What a GST invoice must legally contain
A GST tax invoice must carry sixteen particulars listed in Rule 46 of the CGST Rules, including a consecutive serial number under sixteen characters, the place of supply, HSN or SAC codes and the tax split. Unregistered suppliers and exempt supplies use a bill of supply instead.
Most invoicing disputes in small businesses are not about the money. They are about a document that cannot support the claim it makes. An invoice missing the place of supply, or carrying a serial number that repeats one from four months ago, is a weaker document than the same invoice with those two things right, and the weakness only surfaces when it matters.
Rule 46 of the CGST Rules 2017 lists what a tax invoice has to carry. It is not a long list, and none of it is difficult. It is just easy to leave things out when you are building the document by hand each time.
The particulars
Your details. Name, address and GSTIN of the supplier. The address should be the one registered against that GSTIN, not a convenient postal address.
A serial number. Consecutive, unique within the financial year, no more than sixteen characters, using only letters, numerals, hyphens and slashes. You may run multiple series — one per branch, or one for invoices and another for credit notes — as long as each series is itself consecutive.
The date of issue.
Your customer’s details. Name, address and GSTIN or UIN where they are registered. Where they are not registered and the value exceeds fifty thousand rupees, you still need their name, address, the delivery address and the state with its code.
The place of supply, together with the name of the state, where the supply is inter-state. This is the field people skip most often and it is the one that decides whether you charge IGST or CGST plus SGST.
HSN code for goods or SAC for services. The number of digits required depends on your aggregate turnover in the preceding financial year.
A description of the goods or services.
Quantity and unit for goods, expressed in the unique quantity code.
Total value of the supply.
Taxable value, after any discount that is being given at the time of supply and shown on the invoice.
Rate of tax, split into central, state, integrated tax and cess as applicable.
Amount of tax charged, in the same split.
Whether tax is payable on reverse charge. A yes or no field. It has to be present even when the answer is no.
Signature or digital signature of the supplier or an authorised person.
The three that actually cause problems
The serial number
The requirement is consecutiveness within a financial year, and the reason it exists is straightforward: gaps suggest a suppressed invoice. If you cancel an invoice, do not reuse the number and do not skip it. Keep the number, mark the document cancelled, and retain it. A cancelled invoice you can produce is a complete answer. A missing number is an open question.
Sixteen characters is tighter than it sounds. INVOICE/2026-2027/000001 is twenty-four. Shorten the prefix or use a two-digit year form. Our invoice number generator counts the characters as you build the format.
The place of supply
The default is your customer’s registered location, and for most straightforward supplies the default is right. But sections 10 to 13 of the IGST Act carry a list of situations where it is not.
Services directly related to immovable property follow the property. An interior designer in Lucknow working on a client’s Goa villa has Goa as the place of supply, even though the client lives in Delhi and every meeting happened there. Services performed at an event follow the event location. Goods that are not moved follow where they stand at delivery.
Getting this wrong is recoverable but tedious. Section 77 of the CGST Act and section 19 of the IGST Act let you pay the tax correctly and claim a refund of the wrongly-paid amount. It works. It is also months of correspondence to fix something a dropdown could have got right.
Discount
A discount reduces the taxable value only if it was known at the time of supply and is shown on the invoice. That is the whole test. A discount agreed afterwards, or given as a credit at year end against volume, follows different rules under section 15(3) and generally needs to be linked to the specific invoices it relates to.
Practically: put the discount on the invoice, per line, before the tax is computed. Do not apply it after tax and do not settle it by informal adjustment later.
Tax invoice or bill of supply
You issue a tax invoice when you are registered and making a taxable supply.
You issue a bill of supply when you are making an exempt supply, or when you are registered under the composition scheme. A bill of supply carries most of the same particulars but no tax, and a composition dealer’s bill of supply must state that they are not eligible to collect tax on supplies.
Issuing the wrong document type is a compliance problem even when no tax was wrongly charged. A clinic issuing tax invoices for exempt healthcare services has a paperwork problem to unwind, not just an unusual filing.
When the invoice has to be issued
For goods, at or before removal or delivery. For services, within thirty days of supply. Banking and financial institutions get forty-five days.
The deadline matters more than people assume, because the time of supply under sections 12 and 13 partly depends on the invoice date, and an invoice issued late can pull a liability into a period you did not expect.
Copies
For goods, three copies: original for the recipient, duplicate for the transporter, triplicate for the supplier, each marked as such. For services, two: original for the recipient and duplicate for the supplier.
This is why the old carbon-copy bill book has the markings printed on it. If you issue digitally, the marking convention still applies to the copies you produce.
Getting it right without thinking about it
Every particular above is a field you can forget on a hand-built document and cannot forget on a generated one. That is the entire argument for using a tool rather than a spreadsheet: not that it is faster, but that it cannot leave out the place of supply.
Our free GST invoice generator lays out every Rule 46 particular and works out the CGST/SGST or IGST split from the place of supply you select. It runs entirely in your browser and keeps nothing.
None of this is tax advice. Classification, rates and the treatment of edge cases are questions for your accountant, and rates change by notification. Confirm anything that affects a filing against the position in force on your date of supply.