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 registered supplier to a buyer that records the details of a taxable supply. It 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
Mandatory Fields in a GST Invoice
Under Rule 46 of the CGST Rules, every GST invoice must contain these 16 fields:
Supplier Details
- Supplier name — as registered on GST portal
- Supplier GSTIN — 15-digit GST Identification Number
- Supplier address — registered place of business
Invoice Details
- Invoice number — unique, sequential, max 16 characters (letters, numbers, hyphens, slashes only)
- Invoice date — date of issue (DD/MM/YYYY)
Buyer Details
- Buyer name
- Buyer GSTIN (if registered) — for B2B transactions, this is mandatory
- Buyer address — including state code and state name
- Place of supply — determines whether CGST+SGST or IGST applies
Item Details
- HSN/SAC code — Harmonized System of Nomenclature for goods / Services Accounting Code for services
- Description of goods/services
- Quantity and unit (for goods)
- Taxable value — total value before tax
- Tax rate — CGST, SGST, or IGST rate applicable
- Tax amount — broken down into CGST, SGST, IGST, and Cess (if any)
- 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?
This is the most common point of confusion:
- 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
The Place of Supply field determines which tax applies — not the shipping address. For services, place of supply rules vary by service type (refer to Section 12 and 13 of the IGST Act).
GST Invoice Number Format
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 format: INV/2026-27/001 or RS-2627-0001
Important: If you have multiple branches, each branch can maintain its own invoice series — e.g., KAR-001 for Karur, CHN-001 for Chennai.
Common GST Invoice Mistakes
These errors cause ITC rejections and audit issues:
- 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
GST Invoice Templates
You can create GST invoices using:
- 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
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.
E-Invoicing Rules (2026 Update)
E-invoicing is now mandatory for businesses with annual turnover above ₹5 crore. Key points:
- 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 threshold, start preparing now. Custom billing software can be integrated directly with the IRP API for automated e-invoice generation.
