What Is a GST Invoice? Mandatory Fields Explained

A GST invoice is the document a registered supplier issues to record a sale of goods or services and the GST charged on it. It is also the document your client uses to claim input tax credit (ITC), which is the real reason clients care whether yours is correct. If a mandatory detail is missing or wrong, your client may lose credit on the tax they paid, and a client who loses money because of your paperwork will not be quick to pay you again.

This guide explains what counts as a GST invoice, which fields are mandatory under the law, when you need to issue one, and the mistakes that most often cause trouble for freelancers and small businesses.

Tax invoice, bill of supply and other documents

Not every sale needs a "tax invoice". Which document you issue depends on who you are and what you are selling.

• Tax invoice. Issued by a registered taxpayer who charges GST. This is the one most freelancers and small businesses use.
• Bill of supply. Issued instead of a tax invoice by composition taxpayers, and for supplies that are exempt or nil-rated. No GST is shown on it.
• Invoice-cum-bill of supply. Used when a single document covers both taxable and exempt supplies.
• Receipt voucher. Issued when you receive an advance before the supply happens.
• Credit and debit notes. Issued to reduce or increase the value or tax on an earlier invoice.
• Plain invoice. If you are not registered for GST, you cannot charge or show GST at all. You can issue an ordinary invoice or bill, but it must not look like a tax invoice. (Not sure whether you need to register? Read whether freelancers need GST registration.)

The mandatory fields, one by one

The details required on a tax invoice come from Rule 46 of the CGST Rules. In practice, they fall into a few groups.

1. Who is selling

• Your name, address and GSTIN. The GSTIN is a 15-character number whose first two digits are your state code and the next ten are your PAN.
• Your business name should match what is on your registration certificate. A trade name that differs from the legal name can confuse a client's accounts team and their software.

2. A unique invoice number and a date

• A consecutive serial number, not more than 16 characters, using letters, numerals, a hyphen or a slash, unique for the financial year. INV-001, 2026-27/045 and A/0012 are all fine.
• The date of issue. Do not backdate or leave it blank.
• Do not skip numbers or reuse them. If you cancel an invoice, keep the number and issue a credit note rather than deleting it. Gaps and duplicates are a classic audit flag.

3. Who is buying

• The client's name and address.
• Their GSTIN, if they are registered. A registered client needs it to claim credit, so ask for it up front.
• If the client is unregistered and the invoice value is above ₹50,000, you must also show their address and the name of the state with its code. For smaller invoices to unregistered buyers, name and address are enough in most cases.

4. What is being sold

• A description of the goods or services, specific enough to be understood. "Website development, 8 pages, CMS" is better than "Services".
• The HSN code for goods or SAC code for services. How many digits you must show depends on your turnover, but quoting the full code is the safest habit. See our practical guide to HSN and SAC codes.
• Quantity and unit for goods. For services, a quantity of one with a clear description is common.

5. How much it costs and how the tax is calculated

• The total value of the supply.
• Any discount and the resulting taxable value. Tax is calculated after the discount, so show it clearly.
• The tax rate and amount of each tax: CGST and SGST (or UTGST) for same-state supplies, or IGST for inter-state ones. Our explainer on CGST, SGST and IGST shows how to decide.
• The total invoice value, in figures and, for easy checking, in words. Indian invoices usually write amounts in words using lakh and crore.

6. Where the supply is treated as happening

• The place of supply, together with the name of the state, wherever it differs from the state of the buyer's address.
• Whether tax is payable under reverse charge. If it is, the invoice should say so. If not, "Reverse charge: No" is a harmless addition.

7. Authentication

• The signature or digital signature of the supplier or an authorised person. A system-generated invoice with a clear note, such as "This is a computer generated invoice", is generally accepted for ordinary invoicing.

When do you have to issue the invoice?

The law sets time limits, and they are shorter than most people think.

• For services, issue the invoice within 30 days of providing the service (45 days for insurers, banks and certain financial institutions).
• For goods, issue it before or at the time of removal of the goods or delivery.
• If you take an advance payment, issue a receipt voucher for it. GST on an advance is payable in the month you receive it.
• For continuous services with milestone payments, the invoice is due when each payment falls due or is received, whichever is earlier.

Because GST becomes payable based on when the invoice is issued or when payment is received (not necessarily when your client pays the invoice), issuing late invoices does not postpone your liability. It just adds risk.

E-invoicing: does it apply to you?

Businesses with aggregate annual turnover above the notified threshold (currently ₹5 crore) must generate an Invoice Reference Number and QR code through the government's e-invoice system for B2B invoices. Most freelancers and small studios fall well below this and simply issue ordinary PDF invoices. If your turnover grows, check the current threshold, because it has been lowered in steps.

Common mistakes that cause rejected invoices

1. Wrong or missing buyer GSTIN. One mistyped digit and the client's credit claim fails to match in their return. Copy it from their registration certificate.
2. Wrong tax type. Charging CGST and SGST to a client in another state, or IGST to a client in your own state. Both errors are fixable only with a credit note and a new invoice.
3. Missing HSN/SAC code. An invoice with no code is easy to reject at the client's end.
4. A blank or vague description. "Professional charges" triggers questions that "Logo design, 3 concepts, 2 revisions" does not.
5. Duplicate or non-sequential numbering. Keep a simple running series and never recycle numbers.
6. Charging GST while unregistered. This is not allowed. You cannot collect GST without registration, and an unregistered supplier cannot pass on credit.
7. No place of supply for inter-state sales. Always name the state.

A simple checklist before you hit send

• Does my GSTIN, name and address match my registration certificate?
• Is the invoice number unique and in sequence?
• Is the client's GSTIN correct, and is the invoice addressed to the right entity?
• Is each line described clearly, with an HSN or SAC code?
• Is the tax split right for the two states involved?
• Are taxable value, tax amounts and total correct, and do the words match the figures?
• Are my bank and UPI details on it, with the due date?

Make it easier on yourself

You do not need to build any of this by hand. BillBuddy's free GST invoice generator includes every field above, checks your GSTIN format, works out CGST, SGST or IGST from the two states, writes the total in words and produces a clean PDF in your browser. If you want a starting point for your kind of work, browse the invoice templates by profession.

This article is general information, not tax or legal advice. Rules and thresholds change, so confirm the current position with a chartered accountant.