GST invoice format explained: what every B2B invoice must show

The GST invoice format is not a design choice. A tax invoice under GST must carry a defined set of details, and your client's ability to claim input tax credit depends on them. An invoice with the wrong GSTIN, a missing place of supply or IGST charged where CGST and SGST were due can delay your payment while the client asks for a corrected document.

This guide explains what every B2B tax invoice must show, when you issue a bill of supply instead, how to decide between CGST plus SGST and IGST, how exports under a Letter of Undertaking are shown, and how to number invoices. It is written for founders and finance leads at service and product companies who raise invoices themselves.

The GST invoice format: mandatory fields

For a tax invoice issued by a registered supplier, the GST rules list the particulars it must contain. In practical terms, check every B2B invoice for the following:

About you, the supplier

• Your legal name, address and GSTIN.
• A signature or digital signature of the supplier or an authorised representative.

About the invoice

• A consecutive serial number, unique for the financial year (see numbering below).
• The date of issue.
• Whether tax is payable on reverse charge.

About the client

• The recipient's name, address and GSTIN if registered.
• The delivery address, if different from the billing address.
• Place of supply, with the state name and code, for inter-state supplies.

About what you supplied

• A description of the goods or services.
• The HSN code for goods or SAC for services, to the number of digits required for your turnover.
• Quantity and unit, for goods.
• The total value and the taxable value after any discount.
• The rate and amount of each tax charged: CGST, SGST or UTGST, IGST and any cess, shown separately.

Two practical points are worth adding. First, clients check the GSTIN before anything else, because input tax credit depends on the invoice appearing against their GSTIN in your return. Keep client GSTINs on the client record and verify them when you add a client. Second, service invoices should normally be issued within the time limit after the service is supplied, so do not let a finished month sit unbilled.

Tax invoice or bill of supply?

A tax invoice is used when you charge GST. A bill of supply is used when you do not: for supplies that are exempt from GST, or when you are registered under the composition scheme, which does not allow you to collect tax from the client. A bill of supply carries similar identifying details, but no tax rate or tax amount.

Most service companies selling to other businesses issue tax invoices. If you also make some exempt supplies, keep a separate series or clear labelling so the two are never confused.

CGST and SGST or IGST: place of supply decides

GST is split by whether a supply is within one state or between states:

• Intra-state supply: when your location and the place of supply are in the same state, you charge CGST and SGST, each at half the applicable rate. At an 18% rate, that is 9% CGST and 9% SGST, shown on separate lines.
• Inter-state supply: when the place of supply is in a different state, you charge IGST at the full rate, for example 18% IGST.

The key is the place of supply, not where your client's head office is or where the work was done. For most B2B services, the general rule is that the place of supply is the location of the registered recipient, which in practice usually follows the state of the client's GSTIN. The first two digits of a GSTIN are the state code, so comparing your GSTIN's state code with the client's is a quick first check. Some services, such as those related to immovable property, events or transport, have their own place-of-supply rules, so check these with your CA.

For goods, the place of supply generally follows where the movement of goods ends for delivery to the recipient.

Getting this wrong is costly. If you charge IGST on what should have been CGST and SGST, or the reverse, the usual remedy is to pay the correct tax and claim back the wrong one, which ties up cash and adds paperwork for both sides.

Exports of services under LUT

Exports are treated as zero-rated supplies. A registered exporter can either pay IGST and claim a refund, or export without paying IGST under a Letter of Undertaking (LUT) filed on the GST portal for the financial year. Most service exporters use the LUT route because it avoids blocking cash in refunds.

An export invoice under LUT should:

• Charge no IGST, CGST or SGST.
• Carry a clear endorsement, typically "Supply meant for export under LUT without payment of IGST", along with the LUT reference.
• Show the client's name and overseas address, and the invoice currency and value.

Whether a particular service qualifies as an export depends on conditions, including where the recipient is located and how payment is received. Confirm this for each overseas client, and remember to renew the LUT each financial year.

Invoice numbering rules

Invoice numbers must be a consecutive serial, unique for each financial year, of not more than 16 characters. Letters, numbers, hyphens and slashes are allowed. A common pattern combines a prefix, the financial year and a running number, for example INV/26-27/0042.

• Restart or change the series each financial year so numbers stay unique within the year.
• You may run separate series, for example for exports or different branches, as long as each is consecutive.
• Do not reuse a number, even for a cancelled invoice. Correct an issued invoice with a credit note or debit note, not by editing it.

Gaps and duplicates are the first things an auditor or GST officer notices, so let your system issue the numbers rather than typing them.

A note on e-invoicing

Businesses above a notified turnover threshold must report B2B invoices to the Invoice Registration Portal and print the resulting reference and QR code on the invoice. The threshold has been lowered several times. Check with your CA whether it applies to you; if it does, the fields above still apply, and the e-invoicing requirements sit on top of them.

A quick checklist before you send a B2B invoice

1. Your name, address and GSTIN are correct.
2. The invoice number is the next in the series and the date is right.
3. The client's name, address and GSTIN match their registration.
4. The place of supply is shown, with state name and code.
5. Each line has a description and the correct HSN or SAC.
6. Taxable value is after discounts.
7. CGST and SGST for intra-state, or IGST for inter-state, at the correct rate, each on its own line.
8. For an export under LUT: no GST, and the LUT endorsement is printed.
9. Reverse charge is marked yes or no.
10. The invoice is signed.

After the invoice: receipts and TDS

An invoice is only half the job. Many business clients deduct TDS before paying a service invoice, so the amount received is less than the invoice total. Record each payment against the invoice with the TDS deducted, so receivables show what is truly outstanding and the TDS can be matched against your tax credit statement. Keeping GST output and TDS receivable in registers makes month-end and your CA's work much simpler. If you bill clients from timesheets, our article on timesheet to invoice shows how to close that loop.

Where Techno IQ fits

Techno IQ raises GST invoices from your company details and the client record: it charges CGST plus SGST or IGST by place of supply, shows the place of supply on the invoice, and issues zero-rated export invoices under LUT with the endorsement printed. Payments received are recorded against invoices with any TDS deducted, and GST and TDS registers are kept for your CA.

To see an invoice go from client record to payment received, book a demo of Techno IQ.