GST Invoice Rules: Mandatory Fields, Series, Time Limits and Formats
- A GST invoice must contain 16 mandatory fields; missing even one can make your customer's ITC claim invalid and expose you to penalties.
- The time limit to issue a tax invoice is generally before or at the time of supply for goods, and within 30 days of service completion for most service providers.
- Invoice series must be unique, consecutive, and reset every financial year — random or duplicate numbering is a compliance red flag.
If you run a GST-registered business, your invoice is not just a payment request — it is a legal document. Get it wrong, and your buyer loses Input Tax Credit (ITC), the GST department can deny your refunds, and you face penalties under Section 122 of the CGST Act. This guide covers every mandatory field, the correct invoice series format, time limits for issuance, and the format rules you must follow in FY 2026-27 — with a worked example so you can see exactly how it all fits together.
16 Mandatory Fields on a GST Tax Invoice
Rule 46 of the CGST Rules, 2017 specifies what every tax invoice must contain. Missing fields are the single most common reason ITC claims are disallowed during scrutiny.
| # | Mandatory Field | Why It Matters |
|---|---|---|
| 1 | Name, address, and GSTIN of the supplier | Identifies who is charging GST |
| 2 | Tax invoice number (unique, consecutive series) | Links invoice to GSTR-1 filing |
| 3 | Date of issue | Determines time of supply and ITC eligibility period |
| 4 | Name, address, and GSTIN of recipient (if registered) | Required for B2B ITC claim |
| 5 | Name and address of recipient (unregistered buyer, if value ≥ ₹50,000) | Required for B2C large invoices |
| 6 | HSN code or SAC code | Determines applicable GST rate |
| 7 | Description of goods or services | Identifies what is being supplied |
| 8 | Quantity (for goods) | Needed for per-unit valuation |
| 9 | Total value of supply | Gross amount before discount/tax |
| 10 | Taxable value after discount | Base on which GST is computed |
| 11 | Rate of tax (CGST, SGST, IGST separately) | Determines correct tax head |
| 12 | Amount of tax charged (CGST, SGST, IGST separately) | Must match what is reported in GSTR-1 |
| 13 | Place of supply | Determines whether IGST or CGST+SGST applies |
| 14 | Whether supply is intra-state or inter-state | Ties to correct tax head above |
| 15 | Signature or digital signature of authorised signatory | Authenticates the document |
| 16 | Whether GST is payable on reverse charge | Buyer must self-assess tax if yes |
HSN digit requirement (FY 2026-27): Businesses with annual turnover up to ₹5 crore must mention 4-digit HSN codes. Businesses above ₹5 crore must mention 6-digit HSN codes. Exporters must use 8 digits.
Invoice Series Rules: Numbering That the Department Accepts
Your invoice number must follow a unique, consecutive (serial) series for each financial year. You can use alphanumeric characters, hyphens, or slashes — but the total length cannot exceed 16 characters.
- You may maintain separate series for different business locations or different types of supply (goods vs. services), but each series must itself be consecutive.
- The series resets at the start of every financial year (1 April). So your first invoice of FY 2026-27 can legitimately be INV-2627-001.
- You cannot skip numbers or reuse a number. Gaps in series trigger scrutiny and may be treated as suppressed sales.
- If you issue a credit note or debit note, it must have its own separate series — do not mix it with your tax invoices.
Time Limits for Issuing a GST Invoice
Issuing an invoice late is a compliance violation. Here are the rules under Section 31 of the CGST Act:
| Type of Supply | When Invoice Must Be Issued |
|---|---|
| Goods (movement involved) | At or before the time of removal/delivery |
| Goods (no movement, e.g., immovable property goods) | At or before the time of delivery |
| Services (general) | Within 30 days of date of supply of service |
| Services by banks / NBFCs / insurance companies | Within 45 days of date of supply of service |
| Continuous supply of goods | Before or at the time each successive statement of accounts or payment is due |
| Continuous supply of services | On or before the due date of payment as per contract, or receipt of payment (whichever is earlier) |
| Export of goods | Before or at the time of removal for export |
Worked Example: Raising a Correct B2B Invoice
Let's say Ravi Enterprises (Mumbai, GSTIN: 27XXXXX) supplies 50 units of industrial fans to Mehta Industries (Pune, GSTIN: 27YYYYY). Both are in Maharashtra, so this is an intra-state supply — CGST + SGST applies. GST rate on industrial fans is 18%.
