers — gaps in sequence trigger audit flags
• Missing reverse charge notation — must be explicitly mentioned when applicable
• Rounding errors — tax should be calculated per line item, not on the total

Where Can I Get a Free GST Invoice Template?

You can get GST-compliant invoice templates for free from Excel/Google Sheets templates (best for under 20 invoices/month), Word documents (more branding control), or billing software (essential for 50+ invoices/month). Billing software auto-calculates taxes, maintains sequential numbering, and exports directly to GSTR-1.

• Excel / Google Sheets — download a free template, add your details, and save as PDF. Good for businesses issuing fewer than 20 invoices per month
• Word — more control over layout and branding, but harder to maintain sequential numbering
• Billing software — auto-generates compliant invoices, tracks numbering, calculates taxes, and exports for GST filing. Essential for 50+ invoices/month

When to Use Billing Software vs Manual Invoicing?

Use billing software instead of manual invoicing when you issue more than 30 invoices per month, sell across multiple states, use multiple HSN/SAC codes, need payment tracking, or want automatic GSTR-1/GSTR-3B data. Manual invoicing (Excel/Word) works fine for businesses under 20 invoices per month with a single tax type.

If you answer “yes” to any of these, you need billing software:

• You issue more than 30 invoices per month
• You sell both intra-state and inter-state
• You have multiple HSN/SAC codes
• You need to track payments and outstanding amounts
• You want automatic GST return data (GSTR-1, GSTR-3B)

Our Invoice Generator product handles all of this — GST-compliant invoicing with auto-fill, PDF export, and tax summary reports. For custom billing software tailored to your business workflow, explore our GST billing software service.

What Are the E-Invoicing Rules for 2026?

E-invoicing is mandatory in India for businesses with annual turnover above ₹5 crore (since 1 October 2023). Every B2B invoice must be registered on the Invoice Registration Portal (IRP), which generates a unique Invoice Reference Number (IRN) and QR code before the invoice is sent to the buyer. E-invoices auto-populate GSTR-1, and non-compliance invalidates the invoice so the buyer cannot claim Input Tax Credit.

• Invoices must be registered on the Invoice Registration Portal (IRP) before being sent to the buyer
• The IRP generates a unique Invoice Reference Number (IRN) and QR code
• E-invoices are automatically populated in GSTR-1 — no manual filing needed
• Non-compliance results in the invoice being treated as invalid — buyer cannot claim ITC

If your turnover is approaching the ₹5 crore threshold, start preparing now. Custom billing software can be integrated directly with the IRP API for automated e-invoice generation.