Answer first: a compliant GST tax invoice needs the Rule 46 mandatory fields (parties with GSTINs, a consecutive ≤16-character serial unique in the financial year, date, place of supply, HSN per item, taxable values and the correct CGST/SGST or IGST split, reverse-charge declaration, signature); the right HSN digit count for your turnover; issue at or before removal for goods; the right number of copies; and — if your AATO exceeds ₹5 crore — an IRN and signed QR from the IRP, without which the invoice is treated as not issued. This guide turns that into a checklist your billing team can actually run. Rules evolve by notification; treat this as current to July 2026 and confirm specifics with your CA.
The Rule 46 mandatory fields
Rule 46 of the CGST Rules is the canonical list. On every tax invoice, check:
- Supplier identity — your legal name, address and GSTIN, exactly as registered.
- Serial number and date — consecutive, ≤16 characters, unique within the financial year (rules below).
- Recipient identity — name, address and GSTIN for registered buyers; for unregistered buyers on invoices of ₹50,000+, name, address and place of supply.
- Place of supply and delivery address — with state name and code where the supply is inter-state; this is what justifies the IGST decision.
- Line detail — description, HSN/SAC, quantity with unit, rate, total value, discount if any, and the resulting taxable value.
- Tax detail — the GST rate and the tax amount, shown split as CGST + SGST (intra-state) or IGST (inter-state), plus cess where applicable.
- Reverse charge declaration — state whether tax is payable on reverse charge.
- Signature — the supplier's signature or digital signature; on e-invoices the IRN and signed QR stand in for it.
Amount in words is not itself a Rule 46 item, but it is universal Indian practice and a cheap fraud control — a total that reads "Rupees One Lakh Twenty Thousand Only" in proper Indian numbering is hard to tamper with. Any serious billing system prints it automatically; see GST Tax Invoicing.
Serial number discipline — 16 characters, no gaps unexplained
The serial number is where audits start, because it is where shortcuts show. The rules: ≤16 characters; letters, numerals, hyphen and slash only; consecutive; unique within the financial year. Multiple series are fine — per branch, per document type, per year (INV/26-27/0001) — provided each series is itself consecutive and declared in GSTR-1.
What the rules imply is more demanding than what they say: every number must be accounted for. A cancelled invoice explains its own gap — which is why cancellation must be a formal state that preserves the document, not a delete key. A parallel "kaccha" series, recycled numbers, or a number edited after issue are the three classic red flags, and all three are habits that template-based billing quietly encourages. The pillar guide covers why a single linked document chain is the structural fix.
HSN digits — what your turnover requires
| Aggregate turnover (preceding FY) | B2B invoices | B2C invoices |
|---|---|---|
| Up to ₹5 crore | 4-digit HSN mandatory | Optional |
| Above ₹5 crore | 6-digit HSN mandatory | 6-digit HSN mandatory |
| Notified goods / exports | 8-digit where prescribed | |
The digit rule is easy; consistency is the real work. The same HSN must appear on the invoice, in your item master and in the GSTR-1 HSN summary — three places, one truth. Map every item to its HSN and GST rate once in the master (bulk import beats hand-editing a thousand items), and the invoice line and the return table both inherit it. Mismatched or missing HSN is also a hard rejection at the IRP if you are in the e-invoicing mandate.
The tax split, rounding and amount in words
Three checks close out the money section:
- The split matches the geography. Same state: CGST + SGST, half each. Different state: one IGST at the full rate, justified by the place of supply on the face of the invoice. Exports: zero-rated under LUT or with tax as applicable. This decision should come from the buyer's GSTIN and your tax configuration — never from an operator's memory.
- Rounding is consistent. Section 170 permits rounding the tax to the nearest rupee; whatever rule you adopt must be identical on the printed invoice, in the books and in the return — paise drift across hundreds of invoices is how GSTR reconciliation weekends are born.
- The total is spelled out — amount in words, Indian numbering, matching the figure to the paisa.
Time limits and copies
When to issue: for goods, at or before removal (where supply involves movement) or at delivery; for services, within 30 days of supply — 45 days for banks, insurers and NBFCs. For goods businesses the practical rule is that the paperwork question must be settled before the vehicle moves: either the invoice travels with the goods, or a delivery challan moves them (job work, approval basis, branch transfer) and the invoice follows in the permitted pattern — with the e-way bill matching whichever document is in the cab.
How many copies: goods need three — original for the recipient, duplicate for the transporter, triplicate for the supplier. Services need two. Where e-invoicing applies, the QR-bearing document can travel digitally, but checkpoint practice still favours the classic copy discipline for moving goods.
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Signature, e-invoice IRN and the two QR codes
A physical or digital signature closes an ordinary invoice. Two portal-era additions sit on top:
- The e-invoice IRN and signed QR — mandatory on B2B invoices, exports and B2B credit/debit notes if your AATO has exceeded ₹5 crore in any year since 2017-18. No IRN means the invoice is treated as not issued; the signed QR then also satisfies the signature requirement. Full mechanics in the e-invoicing guide.
