Why the format actually matters
A GST invoice is not just a receipt — it is the legal document that lets your buyer claim input tax credit (ITC) and that ties your sales to your GSTR-1 return. If a mandatory field is missing or wrong, your customer can lose their credit and you can face queries at filing time. For a small shop, that usually shows up as a late-night scramble to re-issue bills before the 11th of the month.
The good news: the format is fixed and well defined. Once you know the fields a tax invoice must carry, you can check any bill in a few seconds — or let billing software fill them in for you so there is nothing to check.
The mandatory fields on a tax invoice
Under the CGST Rules, a tax invoice issued by a registered supplier must contain: your name, address and GSTIN; a consecutive invoice number (unique for the financial year, up to 16 characters, letters/numbers/slash/hyphen only); the date of issue; and the customer details.
For the customer, if they are registered you must show their name, address and GSTIN, plus the place of supply and the state code where the supply is inter-state. If the buyer is unregistered and the invoice value is above ₹50,000, you still need to show their name, address and the state of delivery.
On the line items you need: a description of the goods or services, the HSN or SAC code, quantity and unit, taxable value, the discount if any, the tax rate, and the tax amount split into CGST, SGST/UTGST or IGST. Finally, the invoice must show whether tax is payable on reverse charge, and it must be signed or digitally signed by the supplier or an authorised person.
CGST + SGST or IGST? Read the place of supply
This is the field people get wrong most often. The rule is simple once you frame it around location. If the supplier and the place of supply are in the same state, the sale is intra-state and you charge CGST plus SGST (each half of the total rate). If they are in different states, the sale is inter-state and you charge a single IGST at the full rate.
So a 18% item sold within Telangana carries 9% CGST + 9% SGST; the same item shipped to Maharashtra carries 18% IGST. Getting this wrong does not usually change the total the customer pays, but it puts the tax under the wrong heads in your return, which is exactly the kind of mismatch that triggers notices.
HSN codes and how many digits you need
Every product line should carry its HSN (Harmonised System of Nomenclature) code; services use the SAC code. How many digits you must print depends on turnover: businesses with aggregate turnover up to ₹5 crore need 4 digits, and those above ₹5 crore need 6 digits, on B2B invoices. Many shops simply store the full code against each item once and let it print automatically.
The invoice should also carry an HSN-wise summary — a small table totalling taxable value and tax by HSN. This is the part that must reconcile with the HSN summary in your GSTR-1, so it is worth getting right at the source.
Common mistakes that cost you at filing
The usual culprits: invoice numbers that reset or repeat within a year, a missing or mistyped customer GSTIN, the wrong tax split for an inter-state sale, HSN codes left blank, and round-off differences that do not match the return. Any one of these can hold up ITC for your buyer or flag a mismatch for you.
The reliable fix is to stop treating the invoice as something you assemble by hand. When your billing system reads the place of supply from the party, applies the correct tax heads, keeps the number series consecutive, stores HSN against each item, and rounds off consistently, a compliant invoice is simply the default output. RetailDek does exactly this — you pick the party and the items, and the compliant bill is what comes out.