What a GST invoice legally must contain in 2026

Most invoice disputes at audit aren't about tax rates. They're about fields that were never filled in, and numbering that doesn't hold up.

8 September 2026 · 6 min read

Almost every business we've looked at raises invoices that are 80% correct. The missing 20% is rarely the tax calculation — that part people get right, because it's the part they think about. What goes wrong is the boring structural stuff: a field left blank, a serial number that restarts, a place of supply nobody filled in.

Here's what actually has to be on a tax invoice under the CGST Rules, and where things usually break.

The mandatory fields

• Your name, address and GSTIN
• A consecutive serial number, unique for the financial year, maximum 16 characters, using only letters, numbers, hyphens and slashes
• Date of issue
• The recipient's name, address and GSTIN (or UIN) if they're registered
• For unregistered recipients above the prescribed value: name, address, and address of delivery with state name and code
• HSN code for goods, or SAC for services
• Description, quantity and unit
• Total value, taxable value after any discount
• Tax rate and tax amount, split into CGST, SGST/UTGST, IGST and cess as applicable
• Place of supply with state name, for inter-state supplies
• Address of delivery, where it differs from the place of supply
• Whether tax is payable on reverse charge
• Signature or digital signature of the supplier or an authorised person

Where it usually goes wrong

Numbering with gaps

This is the big one. Your invoice series has to be consecutive for the financial year. If invoice 41 exists and 43 exists, an officer will ask what happened to 42. "It got cancelled" is a fine answer only if you can show the cancelled document. Deleting a row from a spreadsheet is not cancellation.

This gets worse when invoices are raised from more than one place — a laptop in the office and a phone in the field can quietly produce two number 47s. Any system you use should generate the number itself, at the moment of saving, and never let a gap open up.

Place of supply left blank

For an inter-state supply this determines whether you charge IGST or CGST plus SGST. Getting it wrong doesn't just create a bad invoice — it puts the tax in the wrong bucket, which your customer then can't claim properly. And your customer will notice, because their credit won't reconcile.

HSN missing or wrong

The number of HSN digits required depends on your turnover, and the threshold has been tightened over the years. Worth confirming the current requirement for your bracket rather than assuming the rule you learned three years ago still applies.

Delivery address assumed to be the billing address

For equipment and distribution businesses this is common. Head office in one state, installation site in another. If you only print the billing address, the document doesn't reflect the actual transaction.

The part nobody thinks about until audit

A correct invoice is not the same as a correct invoice trail. What gets asked for at audit is usually the sequence: quotation, purchase order, invoice, delivery challan, payment. If those live in four different places — a Word file, WhatsApp, Tally, and a challan book — assembling that trail is a week of work.

When each document is generated from the one before it, the trail assembles itself, because the link was never broken in the first place.

E-invoicing

If your turnover is above the notified threshold, invoices have to be reported to the government portal and carry an IRN and QR code. That threshold has been lowered several times since e-invoicing started, so check where the limit sits today rather than relying on what applied when you last looked. Plenty of businesses cross into the requirement without noticing, because turnover grew and nobody re-checked.

None of this is complicated. It's just detailed, and detail is exactly what gets skipped when invoices are being typed by hand at the end of a long day.