GST Invoice Rules in India: Complete 2026 Guide for Business Owners

Every GST-registered business in India is required by law (CGST Rules, Rule 46) to issue a proper tax invoice for every taxable supply of goods or services. Get this wrong and you face penalties starting at ₹10,000, ineligible input tax credit for your customers, and questions during your next GST audit.

This guide is the practical reference: GST invoice rules in India in 2026, the difference between CGST + SGST and IGST, when each applies, mandatory fields, HSN/SAC code requirements, e-invoicing thresholds, common mistakes that cost businesses lakhs, and the easiest way to generate compliant invoices without paying for software.

What is a GST invoice?

A GST invoice is a tax invoice issued by a GST-registered business in India for the supply of taxable goods or services. Under India's Goods and Services Tax law (CGST Rules 46), it must contain specific mandatory fields — supplier and recipient GSTINs, HSN/SAC codes, taxable value, GST rate, and the split of CGST + SGST (intra-state) or IGST (inter-state). The invoice is the legal record of the transaction and the basis on which your customer claims input tax credit (ITC) on the GST you charged.

If you're GST-registered and you don't issue a compliant invoice, three things go wrong: (1) you face a penalty of ₹10,000 or the tax amount, whichever is higher, (2) your customer can't claim ITC, which makes them less likely to buy from you again, and (3) you can't file accurate GSTR-1 returns, which compounds into more compliance problems.

CGST + SGST vs IGST: which do I charge?

India's GST has three components — Central GST, State GST, and Integrated GST — and which one(s) you charge depends entirely on whether the transaction is intra-state or inter-state.

Intra-state supply (supplier and buyer in the same state): charge CGST + SGST, split equally.

If the GST rate on your item is 18%, you charge:

• CGST: 9%
• SGST: 9%
• Total to customer: 18%

The CGST half goes to the central government; the SGST half goes to your state government.

Inter-state supply (supplier and buyer in different states): charge IGST at the full rate.

If the GST rate is 18%, you charge:

• IGST: 18%
• Total to customer: 18%

All of it goes to the central government, which then settles with the destination state.

How to determine intra-state vs inter-state: Look at the place of supply. For goods, it's where the goods are delivered. For services, it's usually the recipient's location (with some exceptions for events, immovable property, transportation). The first two digits of the buyer's GSTIN identify their state.

When your buyer is unregistered (B2C, no GSTIN), the place of supply for services is generally the supplier's location — so a Mumbai SaaS selling to an unregistered Bangalore consumer charges IGST 18% if it's inter-state, CGST+SGST 18% if intra-state. The rule is the same.

What GST rates apply?

India has five main GST rate slabs:

The exact rate depends on the HSN code (for goods) or SAC code (for services) of what you sell. Most professional services and SaaS fall under 18%. Confirm your specific rate via the official cbic-gst.gov.in portal or with your CA.

What is HSN / SAC and why do I need it on every invoice?

HSN (Harmonized System of Nomenclature) codes classify goods for GST purposes — 4 to 8 digits long. SAC (Services Accounting Code) codes do the same for services — 6 digits, all starting with 99.

India's GST law requires the HSN/SAC code on every invoice line item. The number of digits required depends on your aggregate turnover:

If you don't know your HSN/SAC code, ask your CA. Wrong codes lead to wrong GST rates, which lead to short-payment of tax — which leads to interest, penalties, and ineligible ITC for your customer.

Common SAC codes for service businesses:

Most SaaS businesses operate under 998314 (IT design and development) or 998313 (IT consulting).

What fields are mandatory on a GST invoice?

CGST Rules, Rule 46 specifies the mandatory fields. Missing any of these can invalidate the invoice:

1. Supplier's name, address, and GSTIN
2. Invoice number — consecutive, no gaps, max 16 characters, unique within a financial year
3. Date of issue
4. Recipient's name, address, and GSTIN (if registered)
5. HSN / SAC code for each line item
6. Description of goods or services
7. Quantity and unit for goods (e.g. "10 units", "5 kg")
8. Rate per unit and total taxable value
9. GST rate (e.g. 18%) applied to each item
10. CGST + SGST or IGST amounts split out clearly
11. Place of supply (state name)
12. Total invoice value in figures and words
13. Signature (digital signature certificate or physical) of supplier or authorised representative

Optional but strongly recommended:

• Reverse charge indicator (if applicable)
• PO / reference number from the buyer
• Payment terms ("Due within 30 days")
• Bank details for payment

This is a lot of detail to put on an invoice manually. That's why most businesses use software — or our free GST Invoice Generator that auto-fills all of these fields, calculates CGST + SGST or IGST automatically from seller and buyer state selection, supports multiple line items with per-item HSN/SAC codes and GST rates, and produces a clean printable invoice you can save as PDF. No watermark, no signup.

What is e-invoicing and when do I need it?

E-invoicing is a separate compliance layer on top of regular GST invoicing. It requires registering each B2B invoice on the government's Invoice Registration Portal (IRP) to get an Invoice Reference Number (IRN) and a signed QR code, which then must appear on the invoice.

Who must e-invoice as of 2026:

• Any business with aggregate turnover above ₹5 crore in any financial year since 2017-18
• Threshold has been progressively lowered (₹500 cr → ₹100 cr → ₹50 cr → ₹20 cr → ₹10 cr → ₹5 cr) and may drop further

Who is exempt from e-invoicing:

• Businesses below the threshold
• SEZ units
• Insurance companies
• Banking and financial institutions
• GTAs (Goods Transport Agencies)
• Passenger transport services
• Multiplex cinema operators

If you're under ₹5 crore turnover, you don't need e-invoicing — a regular GST invoice (like the one our free generator produces) is fully compliant.

