GST Invoice Format Rules in India (2026) — What the Law Actually Requires

Most Indian businesses issue GST invoices every day. Most of those invoices are missing one or more fields required by law. The vast majority of the time, nothing happens — until a scrutiny notice lands, a buyer can’t claim ITC, or a GSTR-1 upload fails at the portal.

This guide is a practical checklist, not a legal treatise. Every field below is required under Section 31 of the CGST Act + Rule 46 of the CGST Rules, unless noted. Last refreshed for financial year 2026-27.

The 16 mandatory fields on a GST tax invoice

Here’s the full list a registered supplier must print on every tax invoice issued to a registered or unregistered buyer (for goods or services):

Supplier details (you)

1. Your name — legal business name as registered
2. Your full address — registered address from GSTIN
3. Your GSTIN — 15 characters, printed clearly
4. Your state + state code — 2-digit code (e.g., “24 — Gujarat”)

Invoice metadata

5. Invoice number — unique, sequential, max 16 characters, can include alphanumeric + ”/” and ”-” but nothing else. Must be serialised per financial year — INV/2026-27/001, INV/2026-27/002, etc.
6. Invoice date — in DD-MM-YYYY or DD-MMM-YYYY format
7. Place of supply — with state name + code, when it’s different from the supplier’s state (critical for CGST/SGST vs IGST split)

Buyer details

8. Buyer’s name
9. Buyer’s address — billing + shipping if different
10. Buyer’s GSTIN (if registered) — same 15-char format
11. Buyer’s state + state code (if unregistered) — required when supply is inter-state and invoice value exceeds ₹50,000

Line items

12. HSN code (for goods) or SAC code (for services) — 4-digit if turnover > ₹5 Cr; 6-digit if > ₹50 Cr; optional below ₹5 Cr but strongly recommended
13. Description of goods or services
14. Quantity + Unit Quantity Code (UQC) — e.g., 100 KGS, 50 PCS, 25 MTR. UQC must be from the GSTN-approved list.
15. Total value — per-line taxable value after discount

Tax & total

16. Tax breakup — CGST + SGST (intra-state) or IGST (inter-state), shown as separate amounts per line OR as a summary block; tax rate (5%, 12%, 18%, 28%) must be visible; grand total in figures + words

That’s the minimum. Additionally required for specific cases: e-invoice IRN + QR code (turnover > ₹5 Cr), reverse-charge indicator (if RCM applies), LUT reference (for exports), e-Way bill number (for goods > ₹50k moving).

Invoice number rules — the single most common mistake

Rule 46(b) says the invoice number must be:

• Unique per financial year (starts at 1 on 1 April, runs to 31 March)
• Maximum 16 characters
• Can contain letters, digits, hyphens, slashes only
• No spaces, no special characters (#, *, @ etc.), no Unicode

Common mistakes I see in real SMB invoices:

What businesses do	Why it fails
INV-1 INV-2 … INV-500	Not invalid, but you lose FY context — switch to INV/2026-27/001 so GSTR-9 reconciles easily
Resetting to 1 mid-year	Violates the “sequential” requirement in Rule 46(b)
Bill #001 with a #	# isn’t in the allowed character set
INV 001 with a space	Spaces break validation on the portal
Using the same number on two invoices	Single biggest cause of GSTR-1 upload rejection

What a clean format looks like: MT/2026-27/00147 — short business prefix + FY + zero-padded sequence. Portal accepts it, GSTR-9 reconciles to it, and a 5-year-old invoice is instantly placeable.

HSN / SAC code requirements — updated thresholds

The turnover-based thresholds for mandatory HSN on invoices, as of 2026:

Aggregate turnover (previous FY)	HSN digits required	Applies to
≤ ₹1.5 Cr	Optional	All invoices
₹1.5 Cr – ₹5 Cr	2 digits	All invoices
> ₹5 Cr	4 digits (B2B), 4 digits (B2C)	All invoices
> ₹50 Cr (under e-invoicing)	6 digits	All invoices

Practical advice: even if you’re below the threshold, include 4-digit HSN anyway. It’s one extra column on your invoice, it signals professionalism to B2B buyers, and it auto-populates correctly in your GSTR-1. We wrote a full HSN directory for the top 20 codes Indian SMBs use.

Place of supply — the rule that determines CGST/SGST vs IGST

Wrong place-of-supply is the #1 cause of GSTR-1 rejection on upload.

Quick rule:

• Goods: place of supply = where the goods are delivered (if movement is involved) or where the buyer takes delivery
• Services: place of supply = buyer’s location (if B2B with GSTIN) or service performance location (if B2C)

Tax split:

• Seller state ≠ Place of supply state → Inter-state supply → IGST
• Seller state = Place of supply state → Intra-state supply → CGST + SGST (50/50)

The GSTIN’s first two digits are the buyer’s state code. If your software auto-fills place-of-supply from the buyer’s GSTIN state, you’ll be right 99% of the time. The 1% edge case is “bill to X, ship to Y” — place of supply follows the ship-to, not bill-to. Modern billing software handles this; Tally doesn’t always.

