GST Invoice Rules: What a Valid Tax Invoice Must Contain

The tax invoice is GST's atomic unit: your customer's ITC, your GSTR-1, the e-way bill, the audit trail — all hang off this one document. An invoice missing a mandatory field can cost your buyer their credit and you a penalty; a series maintained sloppily can unravel an assessment. The rules (Section 31, Rule 46) are precise and finite. Here they are, with the practice notes that keep invoices audit-proof.

The mandatory fields — Rule 46's checklist

• Supplier's name, address and GSTIN; a consecutive serial number (within the year, one or more series, 16 characters max)
• Date of issue; recipient's name, address and GSTIN (or delivery address and state for large unregistered-buyer invoices)
• HSN/SAC codes — 4 digits (turnover to ₹5 crore) or 6 digits (above), per current notifications
• Description, quantity, unit, total value; taxable value after discounts
• Rate and amount of tax, shown separately as CGST/SGST or IGST; place of supply with state name for inter-state
• Whether tax is payable on reverse charge; signature or digital signature (relaxed where e-invoiced/IRN-embedded)

Print these into your billing software's template once, and every invoice thereafter is compliant by default — the whole game is template discipline.

Time limits: when the invoice must exist

Goods: at or before removal/delivery. Services: within 30 days of supply (45 for banks/NBFCs). Continuous supplies and milestone contracts follow their payment/event schedules. Late invoicing is not cosmetic — it desynchronises GSTR-1, e-way bills and the buyer's 2B, manufacturing 'mismatches' that you'll explain for years. Advance receipts for services also demand a receipt voucher (and tax) even before the invoice.

Know your other documents

• Bill of supply: for exempt goods/services and composition dealers — no tax shown (composition must print 'not entitled to collect tax')
• Receipt/refund vouchers: for advances taken and returned
• Self-invoice + payment voucher: for RCM purchases from unregistered suppliers
• Delivery challan: for job work, exhibitions, stock movement without sale
• Credit/debit notes: the only lawful way to reduce/increase an issued invoice's value — linked to the original, reported in returns; credit notes with GST effect have a time bar (30 November of the following year)

E-invoicing: who's in and what changes

Businesses above ₹5 crore aggregate turnover (current threshold) must generate IRN-bearing e-invoices for B2B supplies, exports and credit/debit notes — the invoice is reported to the IRP, receives an IRN and QR code, and only then is a legal invoice. Practical rules: generate before or at supply (a 30-day reporting window applies to larger taxpayers), cancellation is only within 24 hours (after that, credit notes), and B2C invoices stay outside e-invoicing but larger taxpayers must print dynamic QR codes on them. Once e-invoiced, your GSTR-1 auto-populates — which means invoice errors now replicate faster, and the template discipline above matters more, not less.

Common defects that cost real money

Missing place of supply on inter-state invoices (buyer's ITC questioned); wrong GSTIN captured from WhatsApp instead of verified from the portal (credit lands in the wrong account — verify GSTINs at onboarding); discount treatment (pre-agreed discounts reduce taxable value on the invoice; post-supply discounts need credit notes with conditions); free samples invoiced casually (ITC reversal interplay); and 'revised' invoices issued like drafts (the law permits revision only in narrow registration-period cases — everything else is credit/debit note territory). Each defect is small; at audit, multiplied across a year, they price like a scheme.

How Aidwish helps

Aidwish configures compliant invoicing for clients — templates with every Rule 46 field, series design, e-invoicing onboarding and staff SOPs — and runs periodic invoice audits so the atomic unit of your GST never becomes the weak point.