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GST invoice rules every small business should know

6 min read · Updated July 2026

A GST invoice is not just a bill — it is a legal document. Get the fields right and your customers can claim input credit, your filings match, and audits are boring. Get them wrong and you invite mismatches, rejected credits, and penalties. Here is what a compliant GST invoice needs, in plain terms.

Note: GST rules and thresholds change over time. Confirm current requirements on the GST portal or with your accountant before finalising your invoice format.

What every tax invoice must contain

  • Your business name, address, and GSTIN.
  • A unique, consecutive invoice number and the invoice date.
  • The customer's name, address, and GSTIN (if they are registered).
  • Place of supply and the state code — this decides whether you charge CGST + SGST or IGST.
  • A clear description of goods or services, quantity, and rate.
  • The HSN or SAC code for what you are selling.
  • Taxable value, the GST rate, and the tax amount split into CGST, SGST, or IGST.
  • The total, and your signature (physical or digital).

CGST + SGST or IGST? The rule that trips people

If the supply is within your state, you split the GST into CGST and SGST. If it goes to another state, you charge a single IGST at the combined rate. Charging the wrong one is the most common mistake, and it means re-issuing the invoice. Our GST calculator helps you split the tax correctly before you type it in.

Common mistakes that cost credit

  • Missing or wrong GSTIN — the customer cannot claim input credit.
  • Non-sequential invoice numbers — a red flag in any audit.
  • No HSN/SAC code — increasingly required and easy to forget.
  • Wrong place of supply — leads to the CGST/SGST vs IGST error above.

Make it once, reuse it forever

The easiest way to stay compliant is a template that already has every mandatory field. Our free invoice generator builds a clean, GST-ready invoice you can download in seconds — no sign-up, no watermark. Pair it with the GST calculator and invoicing becomes a two-minute job that never fails an audit.