1. Why the invoice format matters In India, invoices are not just accounting documents, they are legal records that carry tax liability, determine input tax credit (ITC) eligibility, and are the primary evidence during a GST audit. A non-compliant invoice is worse than a missing one: it creates the appearance of a transaction you cannot substantiate. The Rule 46 checklist in the CGST Rules, 2017 defines exactly what every tax invoice must carry, and the practical consequences of skipping a field can include: the buyer being unable to claim ITC (direct cash loss for them), your business facing penalties under Section 122 of the CGST Act, and, in serious cases, registration suspension. This guide covers the format rules small businesses in India need to follow in 2026, the sector-specific variations, and the most common mistakes we see, with links to the specific generators in BillGenerator that handle each case correctly. 2. Types of invoices under GST Indian GST recognises several distinct invoice-like documents. Using the wrong one is a common error in small businesses: Tax invoice , issued by a GST-registered regular taxpayer for a taxable outward supply. Carries the GST rate and amount. Governed by Rule 46. Bill of supply , issued by composition-scheme dealers (Section 10) and for exempt supplies. No GST is charged; governed by Rule 49. Proforma invoice , not a legal tax document. It is a quotation that describes the proposed supply but does not give rise to any tax liability. Often used to request advance payment from a buyer before the actual supply. Receipt voucher , issued when an advance payment is received before the supply is made. Later adjusted when the tax invoice is issued. Debit and credit notes , used to adjust an already-issued tax invoice. Credit note when reducing the tax liability (discount, return); debit note when increasing it (price revision, additional goods). Picking the right document matters. A proforma marked "Tax Invoice" will confuse the buyer's ITC claim; a bill of supply with CGST and SGST lines will be rejected by a GST officer. For a GST tax invoice, use our free GST invoice generator which handles Rule 46 compliance automatically. The three documents small businesses confuse most often are the tax invoice, the bill of supply and the cash memo. Here is how they differ: 3. Rule 46, mandatory fields Rule 46 of the CGST Rules lists the fields every tax invoice must carry. Miss any of these and the buyer cannot claim ITC: Supplier's name, address, GSTIN , in the header block of the invoice. Serial number , unique within the financial year, up to 16 characters, using only alphanumerics, hyphen, and slash. Recipient's name, address, GSTIN or UIN , for B2B supplies or B2C supplies above ₹50,000 (on request). HSN code for goods or SAC code for services - see Section 5 for the turnover-linked depth. Description, quantity, unit of measure, rate per line item. Total taxable value , any discount, and net value. GST rate per line and total tax amount, split into CGST + SGST (intra-state) or IGST (inter-state) or UTGST (in a Union Territory). Place of supply with state code , essential for inter-state supplies. Address of delivery if different from the place of supply. Reverse-charge indicator , "Tax payable on reverse charge: Yes/No" per Rule 46(p). Signature of the supplier or authorised representative, physical, or digital with a Class-3 DSC. 4. GSTIN, reading the 15 characters Every GSTIN is structured as SS - PPPPPPPPPP - E - Z - C : SS , two-digit state code (e.g. 27 for Maharashtra, 29 for Karnataka, 07 for Delhi, 33 for Tamil Nadu). PPPPPPPPPP , the business's 10-character PAN. E , entity/registration sequence (1-9, A-Z) for multiple registrations under the same PAN. Z , a fixed default character. C , checksum digit (0-9, A-Z) computed from the other 14 characters. A valid GSTIN always satisfies the checksum formula. When you enter a GSTIN on our generator, we validate the checksum live, a green tick means the number is structurally valid (though not necessarily assigned to an active taxpayer; only the GSTN portal can confirm that). 5. HSN & SAC codes by turnover HSN classifies goods; SAC classifies services. Per CBIC Notification 78/2020 , the HSN depth required on your invoice depends on your aggregate turnover in the previous financial year: Turnover > ₹5 crore : 6-digit HSN on every invoice, both B2B and B2C. Turnover ₹1.5 – 5 crore : 4-digit HSN on B2B invoices; optional on B2C. Turnover < ₹1.5 crore : 4-digit HSN required on B2B invoices; optional on B2C. SAC codes always use 6 digits regardless of turnover, and are required on any invoice that includes a service. 6. CGST, SGST, IGST, inter-state vs intra-state The fundamental tax split is determined by the place of supply , not the billing address: Intra-state supply (supplier state = place of supply): charge CGST + SGST , split equally at half of the applicable GST rate each. 18% GST becomes 9% CGST + 9% SGST. Inter-state supply (supplier state ≠ place of supply, or cross-border): charge IGST at the full applicable rate. Union Territory (Chandigarh, Lakshadweep, Andaman & Nicobar, etc.): CGST + UTGST , practically identical to CGST + SGST. Export : zero-rated. Either pay IGST and claim refund via form RFD-01, or export under Letter of Undertaking (LUT) in form RFD-11 without paying IGST. For ride-hailing, hotels, and restaurants, the tax split follows the same logic but applies to sector-specific rates (see Section 11). 7. E-invoicing in 2026 E-invoicing means uploading your invoice to the government-operated Invoice Registration Portal (IRP) before issuing it to the buyer. The IRP returns an Invoice Reference Number (IRN) and a QR code that must be embedded in the printed invoice. From 1 August 2023 , businesses with aggregate turnover above ₹5 crore in any financial year from 2017-18 onwards must e-invoice all B2B supplies, per CBIC Notification 10/2023 . B2C supplies are not (yet) under e-invoicing but mandatory QR code requirements apply above ₹500 crore turnover. Our generator produces a fully printable tax invoice, but if your business is above the e-invoice threshold, you must still upload the invoice to the IRP and get the IRN before issuing it to the buyer. BillGenerator is a bill-formatter, not an e-invoice portal. 8. Invoice numbering best practices Rule 46(b) requires invoice numbers to be unique within the financial year, at most 16 characters, using only alphanumerics, hyphen, and slash. Beyond the rule, here's what works in practice:

