A GST invoice is not just a bill — it is the document your buyer's input tax credit stands on, the row your GSTR-1 reports, and the first thing an officer reads in an audit. An invoice missing a mandatory field can cost the buyer their credit and cost you a penalty under Section 122 (₹10,000 or the tax involved, whichever is higher, for incorrect invoicing).

This guide covers the format rules as they stand in 2026. If you just want a correct invoice now, the free GST invoice generator produces a compliant PDF with every field below.

The 16 Mandatory Fields Under Rule 46

Rule 46 of the CGST Rules lists what every tax invoice must carry:

1. Supplier's legal name, address and GSTIN
2. A consecutive serial number, unique for the financial year (max 16 characters; letters, numerals, hyphen and slash allowed)
3. Date of issue
4. Buyer's name, address and GSTIN if registered
5. For unregistered buyers and invoice value ≥ ₹50,000 — buyer's name, address and state (the delivery address decides place of supply)
6. HSN code for goods / SAC for services (4 digits up to ₹5 crore turnover, 6 digits above)
7. Description of goods or services
8. Quantity and unit for goods
9. Total value of supply
10. Taxable value after discounts
11. Rate of tax — CGST and SGST/UTGST shown separately for intra-state, IGST for inter-state
12. Amount of tax for each component
13. Place of supply with state name, for inter-state supplies
14. Delivery address where it differs from the place of supply
15. Whether tax is payable on reverse charge
16. Signature or digital signature of the supplier or an authorised representative (not required on an e-invoice with IRN)

Amounts should also be stated in words — not strictly a Rule 46 field, but universal practice and expected by buyers' accounts teams.

Tax Invoice, Bill of Supply, or Something Else?

• Tax invoice — the default for registered dealers charging GST. The buyer's ITC depends on it.
• Bill of supply — for composition scheme dealers and exempt supplies. No tax column; a composition dealer must print "composition taxable person, not eligible to collect tax on supplies".
• Receipt voucher — for advances received before supply.
• Credit / debit note — to correct an issued invoice's value or tax; reported in GSTR-1 against the original.
• Revised invoice — for supplies made between the registration effective date and certificate issue.
• Consolidated invoice — allowed for B2C supplies under ₹200 per sale where the buyer does not ask for one, issued daily.

Tax Columns: CGST+SGST or IGST — Never Both

The split is decided by comparing the supplier's state code (the first two digits of the GSTIN) with the place of supply — not by what feels right on the invoice. Same state → CGST + SGST, each at half the rate. Different states → IGST at the full rate. Charging CGST+SGST on an inter-state sale is a real mistake with a painful fix: pay IGST, then claim a refund of the wrongly paid tax. The place of supply checker settles doubtful cases.

Copies Required

• Goods — triplicate: original for the recipient, duplicate for the transporter, triplicate for the supplier.
• Services — duplicate: original for the recipient, duplicate for the supplier.

With e-invoicing the IRN-registered copy with QR code serves the purpose; the transporter can carry it electronically alongside the e-way bill.

Time Limits for Issuing an Invoice

• Goods — on or before removal (where movement is involved) or delivery.
• Services — within 30 days of the supply; 45 days for banks, NBFCs and insurers.
• Continuous supply — on or before each due date of payment or statement.

E-Invoicing in 2026

Businesses with aggregate turnover above ₹5 crore must register every B2B invoice on the Invoice Registration Portal (IRP). The IRP returns an IRN (invoice reference number) and a signed QR code, both of which must appear on the invoice. Reporting must happen within 30 days of the invoice date for taxpayers above ₹10 crore. An e-invoice without an IRN is, legally, not an invoice at all — the buyer's ITC fails with it. The e-invoice JSON generator produces the IRP payload from invoice fields.

Common Invoice Mistakes That Cost Real Money

• Wrong tax split — CGST+SGST charged inter-state (or IGST intra-state); fix is pay-then-refund.
• Missing or 2-digit HSN — ₹25,000 penalty under Section 125, and GSTR-1 Table 12 will not reconcile.
• Broken serial series — unexplained gaps show up in GSTR-1 Table 13 and read as suppressed sales in an audit.
• No reverse-charge declaration — the field is mandatory even when the answer is "No".
• Rounding per line instead of per invoice — Section 170 rounds the final tax, not every row.
• Buyer GSTIN typo — the credit lands in a stranger's GSTR-2B; validate it with the GSTIN validator before issuing.

Frequently Asked Questions

Can I issue an invoice without the buyer's GSTIN?

To an unregistered buyer, yes. To a registered buyer, their GSTIN is mandatory — without it the sale reports as B2C and the buyer gets no credit.

Is a digital signature required?

A physical, digital or electronic signature of the supplier or authorised representative is required — except on e-invoices carrying an IRN, where the IRP's signed QR code stands in for it.

Can invoice numbers restart every year?

Yes — uniqueness is required within a financial year. Most businesses prefix the year (e.g. 2026-27/001) so the restart is self-evident.

What about export invoices?

Add the endorsement "Supply meant for export on payment of IGST" or "…under bond or LUT without payment of IGST", plus the buyer's country and, where applicable, shipping bill details.

Generate a Compliant Invoice Now

The DoAide GST invoice generator builds an invoice with every Rule 46 field — correct tax split from the two GSTINs, HSN per line, amount in words — and downloads it as a PDF. Free, no sign-up. Check the right rate first with the HSN code finder and verify your buyer with the GSTIN lookup.