GST Invoice Format 2026: The Rule-46 Mandatory Fields

Reckonist EditorialPublished July 23, 20265 min read

A GST tax invoice is not just a bill — it is the document your buyer needs to claim input tax credit (ITC). Miss one mandatory field and the invoice is legally defective, and your customer may be unable to recover the tax they paid you. The fields are not a matter of style: they are fixed by Rule 46 of the CGST Rules, 2017. This guide walks through exactly what a compliant GST invoice must carry, how the CGST/SGST-versus-IGST split works, and the e-invoicing caveat that trips up growing businesses.

The Rule-46 mandatory fields

Under Rule 46 of the CGST Rules, a tax invoice for a taxable supply must show every one of the following:

Supplier name, address and GSTIN — the registered legal identity issuing the invoice.
A consecutive serial number, unique for the financial year, containing only letters, digits, hyphens or slashes — and no more than 16 characters.
Date of issue.
Recipient name, address and GSTIN (if the buyer is registered). For an unregistered buyer above ₹50,000, the name, address and the state with its code are still required.
Place of supply together with the state code, specifically for an inter-state supply.
HSN code (Harmonised System of Nomenclature) for goods, or SAC (Services Accounting Code) for services.
Description of the goods or services, with quantity and unit (UQC) for goods.
Taxable value of the supply, after any discount.
Rate of tax and the amount of tax charged, split into CGST and SGST/UTGST (intra-state) or a single IGST line (inter-state).
Whether tax is payable on a reverse-charge basis.
Signature or digital signature of the supplier or an authorised representative.

If any of these is missing, the recipient's ITC claim is at risk — which is why buyers routinely reject non-conforming invoices.

The invoice number rule: ≤ 16 characters, unique per FY

A surprising number of disputes come down to the serial number. Rule 46(b) is strict: the number must be consecutive, unique for that financial year, and at most 16 characters long, drawn only from alphanumerics plus hyphen and slash. A format like INV/2026-27/0001 is valid; a 20-character scheme with spaces or special symbols is not. Reset your series at the start of each financial year (1 April) and never reuse a number.

CGST + SGST or IGST — the place-of-supply rule

The single most misunderstood part of a GST invoice is the tax split, and it is decided by the place of supply, not by where the buyer is registered:

Intra-state supply (supplier and place of supply in the same state) → the GST rate splits equally into CGST (Central GST) and SGST/UTGST (State/UT GST). An 18% rate becomes 9% + 9%.
Inter-state supply (different states, or an export/import) → a single IGST (Integrated GST) is charged at the full rate. 18% becomes one 18% IGST line.

The total tax is identical either way — only the split changes.

Worked example — ₹50,000 taxable at 18% GST

Take a taxable value of ₹50,000 at an 18% GST rate.

Intra-state: CGST = ₹50,000 × 9% = ₹4,500, and SGST = ₹50,000 × 9% = ₹4,500. Total tax ₹9,000 → invoice total ₹59,000.
Inter-state: IGST = ₹50,000 × 18% = ₹9,000. Invoice total is still ₹59,000.

Same ₹59,000 to the buyer; the only difference is two tax lines versus one. Get the place of supply wrong and you charge the wrong tax head — a correction that means a credit note and a re-issue.

HSN / SAC: not optional above the turnover thresholds

Every taxable line needs an HSN code (goods) or SAC (services). The number of digits you must quote scales with turnover — smaller businesses quote fewer digits, larger ones the full code — but the code itself is mandatory on a B2B tax invoice. It is what classifies the supply and fixes the applicable rate, so buyers and the GST system both rely on it being present and correct.

Open the Invoice GeneratorCreate a compliant invoice and download it as a PDF, free and with no sign-up — a GST tax invoice (India, Rule-46), a VAT invoice (UK, HMRC), or a plain invoice (US) — plus estimates, quotes, receipts and purchase orders.

The e-invoicing (IRN / QR) caveat

Here is the trap for a scaling business. Once your aggregate turnover crosses the GST e-invoicing threshold, each B2B invoice must additionally be registered on the government Invoice Registration Portal (IRP), which returns an IRN (Invoice Reference Number) and a signed QR code that must be printed on the invoice. A Rule-46-perfect printed invoice is still not valid for those businesses unless it also carries the IRN and QR.

A stateless, printed-invoice generator produces the Rule-46 fields — it does not connect to the IRP or generate an IRN/QR. If you are within the e-invoicing mandate, generate the IRN/QR through your e-invoicing software or the IRP and add it to the invoice before issuing.

Key takeaways

A GST tax invoice is governed by Rule 46 of the CGST Rules — every mandatory field must be present or the buyer's input tax credit is at risk.
The invoice number must be consecutive, unique per financial year, and ≤ 16 characters (alphanumerics, hyphen, slash only).
The tax split is set by the place of supply: intra-state = CGST + SGST (each half the rate); inter-state = a single IGST at the full rate. On ₹50,000 @ 18% → CGST ₹4,500 + SGST ₹4,500, or IGST ₹9,000 — both a ₹59,000 total.
HSN/SAC codes are mandatory on B2B tax invoices; the digit-count scales with turnover.
Above the e-invoicing turnover threshold, a printed Rule-46 invoice must also carry an IRN and QR code from the IRP — a printed-invoice generator does not produce these.