A valid GST tax invoice needs your name, address and GSTIN, a unique serial number for the financial year, the date, the buyer's details (and GSTIN for B2B), an HSN/SAC code per line, taxable value, the CGST/SGST or IGST rate and amount, place of supply, and a reverse-charge flag. Above ₹5 crore turnover, B2B invoices also need an IRN and QR code from the Invoice Registration Portal.
A GST invoice isn't just a bill — it's a tax document, and Rule 46 of the CGST Rules sets out the fields every one must carry. Here's the complete 2026 format, a worked example, and where small businesses most often trip up.
The required fields
| Field | What it must show |
|---|---|
| Supplier details | Name, address, and GSTIN |
| Invoice number | Consecutive serial number, unique for the financial year, up to 16 characters (letters, numbers, "-" and "/") |
| Invoice date | Date of issue |
| Recipient details | Name, address, and GSTIN if registered. For unregistered buyers where the value is ₹50,000 or more: name, delivery address, and state name and code |
| HSN / SAC code | At least 4 digits (turnover up to ₹5 crore, B2B) or 6 digits (above ₹5 crore) |
| Description and quantity | Description of goods or services, quantity, and unit |
| Values | Total value, and taxable value after any discount or abatement |
| Tax | Rate and amount of CGST + SGST/UTGST, or IGST, plus cess if applicable |
| Place of supply | State name and code — needed for inter-state supplies |
| Delivery address | If different from the place of supply |
| Reverse charge | Whether tax is payable on reverse charge |
| Signature | Signature or digital signature of the supplier or authorised representative |
A worked example
A Surat wholesaler sells 20 units of an item at ₹500 each (taxable value ₹10,000, GST 18%) to two different registered buyers:
| Buyer in Gujarat (intra-state) | Buyer in Maharashtra (inter-state) | |
|---|---|---|
| Place of supply | Gujarat (24) | Maharashtra (27) |
| Taxable value | ₹10,000 | ₹10,000 |
| CGST | 9% = ₹900 | — |
| SGST | 9% = ₹900 | — |
| IGST | — | 18% = ₹1,800 |
| Invoice total | ₹11,800 | ₹11,800 |
The total is identical, but the tax heads aren't — and your GSTR-1 must report them under the right heads. That's why the split has to be decided per invoice, from the buyer's state, not set once as a default.
GST rates in 2026
From 22 September 2025, the GST Council moved most goods and services to two main slabs — 5% and 18% — plus a 40% rate for specified demerit and luxury goods. The 12% and 28% slabs were largely removed, with most 12% items moving to 5% and most 28% items moving to 18%. If your item master still carries pre-September 2025 rates, your invoices and returns won't match — check each item's current rate against the CBIC rate notifications.
HSN/SAC codes aren't optional
Every line item needs the correct HSN (goods) or SAC (services) code. Businesses with aggregate turnover up to ₹5 crore must show at least 4 digits on B2B invoices; above ₹5 crore, at least 6 digits on all tax invoices. Using the wrong code, or a code too short for your turnover bracket, is one of the most common reasons an invoice gets flagged during reconciliation. Our HSN/SAC codes guide covers the rules in detail.
e-Invoicing and the IRN
If your aggregate annual turnover has exceeded ₹5 crore in any financial year since 2017-18, B2B invoices, credit notes, and debit notes must be reported to the Invoice Registration Portal (IRP). The IRP returns an Invoice Reference Number (IRN) and a signed QR code, both of which must appear on the invoice you send. Businesses with turnover of ₹10 crore or more must report each e-invoice within 30 days of its date — the IRP rejects older ones. An invoice without a valid IRN, where one is required, isn't a valid tax invoice.
When the invoice must be issued
- Goods: before or at the time of removal or delivery of the goods.
- Services: within 30 days of the date of supply (45 days for banks, insurers, and NBFCs).
- Unregistered or composition sellers: issue a bill of supply instead of a tax invoice — no GST is charged on it.
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 collected. If the place of supply is in the same state as your registration, GST splits into CGST and SGST at half the rate each. If it's a different state, it's IGST at the full rate. Charging the wrong head usually means paying the correct tax again and claiming a refund of the wrong one — a slow, avoidable process.
Where this goes wrong in practice
- A wrong HSN code typed from memory, or one that's too short for your turnover.
- CGST/SGST applied out of habit to an out-of-state buyer.
- Old 12% or 28% rates left on items after September 2025.
- An e-invoice IRN request skipped — or reported after the 30-day window for ₹10 crore+ businesses.
- Invoice numbers that restart or repeat within the same financial year.
Almost every one of these traces back to manual entry. 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 it configures for you.
Frequently asked
Do I need an HSN code if I'm a small business under the e-invoicing threshold?
Yes, on B2B invoices — at least 4 digits if your turnover is up to ₹5 crore. 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 mean filing a correction or explaining the discrepancy to the tax authority — and the wrong code can also mean the wrong rate was charged.
Is a GST invoice without an IRN valid, if I'm above the e-invoicing threshold?
No — for businesses above ₹5 crore turnover, a B2B invoice without a valid IRN from the IRP isn't treated as a valid tax invoice, and the buyer can't rely on it for input tax credit.
Can the invoice number restart every month?
No. It must be a consecutive serial number that's unique for the whole financial year. You can use multiple series (for example, one per branch), but no number can repeat within the year.
Sources
- CBIC — CGST Rules (Rule 46 tax invoice, Rule 47 time limits) and rate notifications
- GST e-Invoice System (Invoice Registration Portal)
- GST Portal — GSTN advisories
General information, not tax advice. Current as of October 2026 — confirm thresholds and rates with the GST portal or your accountant.