The anatomy of a valid GST invoice

The mandatory fields (Rule 46)

Supplier's name, address and GSTIN. A consecutive, unique serial number for the financial year. Date of issue. Recipient's name, address and GSTIN for B2B (address and state for larger B2C). HSN/SAC code per line. Description, quantity, unit and value, with discounts shown. Tax rate and amount, split correctly. Place of supply for inter-state sales. A reverse-charge note where it applies. Signature or digital signature.

The serial number rule is the one small businesses break most: no gaps, no duplicates, one series per year (multiple series are fine if each is consistent). Your GSTR-1 reports these numbers; mismatches invite notices.

CGST+SGST or IGST — never both

Same state between supplier and place of supply → split the rate as CGST + SGST (18% becomes 9% + 9%). Different states → one IGST line. The buyer's state on a B2B invoice usually decides it. Charging the wrong pair is not cosmetic: the tax lands in the wrong government's account and must be unwound.

HSN digits and e-invoicing

Turnover above ₹5 crore: 6-digit HSN on every invoice, and e-invoicing — IRN plus QR code from the Invoice Registration Portal — is mandatory for B2B; an invoice that needed an IRN and lacks one is not a valid tax invoice at all. Up to ₹5 crore: 4-digit HSN on B2B invoices. Pick codes from the GST portal's directory, not from a supplier's old invoice.

When it's NOT a tax invoice

Composition dealers and wholly exempt supplies use a BILL OF SUPPLY — no tax shown, and for composition dealers the mandatory legend "composition taxable person, not eligible to collect tax on supplies". Post-sale reductions ride on a CREDIT NOTE (s.34) referencing the original invoice, reportable by 30 November after the financial year.