GST invoice format guide: mandatory fields and a worked example

The GST tax invoice is the most important document your business produces — it is what your GST return is built on and what lets your customer claim input credit. Get the format wrong and the consequences are quiet but real: a mismatched return, an input-credit claim the buyer cannot make, a duplicate invoice number that surfaces in an audit. This guide walks through exactly what a compliant tax invoice must contain, the rules around it, and a fully worked example you can hold your own invoices up against.

It is written in plain terms for owners and accounts staff, not tax specialists. If you want the wider context first — what GST billing software is and how the whole sale-to-accounts flow fits together — start with the pillar guide, What is GST billing software?. For the software category itself, see What is GST billing software? — the buyer's guide.

1. Why the invoice format matters

A tax invoice is not just a bill — it is a legal document with a defined structure. Two parties rely on it: the tax authority, which matches it against your return, and your customer, who uses it to claim input tax credit. Both need the same specific fields present and correct.

When the format is right, everything downstream is easy: your GST returns reconcile, your customer's credit flows, and your accounts pick the invoice up cleanly. When it is wrong — a missing GSTIN, the wrong tax split, a repeated number — the problem does not appear at billing time. It appears weeks later at filing, or months later in an audit, when it is far harder to fix. Getting the format right once, in software that enforces it, is the cheapest insurance a business can buy.

2. The mandatory fields on a GST tax invoice

Below are the fields a B2B GST tax invoice should carry. In good billing software almost all of these are filled automatically from your company, item and party masters — you pick the customer and the items, and the rest appears.

Miss any of these on a B2B invoice and you risk a return mismatch or a rejected input-credit claim. This is exactly the kind of checklist a manual template cannot enforce — but software fills from your masters. See GST tax invoicing.

3. Invoice numbering rules

Numbering is where manual billing most often goes wrong, and it is easy to get right with software. The rules are simple:

• Unique for the year. Each invoice number must be unique within the financial year — no duplicates.
• Up to 16 characters. Letters, numbers and the special characters slash (/) and hyphen (-) are allowed.
• A continuous series. Numbers should run in sequence with no unexplained gaps, so an auditor can follow the series.
• A sensible structure. Many businesses use a prefix for the series or branch, the year, then a running number — for example INV/2026-27/00042.

Billing software generates the next number in the series automatically, so duplicates and gaps simply cannot happen — even when several people bill at once. That alone is a common reason businesses move off shared Word or Excel templates. See how the pillar guide frames why manual billing falls short.

4. B2B vs B2C and POS invoices

Not every sale needs the full B2B format. The difference comes down to who the buyer is, and a good system produces the right document for each — from the same items and tax master.

The practical point: a business that serves both — say a distributor with a trade counter, or a retailer with corporate buyers — should not need two systems. Running POS counter billing and B2B invoicing from one item and tax master keeps both correct and both in the same accounts. POS leans on barcode scanners and thermal printers; see barcode & POS hardware.

5. Intra-state vs inter-state — CGST/SGST or IGST

The single rule that decides the tax split is the place of supply versus your location:

• Intra-state (same state as you): the GST rate splits into two equal halves — CGST (central) and SGST (state). An 18% item becomes 9% CGST + 9% SGST.
• Inter-state (a different state): the same total rate is charged as a single IGST. That 18% item is a single 18% IGST.

Software reads the buyer's GSTIN and the place of supply and applies the correct split automatically, per line — so an invoice with a mix of rates is still correct throughout. This is the most common manual error, and the easiest one to eliminate. The pillar guide covers the HSN and GST basics in more depth.

6. Export invoices

An export is a zero-rated supply, and its invoice follows special rules rather than the domestic format:

• Two routes. Export under a Letter of Undertaking (LUT) without charging IGST, or export with IGST paid and later claimed as a refund.
• An endorsement. The invoice carries a line such as "Supply meant for export under LUT without payment of IGST".
• Extra details. Currency and conversion, and shipping-bill and port details where available.
• No CGST/SGST. Domestic GST is not charged; the treatment is zero-rated.

Because these rules differ from a domestic invoice, good software keeps a separate export-invoice variant so the right endorsement and treatment apply automatically — rather than someone editing a domestic invoice and hoping. Export handling sits alongside GST-return and e-way data in GST, e-Way Bill & e-Invoice.

7. e-invoice and QR basics

An e-invoice is not a different invoice — it is a normal tax invoice that has additionally been reported to the government Invoice Registration Portal (IRP), which returns a unique Invoice Reference Number (IRN) and a signed QR code printed on the invoice.

• Who it applies to. Businesses above the notified turnover threshold, for B2B and export invoices.
• What changes. The invoice layout is the same — the IRN and QR are simply added.
• How software helps. Because the invoice data already exists in the system, it can generate the IRN and QR from the same record, with no re-entry.

The QR lets anyone verify the invoice was reported, which is why it appears on the printed copy. Software that produces the invoice can produce the e-invoice, so a business crossing the threshold does not change how it bills — it just switches the feature on. See GST, e-Way Bill & e-Invoice.

8. A worked example

Here is an illustrative B2B tax invoice for a distributor billing a dealer in the same state — so the tax is CGST + SGST. All figures are examples only.

Adding it up, still with illustrative figures:

If the same dealer were in another state, the two GST lines would collapse into a single IGST @ 18% of ₹24,000 = ₹4,320, and the invoice total would be identical at ₹28,320 — only the split changes. That single decision, made automatically from the place of supply, is the heart of getting GST right. Header fields — invoice number INV/2026-27/00042, both GSTINs, place of supply, date and signature — complete the document.

9. How Fast Billing Software gets the format right

Fast Billing Software, built in Pune by Improsys under the Fast Technology brand and available cloud and on-premise, produces every one of these formats from your masters — so the operator picks the customer and the items, and the compliant invoice appears.

10. Frequently asked questions