GST Invoice Format Requirements 2026 — Complete Guide
The Goods and Services Tax (GST), introduced in India on 1 July 2017, replaced a complex web of central and state taxes with a unified indirect tax system. Every GST-registered business supplying goods or services must issue a tax invoice — a legally mandated document that both triggers the obligation to collect GST and allows the buyer to claim Input Tax Credit (ITC). Getting the invoice format wrong can lead to ITC rejection, penalties, and compliance issues during audits.
Mandatory Fields on a GST Tax Invoice
Under Rule 46 of the CGST Rules, 2017, every tax invoice must contain the following 16 mandatory fields:
- Supplier details: Legal name, GSTIN, and registered address of the supplier
- Recipient details: Name, address, and GSTIN of the recipient (for B2B transactions)
- Invoice number: A unique, consecutive serial number (can include letters, numbers, and hyphens — no special characters)
- Invoice date: Date of issue of the tax invoice
- Place of supply: State where goods/services are delivered (determines CGST+SGST or IGST)
- HSN/SAC codes: Mandatory based on annual turnover thresholds (see below)
- Description, quantity, and unit: Clear description of goods or services with measurement unit
- Taxable value: Value of supply before GST
- Tax rate and amount: Applicable GST rate, CGST/SGST/IGST amount separately
- Total amount: Gross amount payable (taxable value + GST)
- Signature: Digital or handwritten signature of the authorised person
Understanding GSTIN — 15-Character Structure Explained
The GST Identification Number (GSTIN) is a unique 15-digit alphanumeric code assigned to every GST-registered taxpayer. Understanding its structure is essential for validating business partners and avoiding fraudulent invoices:
- Digits 1-2 (State Code): Two-digit state code per the Census of India 2011. Maharashtra is 27, Delhi is 07, Karnataka is 29, Tamil Nadu is 33, and so on. Values range from 01 (Jammu & Kashmir) to 37 (Andhra Pradesh).
- Digits 3-12 (PAN Number): The taxpayer's 10-digit Permanent Account Number. The PAN structure itself encodes information — the 4th letter indicates entity type (P=Person, C=Company, H=HUF, F=Firm, A=AOP, B=BOI, G=Government, J=AJP, L=Local authority, T=Trust).
- Digit 13 (Entity Number): Represents the number of registrations the same PAN holder has in the same state. Default is 1 for the first registration. If a company has multiple GSTINs in the same state (for different verticals), digits go up to 9 and then A-Z.
- Digit 14: Always "Z" by default in current GSTIN assignments.
- Digit 15 (Checksum): A single character (digit or letter) computed algorithmically to detect input errors. Our GSTIN validator runs this check in real time.
CGST vs SGST vs IGST — When Each Applies
GST in India is a dual structure — both the Central Government and State Governments levy taxes simultaneously on the same transaction:
- Intra-State Supply (CGST + SGST): When the seller and buyer are in the same state, the GST is split equally between Centre and State. On an 18% GST transaction, 9% goes as CGST to the Centre and 9% as SGST to the state government. The buyer can claim ITC of both CGST and SGST.
- Inter-State Supply (IGST): When the supply crosses state lines, the full GST is charged as IGST (Integrated GST) at the combined rate. IGST is collected by the Centre and then apportioned to the destination state. For an 18% GST inter-state sale, 18% IGST is charged — not split.
- Import of Goods: IGST applies to all imports, levied as if it were an inter-state supply. Customs duty is separate.
HSN and SAC Codes — The Classification System
India adopted the Harmonized System of Nomenclature (HSN) for goods and Services Accounting Code (SAC) for services — both globally standardized classification systems. The number of HSN digits required depends on your annual aggregate turnover:
- Turnover ≤ ₹1.5 Crore: HSN optional for B2C; required 2-digit code for B2B
- ₹1.5 Crore to ₹5 Crore: 4-digit HSN code mandatory
- Above ₹5 Crore: 6-digit HSN code mandatory on all B2B invoices
- Export invoices: 8-digit HSN code required for customs clearance
Common SAC codes for services: IT software services = 998314; Business consulting = 998311; Website development = 998315; Legal services = 998212; Accounting services = 998222. SAC codes always have 6 digits.
E-Invoicing Mandate — Who Must Comply
The Government of India introduced mandatory e-invoicing for large businesses, progressively expanding the mandate. As of October 2023, e-invoicing applies to all B2B transactions for businesses with annual aggregate turnover above ₹5 crore in any preceding financial year from 2017-18 onwards.
Under e-invoicing, you must report every invoice to the Invoice Registration Portal (IRP) before sharing it with the buyer. The IRP validates the invoice, generates an IRN (Invoice Reference Number), and embeds a QR code. Only after IRN generation is the invoice legally valid. Businesses not under the e-invoicing mandate can use standard invoice formats like this generator.
B2B vs B2C Invoice Rules
GST invoice rules differ based on whether you are selling to another registered business (B2B) or an end consumer (B2C):
- B2B invoices: Must include buyer's GSTIN. The buyer needs the tax invoice to claim Input Tax Credit (ITC). All details must match exactly what the buyer has registered — mismatches cause ITC rejection in GSTR-2B.
- B2C invoices (small): For transactions under ₹2.5 lakh, a simplified invoice without buyer's name/address is acceptable. Recipient's GSTIN is not required.
