GST Invoice Format for Services: Mandatory Fields, Sample and Rules (2026)

Small invoice mistakes can block your client's input tax credit. Here are the mandatory fields, SAC code rules and time limit for a GST tax invoice for services.

Most GST invoice problems are not dramatic. A missing GSTIN, the wrong place of supply, an invoice number that repeats, a SAC code with too few digits. Each one looks small when you raise the invoice, and each one can come back as a rejected input tax credit claim for your client or a question from the department.

This guide walks through what a GST tax invoice for a service business needs, shows a sample layout, and covers the time limit that people forget.

What a GST tax invoice is

A tax invoice is the document that records a taxable supply and lets your client claim input tax credit. Section 31 of the CGST Act requires a registered person to issue one, and Rule 46 of the CGST Rules sets out what it must contain. You will see different websites quote different counts of "mandatory fields" (some say 16, some say 21) because they group things differently. Do not worry about the number. Focus on the particulars below.

What to include on the invoice

• The words "Tax Invoice" as the heading.
• Your details: name, address and GSTIN.
• A unique invoice number, consecutive, with no more than 16 characters.
• The invoice date.
• Your client's details: name, address and GSTIN if they are registered.
• SAC code for the service, with the right number of digits (see below).
• Description of the service.
• Taxable value, after any discount.
• Tax rate and tax amount, split into CGST and SGST, or IGST, as applicable (plus cess if any).
• Place of supply, with the state name and code.
• Whether tax is payable on reverse charge.
• Signature or digital signature of you or your authorised representative.

CGST/SGST or IGST: how to decide

This follows the place of supply. If your business and your client are in the same state, the tax splits into CGST and SGST. If they are in different states, you charge IGST. In the sample above, the supplier is in Maharashtra and the client is in Karnataka, so the invoice shows IGST at 18%.

SAC code: how many digits

Every service has a Service Accounting Code, and your invoice should show it. The number of digits depends on your turnover.

Rebuild with the Table button, 2 columns:

The 30-day time limit for services

For services, you must issue the tax invoice within 30 days of the date you supplied the service. Banks, insurers and NBFCs get 45 days. This catches many agencies and consultants who bill at the end of a project, or bill several months of a retainer in one go.

Tax invoice, bill of supply and proforma invoice

Rebuild with the Table button, 3 columns:

If you make a mistake

Do not overwrite an issued invoice. If the value or tax needs to change, you issue a credit note or a debit note, which follows the same format and carries a reference to the original invoice. A wrong invoice number, GSTIN or place of supply should be fixed through the correct document, with your CA's guidance.

If your turnover has crossed Rs 5 crore, your B2B invoices may also need an IRN generated through e-invoicing. We cover that in

MoneyGence includes GST invoicing. Whichever tool you use, check that it covers every item in the list above and that you can set your own numbering series.

Common Mistakes

• Repeating or skipping invoice numbers.
• Using the wrong place of supply, which flips CGST/SGST to IGST or the reverse.
• Typing the client's GSTIN wrongly.
• Using a 4-digit SAC code after crossing Rs 5 crore.
• Issuing the invoice more than 30 days after the service.
• Editing an old invoice instead of issuing a credit or debit note.
• Calling a proforma invoice a tax invoice.

Frequently Asked Questions

What is the GST invoice format for services?

There is no single prescribed layout. The law prescribes what the invoice must contain, such as the heading, supplier and recipient details, a unique number, date, SAC, taxable value, tax and place of supply. You can arrange them in any clear format.

Within how many days must I issue a GST invoice for services?

Within 30 days of supplying the service. For banks, insurers and NBFCs the limit is 45 days.

Is a SAC code mandatory on a service invoice?

Yes on B2B invoices, with 4 digits up to Rs 5 crore turnover and 6 digits above.

How long can a GST invoice number be?

Up to 16 characters. It should be unique and consecutive within the financial year, and may include letters, numbers, a hyphen or a slash.

Can I issue a GST invoice without the client's GSTIN?

Yes, if the client is unregistered. The invoice is then treated as B2C. A registered client's GSTIN should always be included so they can claim credit.