If you've ever had an invoice bounced back by a client's finance team, you know how frustrating it is. You did the work, you sent the bill, and now someone is asking you to reissue it because the GSTIN is missing or the tax split is wrong. Payment gets delayed by two weeks, and you're stuck explaining to your landlord why the rent is late.
The thing is, GST invoice formatting isn't complicated once you know the rules. But the rules are specific, and missing any one of 16 mandatory fields gives the buyer's AP department grounds to reject. This guide walks through every field, the HSN/SAC code system, and how CGST/SGST vs IGST actually works — so your invoices get paid the first time.
What Makes an Invoice "GST-Compliant"?
A GST invoice isn't just a bill with a tax number slapped on it. It's a legally defined document under Section 31 of the CGST Act, 2017, and Rule 46 of the CGST Rules. These rules specify exactly which fields must appear, where they go, and how tax components are displayed.
The rules apply equally to a freelancer billing Rs 5 lakh a year and a manufacturer billing Rs 500 crore. Size doesn't matter — format does.
The purpose is twofold: the invoice records the tax you owe on a supply, and it enables your client to claim input tax credit (ITC) on their own GST return. If the invoice is non-compliant, the ITC chain breaks. Your client can't reduce their tax liability by the amount you charged them, so they have no reason to pay you until you fix it.
The 16 Mandatory Fields
Rule 46 lists 16 fields that must appear on every GST invoice. Here's what they are and why they matter:
- Supplier name, address, and GSTIN — Your registered business name, address, and 15-character GSTIN.
- Invoice number — Sequential, no gaps, single series per financial year. Arabic numerals only.
- Date of issue — For services, must be within 30 days of supply completion.
- Recipient name, address, and GSTIN — Required for B2B. For B2C above Rs 50,000, name and address required but GSTIN optional.
- Place of supply — Determines whether CGST+SGST or IGST applies.
- HSN or SAC code — Mandatory at 4-digit level for businesses above Rs 5 crore.
- Description of goods or services — Clear enough for an auditor to verify.
- Quantity and unit — Pieces, hours, kilograms, etc.
- Total value — Pre-tax amount.
- Taxable value — After discounts, before tax.
- Rate of tax — 0%, 5%, 12%, 18%, or 28%.
- CGST and SGST amounts — For intra-state supplies.
- IGST amount — For inter-state supplies.
- Total invoice value — Taxable value plus all taxes.
- Reverse charge flag — Most supplies are forward charge.
- Signature — Digital or physical.
Miss any of these and the invoice is technically non-compliant. In practice, the most commonly missed fields are recipient GSTIN (on B2B invoices), place of supply, and HSN codes.
HSN and SAC Codes
HSN stands for Harmonized System of Nomenclature — a globally standardized 6-digit classification for goods, developed by the World Customs Organization. India uses an 8-digit version. SAC (Service Accounting Code) is the equivalent for services.
The purpose is simple: tax authorities use these codes to verify you're applying the correct GST rate. If you bill for "software development services" with SAC code 998314, the auditor can confirm that 18% is the correct rate for that category.
When HSN/SAC codes are required depends on your turnover:
- Above Rs 5 crore: 4-digit HSN mandatory on all B2B invoices.
- Rs 1.5 to 5 crore: 4-digit on B2B, 2-digit acceptable on B2C.
- Below Rs 1.5 crore: Not mandatory, but recommended.
For tech and service businesses, the most common SAC codes are 998314 (software development), 998313 (IT consulting), 998361 (hosting), and 998399 (other IT services, the catch-all).
CGST/SGST vs IGST — The Split That Trips Everyone Up
This is the field that causes the most rejections. The tax split depends entirely on whether you and your client are in the same state.
If you're both in Maharashtra, that's an intra-state supply. The total GST rate gets split equally between Central GST and State GST. For 18% GST, that becomes 9% CGST + 9% SGST, shown as separate line items on the invoice.
If you're in Karnataka and your client is in Maharashtra, that's an inter-state supply. The full 18% is charged as Integrated GST (IGST). There's no CGST/SGST split — just one IGST line item.
The buyer claims the same input tax credit either way. The split only affects which government collects the tax (central vs state).
Same Rs 1,00,000 invoice at 18% GST:
| Field | Same State | Different States |
|---|---|---|
| Taxable value | Rs 1,00,000 | Rs 1,00,000 |
| CGST @ 9% | Rs 9,000 | — |
| SGST @ 9% | Rs 9,000 | — |
| IGST @ 18% | — | Rs 18,000 |
| Total | Rs 1,18,000 | Rs 1,18,000 |
PDF Format — What's Required
GST law doesn't mandate physical printing. Digital PDFs are fully accepted, with a few practical requirements.
The text must be selectable, not scanned images. GST auditors use Ctrl+F to find specific fields in the invoice — if the PDF is a scan, they can't search it. Use vector text (which any PDF generator produces by default).
Keep the font at 10pt minimum for body text. HSN codes can be 8pt. Anything smaller is hard to read on printouts.
Invoice numbers must be sequential with no gaps. If you use a generator tool that doesn't track state between sessions, keep a manual log of your last invoice number. Gaps in numbering trigger audit questions.
Digital signatures aren't legally mandatory, but they speed up enterprise procurement approval. For B2B invoices above Rs 1 lakh, consider using a Class 3 digital signature.
Retention period is 6 years from the due date of filing the annual return. Keep digital copies organized — you won't remember which invoice number corresponds to which client two years from now.
Generating Compliant Invoices
You can generate GST-compliant invoices with our free GST Invoice Generator. It handles the CGST/SGST/IGST split automatically based on the supply type you select, includes HSN/SAC code fields per line item, and exports to print-ready PDF. The tool runs entirely in your browser, so your business details and client information never leave your device.
For related documents, the Quotation Generator handles pre-sale estimates and the Receipt Generator handles payment confirmations. All three follow the same GST format rules but serve different stages of the sales cycle.
What Usually Goes Wrong
The most common rejection reason is a missing recipient GSTIN on B2B invoices. Always request the client's GSTIN during onboarding — not when you're about to send the invoice. If they don't have one, document that fact so their AP team doesn't ask for it later.
Wrong supply type is the second most common. If you apply IGST to an intra-state supply (or vice versa), the buyer's ITC claim gets messed up. Double-check the client's state before generating the invoice.
Invoice number gaps are the third. Auditors treat gaps as potential unreported supplies. Use a single series per financial year with no breaks. If you void an invoice, keep a record of why.
Rounding errors are the fourth. Tax amounts must match exactly. Even a Rs 1 discrepancy between CGST+SGST and the expected IGST total causes rejection. Use a tool that calculates the split automatically.
Finally, missing place of supply. Without this field, the buyer's finance team can't verify whether you applied the correct tax split. Always include it.