What a GST Invoice Must Legally Contain
A GST tax invoice must carry your GSTIN, a consecutive serial number unique for the financial year, the date, the recipient's registered details, HSN or SAC codes, taxable value, the CGST/SGST/IGST split, place of supply, and a signature. Most invoices get the tax calculation right and the structural fields wrong.
Here is 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 are 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. An 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 used should generate the number itself, at the moment of saving, and never let a gap open up. See what a Business OS actually is for why keeping documents in one connected system prevents exactly this.
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 the customer then can't claim properly, and the mismatch shows up when their credit doesn't reconcile.
HSN missing or wrong
The number of HSN digits required depends on turnover, and the threshold has been tightened over the years. Worth confirming the current requirement for your bracket rather than assuming an older rule 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 only the billing address is printed, the document doesn't reflect the actual transaction. See the same field list applied to an AMC service contract for a worked example in one vertical.
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, an accounting tool 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 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. Businesses cross into the requirement without noticing because turnover grew and nobody re-checked.
FAQ
What happens if my GST invoice is missing a mandatory field?
It can be treated as a defective invoice, which puts the buyer's input tax credit at risk and can draw questions in an audit. Fix the template, not just the one invoice.
Can I use my own invoice number format?
Yes, as long as the number is consecutive, unique for the financial year, and within 16 characters using only letters, numbers, hyphens and slashes.
Do I need a QR code on every invoice?
Only if your turnover is above the notified e-invoicing threshold. Check the current threshold, since it has changed more than once.
Is place of supply the same as the delivery address?
No. Place of supply decides whether CGST+SGST or IGST applies; delivery address is where the goods or service actually go, and the invoice should show both where they differ.
Who can sign a GST invoice?
The supplier or an authorised person, with a physical or digital signature.
Does a missing signature make the whole invoice void?
It is one of the required particulars, so a system that skips it is non-compliant - but fixing a template going forward is simpler than defending old invoices at audit, so correct it as soon as it is found.
Sources
- Rule 46 of the CGST Rules, 2017 and related mandatory-particulars guidance: Tally Solutions, GST invoice format and mandatory fields
- E-invoicing threshold history: ClearTax, e-invoicing under GST
This is general information, not tax advice. GST rules and thresholds change - confirm the current position with your CA before relying on it.