A valid GST invoice needs your GSTIN, the buyer's details, an HSN/SAC code per item, the correct CGST/SGST or IGST split, and — above the e-invoicing threshold — a valid IRN. Missing any of these risks a rejected filing.
A GST invoice isn't just a bill — it's a tax document, and GST authorities expect a specific set of fields on every one you issue. Here's what's required, and where small businesses most often trip up.
The required fields
- Your business name, address, and GSTIN
- A sequential invoice number and date
- The buyer's name and address (and GSTIN, if they're registered)
- An HSN code for goods or SAC code for services, per line item
- The applicable GST rate and amount, split as CGST + SGST (same state) or IGST (different state)
- The total invoice value, including tax
HSN/SAC codes aren't optional
Every line item needs the correct HSN (goods) or SAC (services) code. The number of digits required depends on your annual turnover — businesses above certain thresholds need more digits of precision. Using the wrong code, or a code that's too short for your turnover bracket, is one of the most common reasons an invoice gets flagged during a GST reconciliation.
e-Invoicing and the IRN
If your turnover is above the e-invoicing threshold set by the GST Council, invoices to other registered businesses (B2B) need to be reported to the government's Invoice Registration Portal (IRP) before they're valid. The IRP returns an Invoice Reference Number (IRN) and a QR code, both of which need to appear on the invoice you actually send. An invoice without a valid IRN, where one is required, isn't a legally complete tax invoice.
CGST/SGST vs IGST
Get this wrong and the tax authority doesn't just see a formatting error — it sees a genuine mismatch in how much of each tax type was actually collected. If the buyer is in the same state as your registered business, GST splits into CGST and SGST at half the rate each. If they're in a different state, it's IGST at the full rate. This needs to be determined per invoice, not assumed from your default.
Where this goes wrong in practice
Almost every issue we see traces back to manual entry: a wrong HSN code typed from memory, a CGST/SGST split applied out of habit instead of checked against the buyer's state, or an e-invoice IRN request skipped because it felt optional that day. Software that fills these fields from your business and item setup — rather than asking someone to remember them per invoice — removes the step where the mistake happens.
StashBill's GST/VAT compliance engine fills in GSTIN, HSN/SAC codes, and the CGST/SGST/IGST split automatically — see the full India GST requirements StashBill configures for you.Frequently asked
Do I need an HSN code if I'm a small business under the e-invoicing threshold?
Yes — HSN/SAC codes are required on GST invoices regardless of whether e-invoicing applies to you; the e-invoicing IRN requirement is a separate, turnover-based rule.
What happens if I use the wrong HSN code?
It can cause a mismatch during GST return reconciliation, which may require you to file a correction or explain the discrepancy to the tax authority.
Is a GST invoice without an IRN valid, if I'm above the e-invoicing threshold?
No — for businesses above the threshold, a B2B invoice without a valid IRN registered on the IRP isn't considered a complete tax invoice.