Complete Guide to GST Invoicing for Small Businesses

If you run a business in India with annual turnover above ₹40 lakh (₹20 lakh for services in special category states), you are required to register for GST and issue GST-compliant invoices. Sounds simple enough, until you encounter the alphabet soup of CGST, SGST, IGST, HSN, SAC, IRN, and a dozen other abbreviations.

This guide breaks it all down in plain language, with practical examples. Whether you are a freelance designer, a retail shop owner, or an e-commerce seller, this article will give you everything you need to invoice correctly and avoid GST notices.

GST Basics: What You Need to Know

GST (Goods and Services Tax) replaced a patchwork of central and state taxes in 2017. It is a destination-based tax, meaning the tax goes to the state where the goods or services are consumed, not where they are produced.

Since the GST 2.0 reform (22 September 2025) there are four main GST slabs: 0%, 5%, 18%, and 40%. Most services fall under 18%. Essential goods are at 5% or exempt. Luxury and sin goods are at 40%. The earlier 12% and 28% slabs were removed.

CGST, SGST, and IGST Explained

This is where most confusion starts. Here is the simple rule:

• Intra-state supply (seller and buyer are in the same state): The GST is split equally between CGST (Central GST) and SGST (State GST). So an 18% GST item shows as 9% CGST + 9% SGST on the invoice.
• Inter-state supply (seller and buyer are in different states): The full GST is charged as IGST (Integrated GST). So the same 18% GST item shows as 18% IGST.
• UTGST: If the buyer is in a Union Territory (Delhi, Chandigarh, etc.), UTGST replaces SGST. The rate is the same.

Example: A software company in Bangalore bills a client in Bangalore. The invoice for ₹1,00,000 shows: Taxable Amount ₹1,00,000 + CGST @9% ₹9,000 + SGST @9% ₹9,000 = Total ₹1,18,000.

The same company billing a client in Mumbai would show: Taxable Amount ₹1,00,000 + IGST @18% ₹18,000 = Total ₹1,18,000. The total is the same; only the tax component split changes.

Mandatory Fields on a GST Invoice

A GST-compliant tax invoice must contain the following fields. Missing any of these can lead to the buyer's input tax credit (ITC) being denied, which will make them very unhappy.

• Invoice number: Unique, sequential, up to 16 characters. Format must be consistent (e.g., INV-2026-0001).
• Invoice date: The date of issue.
• Supplier name, address, and GSTIN: Your business details.
• Buyer name, address, and GSTIN: Required for B2B transactions. For B2C transactions under ₹2.5 lakh, GSTIN is not mandatory.
• HSN/SAC code: For goods, use HSN (Harmonized System of Nomenclature) codes. For services, use SAC (Services Accounting Code). Businesses with turnover above ₹5 crore must use 6-digit codes.
• Description of goods/services: Clear description of what is being sold.
• Quantity and unit: For goods. Not required for services.
• Taxable value: The amount before GST.
• Tax rate and amount: CGST + SGST or IGST, shown separately.
• Place of supply: The state code where the goods/services are delivered.
• Total amount: Taxable value + tax.
• Signature: Digital or physical signature of the supplier or authorised representative.

HSN and SAC Codes: How to Find Yours

HSN codes classify goods. SAC codes classify services. You need to mention the correct code on every invoice. Here are some common ones:

• 9983: Professional, technical, and business services (consulting, legal, accounting)
• 998314: IT and software development services
• 9988: Manufacturing services on physical inputs
• 6109: T-shirts, singlets, tank tops (knitted)
• 8471: Laptops and computers
• 0402: Milk and cream (concentrated or sweetened)

To find your HSN/SAC code, search the CBIC (Central Board of Indirect Taxes and Customs) portal or use IndiaCRM's built-in HSN/SAC lookup, which lets you search by keyword and auto-fills the code on your invoice.

E-Invoicing: Who Needs It and How It Works

E-invoicing is mandatory for businesses with aggregate turnover exceeding ₹5 crore (as of 2025-26). The process works like this:

• You generate the invoice in your billing software (like IndiaCRM).
• The software sends the invoice data to the Invoice Registration Portal (IRP) operated by NIC.
• The IRP validates the data, generates a unique Invoice Reference Number (IRN), digitally signs the invoice, and returns it with a QR code.
• The QR code and IRN must appear on the final invoice shared with the buyer.

IndiaCRM handles this entire flow automatically. You click "Create Invoice," and the system handles the NIC API call, IRN generation, and QR code placement in the background.

Common GST Invoicing Mistakes

These errors are surprisingly common and can lead to notices, penalties, or your buyer losing their ITC claim:

• Wrong GSTIN: A single digit error in the buyer's GSTIN means the invoice will not match their GSTR-2B, and they cannot claim ITC. Always validate GSTIN before invoicing.
• CGST+SGST on inter-state supply: If your buyer is in a different state, you must charge IGST, not CGST+SGST. This is the most common tax calculation error.
• Missing HSN/SAC codes: Mandatory for businesses above ₹5 crore turnover at 6-digit level. Even smaller businesses should include 4-digit codes to avoid scrutiny.
• Incorrect Place of Supply: For services, the Place of Supply is usually the buyer's location. Getting this wrong changes the tax type (IGST vs CGST+SGST).
• Non-sequential invoice numbers: GST rules require sequential numbering. Gaps in your invoice series (jumping from INV-100 to INV-105) can trigger questions during audit.
• Missing reverse charge notation: If the transaction falls under reverse charge mechanism (RCM), the invoice must clearly state "Tax payable under reverse charge."
• Rounding errors: GST amounts should be rounded to the nearest rupee at the invoice level, not at the line-item level. Many manual calculations get this wrong.

How IndiaCRM Automates GST Invoicing

Manual invoicing in Excel or Word is error-prone and time-consuming. Here is what IndiaCRM automates:

• GSTIN validation: Enter a GSTIN and IndiaCRM validates it against the government portal, auto-filling the business name and address.
• Auto tax calculation: Based on the buyer's state, IndiaCRM automatically applies CGST+SGST or IGST. You never have to think about which tax type to use.
• HSN/SAC auto-fill: Your product/service catalog stores HSN/SAC codes. When you add a line item, the code and tax rate are filled automatically.
• Sequential numbering: Invoice numbers are generated automatically with no gaps. You can configure the format (e.g., INV/2026-27/0001).
• E-invoicing: One-click IRN generation via NIC API with QR code placement on the PDF.
• GSTR-1 export: Download your monthly invoice data in the format required for GSTR-1 filing. Hand it to your CA or upload directly to the GST portal.
• Payment tracking: Mark invoices as paid (full or partial), send automated payment reminders via WhatsApp, and generate receipts.

Getting Started

GST invoicing does not have to be stressful. With the right software, you can go from "create invoice" to "sent to client on WhatsApp" in under 60 seconds, fully compliant, professionally formatted, and with a payment link attached.

IndiaCRM includes unlimited GST invoicing in its free, plan. No per-invoice charges, no add-on fees. Create your free account, add your GSTIN, and send your first invoice today.