Understanding GST Invoicing — Rules, Format & E-Invoicing Guide India 2026

A GST tax invoice must contain the following mandatory fields under the CGST Rules 2017: (1) Supplier details: legal name, GSTIN, and address of the supplier. (2) Invoice number: consecutive serial number — unique for the financial year. Cannot repeat within a FY. (3) Invoice date: date of issue. (4) Recipient details: name, address, GSTIN (if registered recipient). For unregistered recipients: name and address only. (5) Description of goods or services: HSN (Harmonised System of Nomenclature) code for goods — mandatory from April 2021 for all B2B. SAC (Services Accounting Code) for services. (6) Quantity (for goods): unit of measurement. (7) Taxable value: value before GST. (8) Applicable GST rate and amount: CGST + SGST (intra-state) or IGST (inter-state) broken out separately. (9) Place of supply: mandatory for inter-state transactions. (10) Signature or digital signature of supplier. Missing any mandatory field: invoice may be considered invalid, and the recipient cannot claim Input Tax Credit (ITC) on it. For high-value B2B transactions: e-invoice (IRN-generated) is mandatory — paper invoice alone is insufficient.

E-invoicing under GST is a system where B2B invoices are authenticated by GSTN’s Invoice Registration Portal (IRP) — which generates a unique Invoice Reference Number (IRN) and QR code. E-invoicing is NOT a separate invoice format — it is the same tax invoice, but with IRP authentication and IRN printed on it. Who must generate e-invoices (as of 2026): businesses with aggregate turnover above Rs5 crore in any preceding financial year since FY 2017-18. Exempted from e-invoicing (regardless of turnover): insurance companies, banking companies and financial institutions, NBFCs, Goods Transport Agencies (GTA), passenger transport services, multiplex cinema operators. E-invoice generation process: prepare invoice in your billing software → push to IRP (Invoice Registration Portal at einvoice1.gst.gov.in) → IRP validates GSTIN, generates IRN (unique 64-character hash), signs digitally, returns JSON with IRN and QR code → print QR code and IRN on your physical/PDF invoice. IRP submission must happen before sending invoice to recipient. If IRP is offline: can generate within 30 days of invoice date (rolling period maintained by GSTN). Penalty for not generating e-invoice when required: invoice may not be eligible for ITC at buyer end — major commercial consequence.

GST invoice time limits by supply type: Supply of goods (non-continuous): issue invoice at or before time of removal or delivery. For goods requiring movement: before or at the time of removal. Supply of services: issue invoice within 30 days of service completion. For banking and insurance: within 45 days. Continuous supply of goods (like gas pipelines, periodic delivery): each statement or payment date, whichever is earlier. Continuous supply of services (telecom, subscriptions): within 30 days of due date or payment date (whichever is earlier). Advance receipt: if payment received before supply, issue receipt voucher. Convert to tax invoice on supply completion. Delayed invoicing consequences: if invoice is not issued within prescribed time: (1) GST is still payable from the point of supply date (not invoice date). (2) Interest at 18% per annum on delayed tax payment. (3) Recipient cannot claim ITC if invoice is backdated or late-issued and not reported in GSTR-1 on time. Practical advice: issue invoices on the day of service delivery or goods dispatch. For monthly recurring services: issue invoice by the last day of the month. Batch invoicing (sending all invoices on the 30th for work done throughout the month) is risky — if anything is missed, you may miss the return deadline.

GST document types and when to use them: Tax invoice: issued by GST-registered supplier for taxable supplies. Contains GSTIN, HSN/SAC, GST breakdown. Recipient can claim ITC on this. Bill of Supply: issued by registered supplier for EXEMPT supplies (goods or services exempt from GST) or by supplier under composition scheme. No GST charged. Recipient cannot claim ITC. Example: doctor (exempt service) issues bill of supply, not tax invoice. Receipt voucher: issued when advance is received before supply. Covers the advance amount. Tax invoice issued later when supply made. Delivery challan: for goods movement without accompanying invoice (e.g., goods sent for job work, loan, or exhibition). Credit note: issued when: taxable value of original invoice needs to be reduced (goods returned, price revision, discount post-supply). Must be linked to original invoice via reference. Reduces supplier GST liability. Recipient must reverse ITC to the extent of credit note. Issue credit note by September 30 of the next FY or filing of annual return (whichever is earlier) — after this: cannot issue GST credit note. Debit note: issued when taxable value of original invoice needs to be increased (additional charges). Increases GST liability.

Top GST invoicing mistakes causing Input Tax Credit rejection: (1) Wrong GSTIN of recipient: ITC can only be claimed if invoice shows correct GSTIN of the claiming business. One digit error = ITC rejection. Always verify recipient GSTIN on GSTN portal before issuing. (2) Invoice not reported in GSTR-1 on time: if supplier does not report the invoice in GSTR-1 by due date, it does not appear in buyer GSTR-2B — buyer cannot claim ITC. Critical for bulk invoicing businesses. (3) HSN code mismatch: HSN code on invoice should match what supplier declares in GSTR-1. Mismatch triggers GST department scrutiny. (4) Tax amount arithmetic error: if tax amount shown on invoice does not match taxable value × rate, ITC is restricted to the lower amount. Use billing software to auto-calculate. (5) E-invoice IRN missing (for applicable turnover): without IRN for businesses above Rs5Cr, invoice is considered invalid for ITC by recipient. (6) Reverse charge mechanism (RCM) error: certain supplies (from unregistered vendors, specific goods/services) attract RCM — supplier should not charge GST; recipient pays directly. Incorrect GST charging by unregistered vendor does not entitle recipient to ITC. (7) Credit note not reported within deadline: credit notes must be reported in GSTR-1 by September 30 of the next year — late credit notes cannot reduce GST liability.