- Unit price: ₹4,000
- Total value (50 units × ₹4,000): ₹2,00,000
- Trade discount (5%): –₹10,000
- Taxable value: ₹1,90,000
- CGST @ 9% on ₹1,90,000: ₹17,100
- SGST @ 9% on ₹1,90,000: ₹17,100
- Total invoice value: ₹2,24,200
The invoice must show the discount of ₹10,000 on the face of the invoice — only then is it excluded from the taxable value. A post-sale discount not mentioned on the invoice cannot reduce GST liability without a corresponding credit note.
Ravi must issue this invoice before or at the time the fans leave his warehouse. He uploads it to GSTR-1 for the month of supply. Mehta Industries can then claim ITC of ₹34,200 (₹17,100 CGST + ₹17,100 SGST) once it appears in GSTR-2B.
To generate an invoice exactly like this with auto-calculated taxes, you can use KyaTax's GST Invoice Generator — it pre-fills HSN codes, splits CGST/SGST vs. IGST based on state, and exports a PDF ready for sharing.
Format Requirements: Paper, Digital, and E-Invoicing
GST law does not mandate a specific template, but it does mandate content. That said, there are format rules:
- E-invoicing (mandatory from 1 August 2023 for turnover above ₹5 crore): If your aggregate turnover exceeded ₹5 crore in any preceding financial year, you must generate invoices through the Invoice Registration Portal (IRP). The IRP returns an IRN (Invoice Reference Number) and a QR code that must be printed on the invoice before sharing with the buyer.
- Businesses below the e-invoicing threshold can issue invoices in any format — Word, Excel, accounting software, or handwritten — as long as all 16 mandatory fields are present.
- A minimum of one original copy must be issued for goods (plus a duplicate for the transporter and a triplicate for the supplier's records). For services, one original is sufficient.
- Digital/electronic invoices are valid if they contain a valid digital signature or are sent in a manner both parties agree to.
Common Mistakes That Get Businesses Into Trouble
1. Wrong or Missing GSTIN of the Buyer
If the buyer's GSTIN is incorrect, the invoice will not appear in the buyer's GSTR-2B. The buyer loses ITC. The supplier is not at fault legally, but the business relationship suffers. Always verify the buyer's GSTIN on the GST portal before issuing.
2. Not Mentioning Place of Supply
Many small businesses omit the Place of Supply field. This determines whether you charge IGST (inter-state) or CGST+SGST (intra-state). A wrong choice means the wrong government gets the tax — a serious error requiring rectification through credit notes and fresh invoices.
3. Issuing One Invoice for Mixed Supplies Without Splitting
If you supply goods taxed at 5% and goods taxed at 18% together, you must show them as separate line items with separate HSN codes and separate tax calculations. A single blended rate is not acceptable and creates mismatches in GSTR-1.
4. Backdating Invoices Across Financial Years
Some businesses issue March invoices in April to defer income. Under GST, the time of supply is determined by the invoice date or the date of completion of service — whichever is earlier. Backdating creates a mismatch between GSTR-1 and books of accounts, which draws audit scrutiny.
5. Forgetting the Reverse Charge Declaration
If your supply falls under reverse charge (for example, legal services by an advocate to a business entity), you must state on the invoice that "Tax is payable on reverse charge basis." Omitting this means the recipient may not self-assess the tax, leading to interest and penalties for both parties.
Do it yourself in minutes — free to try, no login needed.
Open GST Invoice Generator →Frequently asked questions
Can I issue a GST invoice without a GSTIN for the buyer?
Yes, for B2C (unregistered buyer) supplies, you do not need the buyer's GSTIN. However, if the unregistered buyer's transaction value exceeds ₹50,000, you must capture their name, address, and state on the invoice. For B2B supplies, the buyer's GSTIN is mandatory for ITC purposes.
What happens if I issue a GST invoice after the time limit?
Late issuance is an offence under Section 122 of the CGST Act. A penalty of ₹10,000 or the tax amount evaded, whichever is higher, can be levied. Additionally, if the invoice is issued after the relevant GSTR-1 filing deadline, it may not appear in your buyer's GSTR-2B for that month, blocking their ITC temporarily.
Do I need to issue a tax invoice if I am under the Composition Scheme?
No. Composition dealers cannot charge GST and cannot issue a tax invoice. They must issue a "Bill of Supply" instead, which clearly states that the supplier is under the Composition Scheme and that no input tax credit is available to the buyer.
Is a digital or WhatsApp-shared PDF invoice valid under GST?
Yes, a digitally shared PDF invoice is legally valid as long as it contains all mandatory fields under Rule 46. However, if you are above the e-invoicing threshold (turnover exceeding ₹5 crore), the invoice must first be registered on the IRP portal and must carry a valid IRN and QR code before you share it with anyone.
General information for FY 2026-27, not professional advice for your specific case. Rules change — verify against the latest notification or ask a KyaTax expert.
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