Billing & GST

GST Invoice Format in India: Rules, Fields and Mistakes That Invite Notices

Mandatory fields, HSN/SAC codes, B2B vs B2C formats, e-invoicing thresholds and the common mistakes that turn routine invoices into GST notices.

A GST invoice is the most important document your business produces — and also the one most likely to quietly invite a notice. The rules aren't complicated, but they are exact, and small omissions (a missing HSN code, a broken invoice number, a wrong GSTIN) accumulate into compliance risk and blocked input tax credit for your customers. This guide lays out the mandatory fields, the format rules, and the mistakes that put you on the department's radar — so you can get it right at the source.

The mandatory fields on a GST invoice

A tax invoice under GST must carry a specific set of details. Missing any of them can render the invoice non-compliant:

• Supplier details — your name, address and GSTIN.
• A consecutive invoice number — unique for the financial year, with no gaps.
• Date of issue.
• Recipient details — name, address and GSTIN (for B2B); for B2C above the threshold, the address and state.
• HSN code (goods) or SAC code (services) for each line item.
• Description, quantity and taxable value of each item.
• Tax rate and amount — split correctly into CGST + SGST (intra-state) or IGST (inter-state).
• Place of supply — which decides whether it's CGST/SGST or IGST.
• Whether reverse charge applies.
• Signature or digital signature of the supplier.

CGST + SGST vs IGST: getting the split right

This is where manual billing goes wrong most often. The rule is simple but unforgiving:

• Intra-state supply (buyer and seller in the same state) — charge CGST + SGST, split equally.
• Inter-state supply (different states) — charge IGST at the full rate.

The determining factor is the place of supply, not just the buyer's billing address. Getting this wrong means either short-charging tax (a liability for you) or wrongly charging it (a problem for your buyer). Software that knows the buyer's state and applies the correct split automatically removes this error entirely.

The mistakes that invite notices

The department's systems now cross-match invoices automatically, so errors surface fast. The usual culprits:

• Broken or duplicate invoice numbers — a gap looks like a hidden sale.
• Missing HSN/SAC codes — now required even for smaller taxpayers.
• Wrong buyer GSTIN — blocks their input tax credit and bounces back to you.
• Wrong tax split — CGST/SGST where IGST was due, or vice versa.
• Mismatch with your returns — invoices that don't reconcile with GSTR-1 and GSTR-3B.

Every one of these is preventable at the billing stage. See our GST returns guide for how these errors ripple into your filings.

E-invoicing: know if it applies to you

Businesses above the notified turnover threshold must generate e-invoices — invoices registered on the government's IRP portal, which return an IRN and a QR code that must appear on the invoice. Even if you're below the threshold today, building on a system that supports e-invoicing means you won't have to rip and replace when you cross it.

Frequently asked questions

What are the mandatory fields on a GST invoice?

Supplier and recipient details with GSTINs, a consecutive invoice number, date, HSN/SAC codes, item description and value, the correct tax split, place of supply, and signature.

When do I charge IGST instead of CGST + SGST?

Charge IGST for inter-state supplies (different states) and CGST + SGST for intra-state supplies (same state), based on the place of supply.

Is an HSN code mandatory on every invoice?

HSN/SAC codes are now required across most taxpayers; the number of digits depends on turnover. Missing codes are a common notice trigger.

What happens if my invoice numbers have gaps?

Gaps or duplicates in the sequence look like unreported sales and are a classic red flag. Invoice numbers must be unique and consecutive for the financial year.

Get every invoice right, automatically

BecozAI generates fully GST-compliant invoices — correct CGST/SGST/IGST split by place of supply, HSN/SAC codes, unbroken sequential numbering and e-invoicing support — across all 17 industry templates. Also read our complete GST billing guide, then start free for six months and stop worrying about the format.