- The B2C dynamic QR — a separate obligation entirely: businesses above ₹500 crore turnover must print a dynamic payment QR on B2C invoices. SMEs are outside it; do not let a vendor conflate the two.
Credit and debit notes — the same bar applies
Notes are invoices' formal siblings and carry near-identical requirements: the issuer's details, a serial number in a consecutive series, the date, the number and date of the original invoice they adjust, the taxable value and tax being reversed or added, and the recipient's details. A note that floats free of its invoice fails both the rule and the reconciliation — it is precisely the unlinked credit note that turns a sales return into a GSTR mismatch. Systems that raise the credit note automatically against the original invoice on a sales return, as described in Credit & Debit Notes, make this compliance structural rather than procedural.
The one-page checklist
| # | Check | Pass condition |
|---|---|---|
| 1 | Supplier identity | Legal name, address, GSTIN as registered |
| 2 | Serial number | ≤16 chars, consecutive, unique in FY, series declared in GSTR-1 |
| 3 | Recipient & place of supply | GSTIN verified for B2B; state name and code for inter-state |
| 4 | HSN per line | 4 digits (≤₹5cr, B2B) or 6 digits (>₹5cr); matches item master |
| 5 | Taxable values | Qty × rate − discount, per line, arithmetic verified |
| 6 | Tax split | CGST+SGST intra-state, IGST inter-state; rates from tax master |
| 7 | Reverse charge | Declared yes/no |
| 8 | Rounding & words | One rounding rule; amount in words matches to the paisa |
| 9 | IRN & QR (if mandated) | Generated before issue; printed on the buyer's copy |
| 10 | Timing & copies | Issued at/before removal (goods); 3 copies goods, 2 services |
| 11 | Signature | Signed or digitally signed — or IRN/QR standing in |
| 12 | Linkage | Invoice references its dispatch/order; notes reference their invoice |
How software enforces the checklist
Every row above is either a memory task or a system property — and the difference is the whole argument for billing software. In Fast Billing Software, the party master carries verified GSTINs (bulk-imported once), the item master carries the HSN and rate map, the CGST/SGST/IGST decision is computed from the buyer's state, rounding and amount in words are automatic, series are consecutive by construction with cancellations preserved as formal states, and every invoice is raised against a real dispatch with a used-quantity guard — so row 12, the one auditors care most about, is enforced by the data model itself. The same confirmed documents feed e-invoice, e-way bill and GST needs and post to Tally as vouchers, and the pricing does not tax you per document for the discipline.
What a clean invoice trail looks like when someone asks
An officer picks invoice 0847 from last October. The firm produces it in seconds: the invoice shows its dispatch reference and HSN lines; the series runs unbroken around it, with 0846's cancellation on record and its quantity re-billed on 0851; the credit note from November points back at 0847; the same figures sit in GSTR-1, in the Tally sales register and on the signed copy the buyer holds. The audit moves on, because there is nothing to reconcile — the documents were born agreeing.
Frequently asked questions
What fields are mandatory on a GST tax invoice?
Rule 46 requires: supplier name, address and GSTIN; a consecutive ≤16-character serial unique in the financial year; date; recipient details with GSTIN if registered; delivery address and place of supply with state code for inter-state supplies; HSN/SAC per item; description, quantity, value and taxable value; the GST rate and tax split as CGST+SGST or IGST; a reverse-charge declaration; and signature or digital signature — which the IRN and signed QR replace on e-invoices. Amount in words is standard practice though not a Rule 46 field.
What are the GST invoice serial number rules?
≤16 characters; letters, numerals, hyphen and slash only; consecutive; unique within the financial year. Multiple series are allowed if each is consecutive and declared in GSTR-1, and every gap must be explainable — a cancelled invoice accounts for its number. Recycled numbers, parallel unofficial series and post-issue edits are the classic red flags.
How many HSN digits must my invoices show?
Turnover up to ₹5 crore: 4-digit HSN on B2B invoices, optional for B2C. Above ₹5 crore: 6-digit on all invoices. 8-digit where prescribed for notified goods and exports. The invoice, the item master and the GSTR-1 HSN summary must use the same codes — map items to HSN once in the master and let everything inherit it.
What is the time limit for issuing a GST invoice?
Goods: at or before removal where the supply involves movement, or at delivery. Services: within 30 days — 45 for banks, insurers and NBFCs. Practically, for goods the paperwork must be settled before the vehicle leaves: the invoice travels with the goods, or a delivery challan moves them and the invoice follows in the permitted pattern.
How many copies of a GST invoice are required?
Goods: three — original for the recipient, duplicate for the transporter, triplicate for the supplier. Services: two. Under e-invoicing the QR-bearing document can travel digitally, but the copy discipline for physical goods movement remains what checkpoints expect.
Do all my invoices need an IRN and QR code?
Only if your AATO has exceeded ₹5 crore in any year since 2017-18, and only on B2B invoices, exports and B2B credit/debit notes. B2C invoices never carry an IRN — the separate dynamic payment QR applies only above ₹500 crore turnover. If you are mandated and skip the IRN, the invoice is treated as not issued. Confirm your position with your CA.