GST Invoice Format 2026: Mandatory Fields, Rules & Time Limits

A GST invoice is not just a bill. It is the document that carries input tax credit from you to your customer — and if it is defective, your customer cannot claim that credit. Which means an invoice error is not your problem in the abstract; it is a direct cost to the person paying you, and they will notice.

This guide covers what a valid GST invoice must contain, when it has to be issued, and the mistakes that cause the most trouble. If you are earlier in the process, start with what GST is or the registration guide.

The mandatory fields

A tax invoice must carry all of the following. Missing any of them makes the document defective:

Field	Notes
Supplier name, address, GSTIN	Exactly as registered
Invoice number	Consecutive, unique within the financial year, up to 16 characters
Date of issue	Governs the tax period it falls in
Recipient name, address, GSTIN	GSTIN required where the recipient is registered
HSN / SAC code	Digit count depends on your turnover
Description, quantity, unit	Specific enough to identify what was supplied
Taxable value	After any discount given at the time of supply
Tax rate and amount	CGST + SGST, or IGST, shown separately; plus cess if applicable
Place of supply	Decides whether the supply is intra-state or inter-state
Reverse charge indicator	State whether tax is payable on reverse charge
Signature	Physical or digital signature of an authorised person

Where the recipient is unregistered and the value crosses the prescribed threshold, the invoice must additionally carry the recipient's name, address and the state and its code.

Place of supply — the field that decides the tax

This is the field to get right, because it determines which tax you charge. Intra-state supply attracts CGST plus SGST; inter-state supply attracts IGST. Charging the wrong one is not a rounding issue — the buyer cannot claim credit for a tax that should not have been charged, and you have to correct it.

For goods it usually follows delivery. For services the rules vary by service type, and the location of the recipient often governs rather than where the work was performed. Consultants working remotely for a client in another state routinely get this wrong.

The GST calculator handles the CGST/SGST versus IGST split once you know which applies.

Tax invoice versus bill of supply

Two different documents for two different situations, and issuing the wrong one is a common early mistake:

• Tax invoice — a registered supplier making a taxable supply. Shows GST separately. Confers input tax credit.
• Bill of supply — exempt supplies, or a supplier under the composition scheme. Shows no GST. Confers no credit.

A composition dealer must also state on the document that they are a composition taxable person and not eligible to collect tax. Issuing a tax invoice showing GST when you are not entitled to collect it is a substantive problem, not a formatting one.

When the invoice must be issued

The timing rules are stricter than most small businesses realise:

• Goods — at or before removal of the goods where movement is involved, or at delivery where it is not.
• Services — generally within thirty days of the supply, with a longer window for certain financial-sector suppliers.
• Continuous supply — tied to the due date of payment or the completion of an event under the contract.

Issuing late does not defer your liability. The tax is payable for the period in which the supply occurred, so a batch of invoices raised at quarter end for work done two months earlier can put you in the wrong period and out of step with your returns.

Copies, numbering and revisions

Copies. For goods, three — original for the recipient, duplicate for the transporter, triplicate for the supplier. For services, two. In practice most businesses issue electronically, but the labelling convention persists.

Numbering. Consecutive and unique within the financial year. You may run more than one series — by branch, by product line — but every series must be gapless. A missing number is the sort of thing that invites a question during scrutiny, and "we cancelled that one" is a much better answer when you have the cancelled document.

Getting it wrong. Two correction documents exist. A credit note reduces the value where you over-charged, the supply was returned, or the service was deficient. A debit note increases it where you under-charged. Both must reference the original invoice, and credit notes carry a time limit tied to the annual return — after that the adjustment is no longer available, so do not sit on disputed invoices indefinitely.

E-invoicing

Businesses above a turnover threshold must report invoices to the Invoice Registration Portal and obtain an Invoice Reference Number and QR code before issuing. The invoice is not valid without it.

The threshold has been progressively lowered since e-invoicing was introduced, bringing steadily smaller businesses into scope. If your turnover is anywhere near the current limit, check it each year rather than assuming you are still outside — this is the compliance change most likely to catch a growing business unprepared.

E-invoicing does not replace your returns. It changes how the invoice is generated and authenticated, and the data flows through to your returns automatically, which is precisely why errors at invoice stage now propagate rather than being quietly fixed later.

Delivery challans, advances and exports

Three situations where a plain GST invoice is not the right document:

Moving goods without a supply. Sending stock to a job worker, moving goods between your own branches, or shipping on approval — none is a supply at that moment, so a delivery challan is issued instead. The invoice follows when the supply actually happens.

Receiving an advance. Taking money before supplying services means issuing a receipt voucher, not an invoice. If the deal then falls through and you refund the advance, a refund voucher closes it out. Businesses that take deposits routinely skip this and reconcile badly at year end.

Exports and SEZ supplies. These are zero-rated, and the invoice must carry an endorsement stating whether it is a supply made with payment of integrated tax or under a bond or letter of undertaking without payment. Which one you use determines how you recover the tax — as a refund of IGST paid or as a refund of accumulated input credit. Choosing the route before you invoice, rather than after, saves a great deal of trouble.

Records and how long to keep them

A GST invoice is not finished when it is sent. The supporting records must be retained for a substantial period from the due date of the annual return for that year — long enough that "we changed accounting software" is not an acceptable explanation for their absence.

Keep the invoice itself, the credit and debit notes against it, delivery challans, e-way bills where applicable, and the payment record. The reconciliation that matters is between your invoices, your returns and your customer's claimed credit. When those three agree, scrutiny is a formality; when they do not, the burden of explaining the gap is yours.

The errors that cost your customer money

1. Wrong or missing GSTIN. The most common, and it directly blocks your customer's input tax credit.
2. CGST+SGST where IGST was due. A place-of-supply error, and it means a correction rather than an adjustment.
3. Missing HSN or the wrong digit count for your turnover band.
4. Gaps in the number series. Cheap to avoid, awkward to explain.
5. Discounts applied after the invoice without a credit note — the taxable value must reflect discounts given at the time of supply.
6. Not stating reverse charge where it applies, leaving the recipient unsure of their obligation.

None of these are complicated. They are all the result of a template that was set up once and never checked against the rules. If you are producing invoices by hand, the invoice generator lays the required fields out for you, and the HSN lookup covers the code you are most likely to guess at.