Reset on 1 April each year. FY 2025-26 starts fresh from INV-001 or however you format it. Use a prefix for the year : 2526/INV/001 or FY26-INV-001 . Makes audit trails clearer. Never skip or re-use a number. Skipped numbers raise audit flags; re-used numbers are a compliance violation. If you issue both invoices and credit notes , keep separate sequences. Don't interleave. 9. Signatures, digital and physical Every invoice must be signed by the supplier or an authorised representative. Three options: Physical signature , signed ink on a printed copy. Required if you issue paper invoices. Digital signature (DSC) , Class-3 DSC is the standard for invoice signing in India. Required for electronic invoices where the DSC is the only authentication. Scanned signature image , a scanned ink signature embedded in the PDF. Commonly used by small businesses. Legally weaker than a DSC but widely accepted in practice. For e-invoices uploaded to the IRP, the IRP itself digitally signs the invoice payload and returns the signed QR, you don't need a separate DSC for the print. 10. State-specific quirks While GST is central, a few rules layer on at the state level: E-way bill , required for inter-state movement of goods valued above ₹50,000. Some states (Kerala, Punjab, West Bengal) also require it for intra-state movement above the same threshold. The e-way bill carries its own number that should be referenced on the invoice. Professional tax , levied by Maharashtra, Karnataka, West Bengal, and a few other states. Usually not on the invoice, but worth knowing if you're self-employed. Stamp duty on rent receipts , the ₹1 revenue stamp rule for cash rent above ₹5,000 per month is under the Indian Stamp Act, 1899, but the Act is state-administered. Each state can set its own rate. 11. Sector-specific nuances Different sectors have different mandatory fields and GST rates. Use these specific generators for the right format out of the box: Hotel bill , 12% GST up to ₹7,500/night tariff; 18% above. F&B on the folio taxed separately. Restaurant bill , 5% for non-AC standalone; 18% for AC or liquor-licensed. Service charge is voluntary per CCPA 2022. Cab / taxi receipt , 5% GST without ITC. Aggregator (Ola/Uber) collects the GST, not the driver. Medical bill , healthcare by clinical establishments is GST-exempt under Notification 12/2017. Doctor registration number is the key insurance-claim field. Petrol / fuel bill , petrol and diesel are outside GST (state VAT + central excise). Most often used for expense reimbursement. Rent receipt , for HRA exemption under Section 10(13A). Landlord PAN above ₹1 lakh annual rent; ₹1 revenue stamp on cash above ₹5,000 per month. 80G donation receipt , cash donations above ₹2,000 do not qualify for 80G. Donor's PAN required above ₹10,000. Form 10BE cross-reference filed by 31 May. 12. Common mistakes that cost money Quoting the buyer's state code on the GSTIN but the wrong place of supply. Results in the wrong tax split - IGST vs CGST/SGST. Buyer's ITC chain breaks. Using CGST+SGST on an export invoice. Exports are zero-rated; only IGST or LUT applies. Reusing invoice numbers across financial years. Must be unique per FY. Issuing a "Tax Invoice" as a composition dealer. Composition dealers issue Bill of Supply, not Tax Invoice. Skipping the reverse-charge declaration. For Section 9(3)/9(4) supplies, Rule 46(p) makes it mandatory. Missing HSN/SAC or using a code that doesn't match the line item. Common on legacy invoicing software. Accepting cash above ₹2,000 on an 80G donation. Disqualifies the deduction entirely. Charging GST on exempt healthcare. Notification 12/2017 makes it exempt, adding GST just annoys the patient and doesn't go to the government. Do I need GST registration to issue an invoice? No. Unregistered small businesses below the ₹20 lakh threshold can issue invoices without GST, but they cannot call them "tax invoices" and cannot charge GST. They can issue a plain invoice or bill of sale. Can I issue a handwritten invoice? Legally yes, provided all Rule 46 fields are present. Practically, software-generated invoices are more reliable and auditable. What's the difference between a proforma and a tax invoice? A proforma is a quotation, no tax liability is created. A tax invoice creates the tax liability and the buyer's ITC eligibility. Never call a proforma a "Tax Invoice". Do I need a separate invoice series for exports? Not strictly, but it's best practice. Export invoices require different fields (currency in foreign denomination, shipping bill number, LUT reference) and a separate series keeps the audit trail clean. How long do I keep old invoices? Under Rule 56, every registered person must retain GST records for at least six years from the due date of the annual return. Keep both digital and printed copies. Can I edit an issued invoice? No. Once issued, a tax invoice cannot be edited. Corrections are made via credit or debit notes that reference the original invoice number. Is a scanned signature legally valid? Yes for practical purposes, widely accepted by buyers and auditors, but a Class-3 DSC is legally stronger and required for e-invoice environments. What's the consequence of a wrong GST rate? Undercharging means you owe the difference plus interest under Section 50. Overcharging means the buyer can't claim ITC and can sue you for refund. Always verify the rate against the current GST schedule. Skip the formatting, generate compliant invoices in 30 seconds BillGenerator handles Rule 46 fields, GSTIN validation, HSN depth, CGST/SGST/IGST split, and sector-specific rates automatically.