Complete GST Invoice Guide 2026: Everything Indian Businesses Need to Know

Creating GST-compliant invoices is mandatory for all registered businesses in India. This comprehensive guide covers everything you need to know about GST invoices, including format, mandatory fields, tax calculation rules, and how to automate the process.

What is a GST Invoice?

A GST invoice, also called a tax invoice, is a legal document issued by a GST-registered seller to a buyer. It serves as proof of sale and enables the buyer to claim Input Tax Credit (ITC). Under GST law, every registered business must issue a proper tax invoice for taxable supplies.

Mandatory Fields in a GST Invoice

As per Rule 46 of the CGST Rules, 2017, a GST invoice must contain the following mandatory fields:

1. Supplier Information

• Business Name: Your registered business name
• Address: Complete business address with pincode
• GSTIN: 15-digit GST Identification Number
• State Code: Two-digit state code (e.g., 27 for Maharashtra)

2. Customer/Recipient Information

• Name: Legal name of the buyer
• Address: Billing address
• GSTIN: Customer's GST number (if registered for B2B)
• State: Customer's state (determines CGST/SGST vs IGST)
• Place of Supply: Location where goods/services are delivered

3. Invoice Details

• Invoice Number: Unique sequential number (e.g., INV-001)
• Invoice Date: Date of issue
• Due Date: Payment due date (if applicable)

4. Item/Service Details

• Description: Name and description of goods/services
• HSN/SAC Code: 4 or 8-digit classification code
• Quantity: Number of units
• Unit Price: Rate per unit
• Total Amount: Quantity × Unit Price
• Discount: Any discount applied

5. Tax Information

• Taxable Value: Amount before tax
• GST Rate: Applicable tax rate (5%, 12%, 18%, 28%)
• CGST Amount: Central GST (for intra-state)
• SGST Amount: State GST (for intra-state)
• IGST Amount: Integrated GST (for inter-state)
• Total Tax Amount: Sum of all taxes

6. Final Amount

• Grand Total: Taxable value + Total tax
• Amount in Words: Total amount spelled out

Understanding CGST, SGST, and IGST

One of the most important aspects of GST invoicing is understanding when to apply CGST+SGST vs IGST:

Intra-State Transaction (Same State)

When both supplier and customer are in the same state, GST is split equally between:

• CGST: Central GST - Goes to the Central Government
• SGST: State GST - Goes to the State Government

Example: 18% GST = 9% CGST + 9% SGST

Inter-State Transaction (Different States)

When supplier and customer are in different states, only IGST is charged:

• IGST: Integrated GST - Goes to the Central Government, later distributed

Example: 18% GST = 18% IGST

GST Tax Slabs for Different Products

India has 5 GST tax slabs. Here's a breakdown:

HSN and SAC Codes Explained

HSN (Harmonized System of Nomenclature) and SAC (Services Accounting Code) are international classification systems:

HSN Codes (For Goods)

• 2-digit: Businesses with turnover < ₹5 crore
• 4-digit: Businesses with turnover ₹5-500 crore
• 6/8-digit: Businesses with turnover > ₹500 crore or exports

SAC Codes (For Services)

• Required: All B2B invoices for services
• Format: 6-digit code (e.g., 998314 for IT services)

Invoice Numbering Rules

Invoice numbers must be:

• Sequential: Continuous series without gaps (e.g., INV-001, INV-002, INV-003)
• Unique: No duplicate numbers within financial year
• Alphanumeric: Can include letters and numbers (e.g., MH-2026-001)
• Multiple Series: You can maintain separate series for different branches

Time Limit for Issuing GST Invoices

Invoices must be issued within specific time frames:

Supply Type	Time Limit
Supply of Goods	At or before removal of goods
Supply of Services	Within 30 days from supply date
Banking/Insurance/Financial Services	Within 45 days from supply date
Continuous Supply (12 months or less)	Within 30 days from due date

Common GST Invoicing Mistakes to Avoid

1. Wrong GSTIN

Double-check both your GSTIN and customer's GSTIN. Wrong GSTIN can cause ITC rejection for your customer.

2. Incorrect State Code

Using wrong state code will mess up your CGST/SGST vs IGST calculation, leading to GST mismatch.

3. Missing HSN/SAC Codes

Mandatory for B2B invoices. Absence can be flagged during GST return filing.

4. Calculation Errors

Always calculate discount before GST. Formula: (Subtotal - Discount) × GST Rate = Tax Amount

5. Non-Sequential Numbering

Gaps in invoice numbers are red flags during audits. Use automated systems to ensure continuity.

How to Create GST Invoices Automatically

Manual invoice creation is time-consuming and error-prone. Modern invoicing software like Billmator can automate the entire process:

Export Invoices (Zero-Rated Supplies)

For exports, special rules apply:

• GST Rate: 0% (zero-rated supply)
• IGST: 0% or paid under bond/LUT
• Additional Details: Shipping bill number, port code, export date
• Currency: Can be in foreign currency

Digital vs Paper Invoices

Both are legally valid under GST law. However, digital invoices have advantages:

• Instant delivery via email/WhatsApp
• Easy storage and retrieval
• Environment-friendly
• Reduced printing costs
• Better tracking and analytics

GST Invoice for Different Business Types

Freelancers & Consultants

Must register for GST if turnover exceeds ₹20 lakhs (₹10 lakhs for special category states). Services usually fall under 18% GST.

E-commerce Sellers

Must collect GST at source (TCS) through the marketplace. GSTIN mandatory regardless of turnover.

Restaurants

5% GST on restaurant services (no ITC). Different rates for AC vs non-AC restaurants may apply.

Retailers

B2C invoices can be simplified if value is less than ₹200. Can issue consolidated invoice at end of day.

Frequently Asked Questions

Q: Can I issue invoice without GSTIN?

No, only GST-registered businesses can issue tax invoices. Unregistered businesses can issue bills of supply (without GST).

Q: What if I discover an error in issued invoice?

Issue a debit note (for undercharged amount) or credit note (for overcharged amount) referencing the original invoice.

Q: How long should I keep invoice copies?

Minimum 72 months (6 years) from the due date of filing annual return for that year.

Q: Can I edit or cancel an invoice?

Once uploaded to GSTR-1, you cannot edit. You must issue a credit note. Cancellation requires proper documentation.

Conclusion

Creating GST-compliant invoices is crucial for every registered business in India. While the rules may seem complex, using the right invoicing software can automate 90% of the work - from calculating taxes to generating sequential numbers to creating professional PDFs.

Billmator makes GST invoicing effortless with voice input, automatic tax calculation, and instant PDF generation. Download the app today and create your first invoice in just 2 minutes!