What are the most common GST invoice mistakes?

These are the ones I see most often costing businesses real money:

1. Charging IGST when CGST + SGST was correct (or vice versa). Common when the buyer's state in your records doesn't match their actual GSTIN. Always derive intra/inter-state from the buyer's GSTIN, not from your CRM.

2. Wrong HSN/SAC code. Triggers wrong GST rate calculation. If you charge 12% on an 18%-rated service, you've under-collected tax. The shortfall comes out of your pocket plus interest plus penalty.

3. Sequential invoice numbers with gaps. Auditors look for this. If you have INV-001, INV-002, INV-005, INV-006 — where are INV-003 and INV-004? Sequential numbering without gaps is a requirement.

4. Forgetting "place of supply". Required field. Without it, the buyer can't claim ITC and the invoice is technically incomplete.

5. Missing the total amount in words. "₹1,18,000 (One Lakh Eighteen Thousand Rupees Only)" is mandatory. Lots of invoices have it in digits only.

6. Forgetting reverse charge marking. If reverse charge applies (specific notified services), the invoice must say so. Buyer pays GST instead of supplier.

7. Issuing invoices to B2C as if they were B2B. B2C invoices below ₹50,000 don't need the recipient's GSTIN (because the recipient isn't registered). But they still need a name and address.

8. Late issuance. Invoices must be issued within 30 days of supply for services, immediately for goods. Late invoices invite penalty.

When can I revise or cancel a GST invoice?

You can't really "edit" a GST invoice after issuing it. You can:

1. Cancel via credit note. If the invoice was issued incorrectly or the sale was reversed, issue a credit note referencing the original invoice. Credit notes reduce your output tax liability.

2. Supplement via debit note. If you under-charged on the original invoice, issue a debit note for the differential amount.

3. Re-issue with a new invoice number. For pre-shipment cancellations, void the original and issue a fresh invoice with a new number. Document the void clearly.

You CANNOT renumber, backdate, or quietly replace an issued invoice. Auditors will catch it.

How do I file GST returns from my invoices?

Your invoices feed three monthly/quarterly returns:

The data flows from invoices → GSTR-1 → buyer's GSTR-2B. If your invoice has wrong fields or wrong HSN, it propagates errors through the whole chain.

FAQ

Is a Tax Invoice the same as a GST Invoice?

Effectively yes. "Tax invoice" is the term used in the CGST Rules; "GST invoice" is the common business term. Both refer to the same document — a tax invoice issued under India's GST law. There's also a separate document called a "bill of supply" issued by composition-scheme dealers and for exempt supplies — that one doesn't show GST split because GST isn't charged.

Can I issue a GST invoice without a GSTIN?

No. A GST invoice requires a valid GSTIN from the issuer (you). If you're not GST-registered (turnover below threshold, currently ₹40 lakh for goods or ₹20 lakh for services in most states), you cannot issue a tax invoice — you issue a regular bill or receipt instead, with no GST charged.

What's the difference between B2B and B2C GST invoices?

B2B invoices need the recipient's GSTIN (so they can claim ITC). B2C invoices below ₹50,000 don't require the recipient's GSTIN (because the recipient isn't registered) but still need name and address. B2C invoices ₹50,000 and above DO need to show the place of supply prominently.

Do I charge GST on exports?

Exports are zero-rated under GST. You can either: (a) export with payment of IGST and claim refund later, or (b) export under a Letter of Undertaking (LUT) and don't pay IGST upfront. The invoice still needs all the standard fields but the IGST amount is zero. Get the LUT from your CA — it's an annual filing.

How many invoices can I issue per month?

No limit. The invoice number must just be unique within the financial year and consecutive (no gaps). Use a prefix like INV-FY25-26-NNNN to make sequence management easy.

Can I send a GST invoice digitally?

Yes. Email PDF is universally accepted. The invoice must have a digital signature (DSC) only if you're an e-invoicing-mandated business above the threshold. Below the threshold, an emailed PDF with the supplier's name + GSTIN is fully valid — no DSC required.

What happens if my customer doesn't pay?

Late payment doesn't affect your GST liability. You still owe the GST on the invoice in the month it was issued (since GST is on supply, not on payment). If the customer never pays, you can claim bad debt as a deduction for income tax purposes but the GST remains payable.

Do I need to keep paper copies?

No. Digital records are sufficient. CGST Rules require you to retain all invoices for 6 years from the end of the relevant financial year. Cloud storage is fine. Make sure backups are reliable.

TL;DR

• GST invoice is the mandatory tax document under CGST Rule 46 for every GST-registered supply
• CGST + SGST for intra-state, IGST for inter-state — derived from buyer's GSTIN, not your CRM
• HSN/SAC code mandatory per line — 4 digits if turnover < ₹5cr, 6 digits if above
• 13 mandatory fields per Rule 46 — miss any and the invoice is non-compliant
• E-invoicing mandatory above ₹5 crore turnover; not required below
• Use our free GST Invoice Generator — auto CGST+SGST/IGST calc, HSN codes, multiple line items, print-to-PDF, no watermark, no signup

This is informational, not legal advice. For edge cases (RCM, export with LUT, composition scheme, sector-specific exemptions) consult your CA. The free generator handles the 95% case for normal B2B and B2C invoices — it's what most small businesses actually need.