Common compliance mistakes that trigger scrutiny

These aren’t hypothetical — they show up in actual GST audit notices:

1. Missing HSN on B2B invoices above threshold

Even if you’re at ₹5 Cr turnover and issuing B2B invoices, some businesses still skip HSN. Your buyer can’t claim ITC without it. They call you, you re-issue, compliance headache.

2. Wrong place-of-supply → wrong tax split

Surat seller ships to Mumbai buyer, marks place-of-supply as Gujarat (24) out of habit. Charges CGST+SGST. Buyer tries to claim as IGST-credit, mismatch, rejection, angry call.

3. Invoice number discontinuity

Issue INV/001 to INV/050, then accidentally issue INV/052 (skipping 051). GSTR-1 will show the gap; department might ask you to explain INV/051.

4. Amount in words doesn’t match figures

“₹1,47,200 (Rupees One Lakh Forty Seven Thousand Two Hundred only)” vs an actual figure of ₹1,47,200. This trips up scrutiny AI tools automatically. A manual-entry workflow causes this; auto-computed workflows don’t.

5. Credit note issued without reference to original invoice

Rule 53(1A) requires a credit note to clearly reference the original invoice number + date. Just issuing a “CN/001” without cross-reference breaks the ITC chain for your buyer.

6. Signature / DSC missing

Physical invoices need an authorised signatory signature. Digital invoices need a valid DSC or an e-signature via Aadhaar. “Printed invoice without signature” is technically invalid, though rarely challenged.

7. Using the word “Invoice” when it should be “Tax Invoice”

Rule 46 requires the document to be titled “Tax Invoice” (not “Invoice” or “Bill”). Composition dealers use “Bill of Supply”. Many businesses get this wrong; it’s a small fix with zero cost.

E-invoicing — who needs it (2026)

As of 2026, mandatory e-invoicing applies to businesses with aggregate turnover of ₹5 Cr+ in any previous financial year since 2017-18. For these:

• Every B2B / export invoice must have an IRN (Invoice Reference Number) generated from the government IRP portal before issue
• QR code printed on the invoice
• Once IRN is generated, invoice data auto-populates into GSTR-1

B2C invoices don’t require IRN — only B2B + exports.

Below ₹5 Cr turnover, e-invoicing is optional but moving towards universal. 21bill will add native IRN generation in Q3 2026. Today, above-threshold businesses use Tally or ClearTax Pro for this step; we’d recommend staying on those until our integration ships.

A compliant invoice — in plain terms

If you stripped down Rule 46 to the practical essentials, a valid GST invoice has:

TAX INVOICE

[Your Business Name]                          Invoice #: MT/2026-27/00147
[Your Address Line 1]                         Date:      21-Apr-2026
[Your City, Pincode]                          
GSTIN: 24ABCDE1234F1Z5                        
State:  Gujarat (24)                          Place of Supply: Maharashtra (27)

Bill To:                                      Ship To: (if different)
[Buyer Name]                                  [Buyer Name]
[Buyer Address]                               [Shipping Address]
GSTIN: 27XYZAB5678G2H9
State:  Maharashtra (27)

┌──────┬───────────────────┬──────┬─────┬──────┬──────┬─────────────┐
│ Item │ Description       │ HSN  │ Qty │ Rate │ Disc │ Taxable Val │
├──────┼───────────────────┼──────┼─────┼──────┼──────┼─────────────┤
│ 1.   │ Cotton saree      │ 5208 │ 100 │ 850  │  -   │   85,000    │
│ 2.   │ Polyester fabric  │ 5407 │ 500 │  85  │  -   │   42,500    │
└──────┴───────────────────┴──────┴─────┴──────┴──────┴─────────────┘

Subtotal:                                                  ₹1,27,500
IGST @ 5% (inter-state — Gujarat to Maharashtra):             ₹6,375
Grand Total:                                               ₹1,33,875

Amount in Words: Rupees One Lakh Thirty Three Thousand Eight Hundred
                 Seventy Five Only.

Bank Details: ...
Terms: Payment due within 30 days.

Authorized Signatory: ____________________

Any modern GST-compliant billing tool (21bill, Zoho Books, Tally) generates this correctly when you enter the right data. The problem is almost always the data, not the tool.

The compliance-by-software principle

Here’s the truth: you shouldn’t be memorising Rule 46. That’s what software is for.

A good billing tool enforces compliance by design:

• Sequential invoice numbering with no gaps possible
• HSN auto-populated from product master
• Place-of-supply auto-computed from buyer GSTIN
• CGST/SGST vs IGST auto-split
• Amount-in-words auto-generated from grand total
• “Tax Invoice” title hard-coded
• Credit notes auto-reference original invoice

If your current tool requires manual vigilance on any of these, you’re one mistake away from a scrutiny letter. Upgrade it.

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21bill ships with Rule 46 compliance by default. Pick a customer, pick items, hit save — the PDF that comes out is audit-clean. 21bill is invite-only; request access to evaluate it.

⚠️ Not tax advice. Rules change via CBIC notifications. Confirm with a CA before any legal filing.