GST Invoice in India — Format, Rules & Common Mistakes (2026 Guide)

What is a GST invoice?

A GST invoice is a tax document issued by a registered seller to a buyer that records the supply of goods or services and the tax charged. Under Section 31 of the CGST Act, every registered person must issue an invoice for every taxable supply.

Two things make a GST invoice different from a regular bill:

1. It shows the GSTIN (Goods and Services Tax Identification Number) of the seller, and the buyer's GSTIN if they're registered.
2. It separates the value of the supply from the tax (CGST/SGST or IGST) so the buyer can claim Input Tax Credit (ITC) — i.e. offset that tax against their own output tax.

When do you need to issue one?

If you are registered under GST and your annual turnover exceeds the threshold, you must issue a GST invoice for every taxable supply. As of 2026:

• Goods: registration mandatory if turnover > ₹40 lakh (₹20 lakh in special-category states)
• Services: registration mandatory if turnover > ₹20 lakh (₹10 lakh in special-category states)

For services, the invoice must be issued within 30 days of providing the service. For goods, it must be issued at or before delivery or removal of goods.

16 mandatory fields on every GST invoice

Section 31 read with Rule 46 of the CGST Rules prescribes the fields. Miss any, and the invoice is technically invalid. The fields:

1. Name, address & GSTIN of the supplier
2. Invoice number — unique for the financial year, max 16 characters, alphanumeric, no special characters except hyphen and slash
3. Invoice date
4. Name, address & GSTIN of the recipient (if registered)
5. If recipient is unregistered and invoice value exceeds ₹50,000 — recipient's name, address, state and state code
6. HSN code (goods) or SAC code (services)
7. Description of goods/services
8. Quantity (with unit of measure)
9. Total value of supply
10. Taxable value (after discount, before tax)
11. Rate of GST — CGST, SGST, IGST, UTGST, Cess
12. Amount of tax — broken down by component
13. Place of supply with state name and code
14. Address of delivery (if different from place of supply)
15. Whether tax is payable on reverse charge (Yes/No)
16. Signature or digital signature of the supplier or authorised signatory

HSN & SAC codes — when and how many digits?

HSN (Harmonised System of Nomenclature) is used for goods. SAC (Services Accounting Code) is used for services. Both are 4–8 digit numeric codes.

How many digits you must show depends on your turnover:

Common SAC codes for service freelancers:

• 998314 — IT design and development services (web dev, app dev)
• 998313 — IT consulting and support services
• 998365 — Marketing & advertising services
• 999293 — Other educational services (coaching, training)

Tax invoice vs Bill of Supply — which to issue?

This trips up nearly every new business owner.

• Tax invoice — issued by a regular registered person for a taxable supply. Shows CGST, SGST or IGST separately.
• Bill of Supply — issued in two situations:
  • By a Composition Scheme dealer (turnover up to ₹1.5 crore; pays flat low GST and cannot collect tax from customers).
  • For an exempt supply (e.g. educational services to a school, healthcare).

If you're under the Composition Scheme, you cannot issue a tax invoice or show GST separately — only a Bill of Supply. Many small dealers issue the wrong document type and get pulled up in audit.

CGST, SGST, IGST — which applies?

Determined by the place of supply:

• Same state (intra-state) → CGST + SGST, each at half the total GST rate. e.g. 18% = 9% CGST + 9% SGST.
• Different state (inter-state) → IGST, at the full rate. e.g. 18% IGST.
• Union Territory → CGST + UTGST.
• Exports → Zero-rated (0% IGST, with refund of input tax).

When is e-invoicing required?

e-Invoicing under GST means generating invoices through the government's Invoice Registration Portal (IRP), which returns a unique IRN (Invoice Reference Number) and a QR code that must be printed on the invoice.

As of 2026, e-invoicing is mandatory if your aggregate annual turnover exceeds ₹5 crore in any preceding financial year since 2017–18.

If you're below ₹5 crore, e-invoicing is optional — but a manually issued GST invoice (paper or PDF) is perfectly valid.

7 expensive mistakes we see repeatedly

1. Wrong invoice numbering — using "INV/2026/001" then "Inv-2026-002". The numbering scheme must be consistent across the financial year.
2. Missing place of supply — this single missing field can block your customer's ITC claim.
3. Charging CGST/SGST instead of IGST for inter-state supply. Refunding and re-invoicing is painful.
4. Wrong HSN/SAC code — pick a code that doesn't fit your service and the tax rate may be wrong too.
5. Tax invoice issued by a Composition dealer — illegal; you can only issue Bill of Supply.
6. No reverse-charge mention when applicable (e.g. import of services, GTA services).
7. Late issuance — for services, issuing the invoice more than 30 days after the service is a violation.

Recommended format & structure

A clean, audit-ready GST invoice typically lays out:

1. Header — your business name (large), logo, address, GSTIN, PAN, contact.
2. Invoice meta — Invoice No., Date, Place of Supply, State Code, Reverse Charge (Yes/No).
3. Bill To / Ship To — recipient name, address, GSTIN.
4. Line items table — Sl. No., Description, HSN/SAC, Qty, Unit, Rate, Discount, Taxable Value, GST Rate, CGST, SGST, IGST, Amount.
5. Totals — Subtotal, total CGST, total SGST/IGST, round-off, Grand Total.
6. Amount in words — e.g. "Rupees Forty-Five Thousand Two Hundred Only".
7. Bank details / payment terms — A/c name, A/c no., IFSC, UPI ID, due date.
8. Authorised signature & declaration ("Certified that the particulars given above are true and correct.").

Record-keeping & retention

Under GST law, you must preserve invoices and related records for 72 months (6 years) from the due date of furnishing the annual return for the year. Both physical and electronic records are accepted. We recommend:

• Store invoices as PDFs in a cloud folder organised by FY (e.g. 2025-26/).
• Name files predictably: INV-2025-26-001_clientname.pdf.
• Keep a single master spreadsheet of all invoices issued — date, number, recipient GSTIN, taxable value, tax. Reconcile monthly.

Quick FAQ

Can I issue a handwritten GST invoice?

Yes, the law doesn't mandate any particular medium. But for clarity, audit trails and your customer's ITC claim, a computer-generated or PDF invoice is far better.

Do I need a separate invoice series for B2B and B2C?

No. A single continuous series for the financial year is fine. Some businesses prefer separate prefixes (e.g. B2B/, B2C/) for internal accounting, which is allowed.

What is the difference between CGST and SGST?

CGST goes to the Central government; SGST goes to the State government. Together they equal the total GST rate for intra-state supplies. For inter-state, the same total is collected as a single IGST, which is then shared between Centre and State.

Is signature compulsory on every invoice?

Yes — either a physical signature or a Digital Signature Certificate (DSC). However, e-invoices (IRP-generated) carry a digital signature automatically and need no further sign.