GST Invoice Rules in India: Every Field a Valid Tax Invoice Needs

A GST tax invoice isn't just a bill — it's the legal document that lets your registered buyer claim input tax credit and the record your own GST returns are built on. Get the fields wrong and the consequences are real: buyers reject the invoice and delay payment, mismatches surface in GSTR reconciliation, and repeated sloppiness invites notices.

The rules (Section 31 of the CGST Act and Rule 46 of the CGST Rules) are actually quite specific about what an invoice must contain. Here's the complete practical checklist, the CGST/SGST/IGST logic, and the errors that cause most rejections. One caveat up front: this is a working guide, not legal advice — your CA has the final word on your specific situation.

The mandatory fields

A valid tax invoice from a registered supplier needs:

1. Your details: legal name, address, and GSTIN.
2. A unique, consecutive invoice number — up to 16 characters, letters/numbers/dash/slash, unique within the financial year. A series like INV-2026-001 onward is ideal. Gaps and duplicates both cause trouble.
3. Date of issue.
4. Buyer's details: name and address; if the buyer is registered, their GSTIN is mandatory — without it they cannot claim credit.
5. Place of supply with state name — this single field decides which taxes apply (next section).
6. HSN code for each item. Mandatory digits depend on your turnover: 4 digits for most businesses up to ₹5 crore, 6 digits above. Goods and services both have codes (services use SAC).
7. Description, quantity, and unit for each line.
8. Taxable value per line — after any discount shown on the invoice.
9. Tax rate and amount, shown separately by type — CGST and SGST as two lines, or IGST as one. A single "GST 18%" line on an intra-state invoice is technically non-compliant.
10. Total value, ideally with the amount in words.
11. Signature or digital signature of the supplier or authorized person.

The GST Invoice Generator bakes this structure in: GSTIN fields for both parties, per-item HSN, separate tax lines, automatic round-off, the total written out in Indian words (lakh/crore), and a signatory block — generated in your browser, so client data stays on your machine, and with autosave so a closed tab doesn't eat your invoice.

CGST + SGST or IGST? One question decides

Compare your state with the place of supply:

• Same state → CGST + SGST, each at half the rate. An 18% supply within Delhi is CGST 9% + SGST 9%.
• Different states → IGST at the full rate. Delhi to Mumbai is IGST 18%.

The total tax is identical either way; what changes is which line items appear — and reconciliation cares deeply about that difference. In the generator this is a single toggle, and it splits or combines automatically.

For goods, place of supply is generally where the goods are delivered. Services have their own rules with exceptions, which is exactly the kind of detail to confirm with your CA when you start invoicing something new.

Invoice types people confuse

Bill of supply — what composition-scheme dealers and sellers of exempt goods issue. No tax is shown, and the buyer gets no credit.

Quotation / proforma — a price offer before the sale. It is not a tax invoice, has no GST consequences, and shouldn't carry your tax-invoice numbering. The generator's Quotation tab exists for exactly this.

Receipt — acknowledges money received; again, not a tax invoice. Use the Receipt tab.

Not registered yet? Then you cannot issue a tax invoice or charge GST at all — collecting GST without a GSTIN is an offence. Issue a regular invoice (the generator's "No tax" mode) until registration; the threshold is ₹40 lakh turnover for goods in most states (₹20 lakh for services, lower in special-category states).

Mistakes that get invoices bounced

• Wrong tax split — IGST on an intra-state sale or vice versa. The mismatch surfaces in the buyer's GSTR-2B and they'll come back to you for a correction.
• Missing buyer GSTIN on a B2B sale — no credit for them, awkward email for you.
• Duplicate or reused invoice numbers — keep one consecutive series per year.
• Rounding done per-line instead of on the total — show round-off as its own line (the generator does).
• Charging GST on the full value after a discount was given but not shown — invoice-level discounts must appear on the invoice to reduce taxable value.

Record-keeping

Keep invoices (issued and received) for 72 months from the annual return due date. A simple discipline that works: export every invoice as PDF, name it INV-2026-001-ClientName.pdf, and at month-end merge the month into one PDF for your records folder. Your future self at filing time will be grateful.

Frequently asked questions

Is a digitally generated invoice without a physical signature valid? Yes — invoices can carry a digital signature or be issued under the supplier's authorization; e-invoicing mandates (for businesses above the notified turnover) work entirely electronically. For small sellers, printing and signing remains the simple route.

Do I put GST on an invoice to an unregistered consumer? Yes, if you're registered — tax applies regardless of the buyer's status. You just don't need their GSTIN.

What rate do I charge? Whatever your product's HSN attracts — 0/5/12/18/28%. Look up your HSN on the CBIC portal and confirm once with your CA; charging the wrong slab is expensive in both directions.

A simple numbering and filing system that survives an audit

Use one series per financial year per document type: INV-2627-001 for invoices, QTN-2627-001 for quotations, RCT-2627-001 for receipts — the year prefix prevents accidental duplication across years, and separating quotations from invoices keeps your tax series clean (quotations are not tax documents and must not consume tax-invoice numbers). Save every PDF with number and client in the filename, keep a monthly folder, and merge each month into a single archive PDF. When reconciliation season arrives — or in the rare event of an audit — your evidence is one sorted folder, not a shoebox of screenshots.

Related tools

• GST Invoice Generator — compliant-format invoices, quotations and receipts
• GST & VAT Calculator — add or extract tax, with the CGST/SGST split
• Marketplace Profit Calculator — fees and tax-on-fees per order