- B2C invoices (large): For B2C transactions above ₹2.5 lakh in intra-state supply or any amount in inter-state supply, the buyer's full name, address, and state must be on the invoice.
Invoice Numbering Rules Under GST
Invoice numbers must be unique and consecutive within a financial year. The invoice series resets each April 1. You can use alphanumeric characters and hyphens in the invoice number (e.g., INV-2026-001, GST/26-27/001, VH-20260001). The series cannot have blank gaps — if you cancel invoice number 5, the next invoice should be number 6, and invoice 5 should be marked cancelled with a Credit Note. The GSTN portal may flag non-sequential invoice series during GSTR-1 reconciliation.
Amount in Words — Why It Matters
Writing the invoice total "in words" (e.g., "Rupees Fifty Thousand Four Hundred Eighteen Only") is a traditional accounting practice that remains best practice on GST invoices, though not strictly mandated by the CGST Act. It prevents fraud — a figure of ₹5,000 is easy to alter to ₹50,000, but altering both the numeral and words simultaneously is much harder. Our generator auto-converts your grand total to Indian words including lakhs and crores.
Frequently Asked Questions
What details are mandatory on a GST tax invoice?
A valid GST tax invoice must include: supplier's legal name, GSTIN, and address; buyer's name and address (and GSTIN if registered); a unique invoice number; date of issue; HSN/SAC codes for each item; description, quantity, and unit of measurement; taxable value; applicable GST rates (CGST+SGST for intra-state, IGST for inter-state); total tax charged; and total amount payable. For B2C invoices above ₹2.5 lakh, the recipient's full details are also required.
How do I validate a GSTIN (GST Identification Number)?
A GSTIN is always 15 characters: the first 2 digits represent the state code (01-37), characters 3-12 form the taxpayer's PAN (5 uppercase letters + 4 digits + 1 letter), character 13 is the entity number (1-9 or A-Z for multiple registrations in the same state), character 14 is always "Z", and character 15 is a checksum digit. Our validator checks all these criteria in real time as you type.
When do I charge CGST+SGST versus IGST?
CGST (Central GST) and SGST (State GST) apply when the supplier and recipient are in the same state — called intra-state supply. IGST (Integrated GST) applies when supply crosses state borders — inter-state supply. The total GST rate is the same (e.g., 18%), but in intra-state transactions it is split 9% CGST + 9% SGST, while in inter-state transactions the full 18% is IGST. Our tool auto-detects the correct type by comparing seller and buyer states.
What is an HSN code and is it mandatory?
HSN (Harmonized System of Nomenclature) codes classify goods, while SAC (Services Accounting Code) classifies services. As of April 2021, businesses with annual turnover above ₹5 crore must use 6-digit HSN codes; those between ₹1.5 crore and ₹5 crore must use 4-digit HSN codes; businesses below ₹1.5 crore (but GST registered) must use 2-digit HSN codes. Composition taxpayers are exempt from HSN requirements.
Can I issue a GST invoice if I am not GST registered?
No. Only GST-registered businesses can issue a "Tax Invoice" and collect GST from customers. If you are not registered (e.g., your annual turnover is below ₹20 lakh for services, ₹40 lakh for goods), you must issue a "Bill of Supply" instead and cannot charge GST separately. However, if you voluntarily register for GST, you can issue tax invoices regardless of your turnover.
What is the GST registration threshold in India?
The mandatory GST registration threshold for most states is ₹40 lakh annual aggregate turnover for goods and ₹20 lakh for services. For special category states (North-Eastern states, Uttarakhand, Himachal Pradesh, J&K), the threshold is ₹20 lakh for goods and ₹10 lakh for services. Some businesses must register regardless of turnover: e-commerce operators, inter-state suppliers, and casual taxable persons.
What is the e-invoicing mandate and does it apply to me?
E-invoicing (electronic invoicing) is mandatory for B2B transactions for businesses with annual aggregate turnover above ₹5 crore (as of August 2023). Under e-invoicing, each invoice must be reported to the Invoice Registration Portal (IRP) before being issued, and an IRN (Invoice Reference Number) and QR code are generated. Our generator creates standard-format invoices but does not integrate with IRP — businesses under the e-invoicing mandate must use certified software.
How do I calculate GST on a composite supply?
A composite supply involves two or more supplies where one is the principal supply (e.g., catering includes food and serving). The GST rate of the principal supply applies to the entire composite supply. For mixed supplies (where no single item is dominant), the highest GST rate among all components applies. Mixed and composite supplies cannot be split for separate GST billing.
What is reverse charge mechanism (RCM) under GST?
Under Reverse Charge Mechanism (RCM), the liability to pay GST shifts from the supplier to the recipient. RCM applies in specific situations: purchase of goods/services from unregistered suppliers (if the transaction crosses ₹5,000/day), import of services, and specific notified categories (legal services from advocates, goods transport agencies, etc.). When RCM applies, the recipient must issue a self-invoice and pay GST directly to the government.
Can I modify or cancel a GST invoice after issuing it?
You cannot simply delete or overwrite a GST invoice — doing so would create discrepancies in GSTR-1 and GSTR-2A. Instead, issue a Credit Note to partially or fully reverse a tax invoice. A Credit Note reduces your tax liability and the buyer's input tax credit. For billing errors that require upward revision, issue a Debit Note. Both Credit Notes and Debit Notes must be reflected in your GSTR-1 filing.