A GST invoice is not just a bill — it's a legally binding document that determines whether your buyer can claim Input Tax Credit, whether you'll survive a GST audit, and whether you're actually compliant with the law.
I've reviewed invoices from dozens of Indian businesses and consistently find the same mistakes — missing fields, wrong state codes, incorrect invoice numbering. These aren't just technicalities. A defective invoice can cost your buyer their entire ITC claim and trigger a notice for you.
Here's everything you need to know about what goes on a proper GST invoice in 2025.
The Legal Basis: Section 31 and Rule 46
The requirement to issue a tax invoice is governed by Section 31 of the CGST Act, 2017. The specific format and content requirements are detailed in Rule 46 of the CGST Rules, 2017.
Understanding that these requirements are law — not suggestions — is important. Failure to issue a proper invoice attracts penalties under Section 122 of the CGST Act: ₹10,000 or the amount of tax evaded, whichever is higher.
When Must You Issue a GST Invoice?
| Situation | Invoice Required? | |-----------|------------------| | B2B taxable supply | Yes — always | | B2C supply above ₹2,00,000 (interstate) | Yes | | B2C supply below ₹2,00,000 (interstate) | Consolidated at day end allowed | | B2C intrastate any amount | Consolidated at day end allowed | | Export supply | Yes — with "Supply meant for export" declaration | | Exempt supply | Bill of Supply (not tax invoice) | | Composition dealer | Bill of Supply (not tax invoice) |
Time limits for issuing invoices:
- Goods: Before or at the time of removal/delivery
- Services: Within 30 days of supply (45 days for banking and insurance)
All Mandatory Fields Under Rule 46
Here is every field that must appear on a valid GST tax invoice:
Supplier Information
- Full name of the supplier
- Complete address with PIN code
- GSTIN of the supplier (15-character format)
Invoice Identification
- Invoice serial number — must be unique for each financial year, maximum 16 characters, consecutive
- Date of invoice
Recipient Information (B2B Invoices)
- Name of the recipient
- Address of the recipient
- GSTIN or UIN of the recipient (mandatory for registered buyers claiming ITC)
- State name and state code (2-digit code: 27 for Maharashtra, 07 for Delhi, etc.)
Item Details (For Each Line Item)
- Description of goods or services — must be specific enough to identify the item
- HSN Code for goods (see table below for required digits)
- SAC Code for services (6 digits always required)
- Quantity and unit of measurement
- Total value before discount
- Taxable value after discount (this is the amount GST is calculated on)
Tax Information
- Applicable GST rate (%)
- CGST amount (for intrastate transactions)
- SGST/UTGST amount (for intrastate transactions)
- IGST amount (for interstate transactions)
- Total amount of tax charged
Additional Mandatory Fields
- Place of supply — the state where the supply is deemed to occur
- Whether the supply attracts Reverse Charge (mandatory field — write "No" if not applicable)
- Signature or digital signature of the supplier or authorized representative
HSN Code Requirements Based on Turnover
| Annual Aggregate Turnover | HSN Digits Required | |--------------------------|---------------------| | Up to ₹5 crore | 4-digit HSN | | Above ₹5 crore | 6-digit HSN | | Exporters | 8-digit HSN | | Services (all taxpayers) | 6-digit SAC |
Not sure which HSN code applies to your product? Use our HSN Code Finder to search by product name.
Invoice Serial Number Rules — Often Got Wrong
The invoice serial number must be:
- Unique within each financial year (April 1 to March 31) — you can reuse the same number in the next FY
- Consecutive — you cannot skip numbers arbitrarily
- Maximum 16 characters including slashes and hyphens
- Can include alphabets, numbers,
/and-only
Valid examples:
INV/2025-26/001GST-2526-0001FIRM/JAN/2026/001
Invalid examples:
INV 001(contains space)INVOICE#001(contains #)
B2B vs B2C Invoice — What Changes?
| Requirement | B2B Invoice | B2C Invoice | |------------|-------------|-------------| | Buyer GSTIN | Mandatory | Not applicable | | State code of buyer | Mandatory | Not required | | Place of supply | Mandatory | Mandatory | | HSN code | Mandatory (digits per turnover) | Mandatory | | Invoice number | Must be unique | Must be unique | | ITC claimable by buyer | Yes | No |
E-Invoice Requirement (2025 Update)
From April 1, 2025, if your annual aggregate turnover exceeds ₹10 crore, you must generate e-invoices through the Invoice Registration Portal (IRP). Key points:
- The IRP generates a unique IRN (Invoice Reference Number) — a 64-character hash
- A signed QR code is returned and must be printed on the invoice
- E-invoices must be reported to IRP within 30 days of the invoice date
- Without a valid IRN, B2B invoices are considered invalid for ITC purposes
Common Costly Mistakes on GST Invoices
Mistake 1: Missing reverse charge declaration
Even if reverse charge doesn't apply, you must write "Whether tax is payable on reverse charge: No." Leaving this blank is a technical defect.
Mistake 2: Wrong place of supply for services
For services, place of supply is usually the location of the recipient — not where the service is physically provided. Getting this wrong means wrong CGST+SGST vs IGST determination.
Mistake 3: Consolidated description ("Miscellaneous items")
Tax authorities can reject ITC if item descriptions are vague. Be specific: "Software development services for e-commerce platform" rather than "Services."
Mistake 4: Not issuing invoice within time limit
For services, if you issue an invoice more than 30 days after the supply date, the invoice date should still reflect the actual supply date — and you may need to pay late interest.
Mistake 5: Using last year's invoice numbers in the new FY
Invoice series must restart on April 1 each year. Many businesses forget this and continue old series.
Generate Compliant Invoices Free
Creating a fully compliant GST invoice manually is error-prone, especially when you need to track HSN codes, state-wise tax components, and invoice series. Our GST Invoice Generator handles all of this automatically:
- Auto-determines CGST+SGST vs IGST based on states
- Validates GSTIN format
- Maintains invoice number sequence
- Generates PDF download instantly
- 5 invoices per month free — no signup required
Conclusion
A GST invoice is your legal protection as a supplier and your buyer's gateway to ITC. Missing even one mandatory field — especially buyer GSTIN, HSN code, or place of supply — can result in real financial loss.
The good news: once you have a properly designed template, you just fill in the details each time. Use our free invoice generator to ensure every invoice you create is compliant from day one.
This article covers GST invoice requirements as per CGST Act 2017 and CGST Rules 2017, updated for FY 2025-26. Consult a CA for specific compliance advice.