General information, not professional advice. GST rules, rates and due dates change by notification. Check the current position on the GST Portal or with your CA before you act on anything here.
The short version
- A tax invoice needs the supplier's name, address and GSTIN, a serial number, the date, the buyer's details, HSN or SAC, quantity, taxable value, rate, tax split, place of supply and a signature.
- The serial number can be at most 16 characters, must be unique for the financial year and must run in an unbroken series.
- Goods: issue it at or before removal or delivery. Services: within 30 days of supply.
- Composition dealers and exempt supplies get a bill of supply instead, with no tax on it.
- Cancelling an invoice keeps the number in the series; the right correction for a change in value is a credit or debit note.
What a tax invoice is for
Your buyer claims input tax credit on the strength of this document, so the information on it is not decoration. A field missing here is a query for them later, which is why the format is prescribed rather than left to taste.
If you are not registered, you issue a plain bill or cash memo instead, and if you are under composition you issue a bill of supply. Which document applies to you is a two-question decision.
The fields Rule 46 asks for
- Your name, address and GSTIN
- A serial number for the invoice (see below)
- The date of issue
- The recipient's name and address, and their GSTIN if they are registered
- For an unregistered recipient where the value is ₹50,000 or more: their name, address, the delivery address, and the state with its code
- HSN code for goods or SAC for services on each line
- Description, quantity with unit, and total value of the supply
- Taxable value after any discount
- Rate and amount of tax, split into CGST and SGST, or IGST, plus cess where it applies
- Place of supply with the state name, for inter-state supplies
- Delivery address, where it differs from the place of supply
- Whether tax is payable on reverse charge
- Signature or digital signature of the supplier or an authorised person
How many digits of HSN you print depends on your turnover, not on the item. Up to ₹5 crore aggregate turnover in the previous year it is 4 digits on B2B invoices, above that it is 6 digits on everything, and exports use 8. The HSN and GST rate finder gives you the code and the rate for everyday items.
The serial number rule
Three conditions, and each one catches somebody:
- 16 characters maximum. Letters, numerals and the characters hyphen and slash only. "SHREE/ENTERPRISES/2026-27/0001" is 30 characters and is not a valid invoice number.
- Unique for the financial year. Numbering restarts on 1 April, and two invoices in one year may not share a number.
- Consecutive. One unbroken series, or clearly defined multiple series. A gap invites the question of what was in it.
Which is why a cancelled invoice keeps its number and is reported as cancelled, rather than being deleted and reused.
When to issue it
For goods, at or before the time the goods are removed for delivery, or made available to the recipient. For services, within 30 days of the supply. Banks, insurers and financial institutions get 45 days.
For continuous supply, the invoice follows the contract: at each due date of payment, or at each statement, depending on how the contract is written.
How many copies
For goods, three: the original for the recipient, a duplicate for the transporter, and a triplicate for you. For services, two: original for the recipient and duplicate for you. Digital copies are fine; a PDF sent on WhatsApp counts as the recipient's copy, and your accounting record is the copy you keep.
A worked example
| Line | HSN/SAC | Qty | Taxable | Tax |
|---|---|---|---|---|
| Refined oil 1L | 1512 | 12 | ₹1,600.00 | CGST 2.5% ₹40.00 + SGST 2.5% ₹40.00 |
| Packing service | 998540 | 1 | ₹500.00 | CGST 9% ₹45.00 + SGST 9% ₹45.00 |
| Invoice total | ₹2,100.00 | ₹170.00, total ₹2,270.00 | ||
Goods and services can sit on one invoice, each with its own code and rate. What must not happen is one blended rate across the bill.
Five mistakes that cost your buyer their credit
- A wrong or missing buyer GSTIN. The invoice will not reach their GSTR-2B, so they cannot claim it.
- The wrong place of supply. Charging CGST and SGST on an inter-state sale means the buyer holds credit of the wrong kind.
- An invoice number over 16 characters, which the portal rejects when the return is filed.
- No HSN, or the wrong depth for your turnover band.
- Editing an issued invoice. Once it is out, a change in value is a credit or debit note, not a quiet correction.
How AawazAI fills this in
You say "Mehta Traders ka bill banao, 12 tel aur ek packing charge 500". The app picks the document type from your registration, fills your GSTIN and address from Settings, pulls the buyer's registered name from their GSTIN, takes HSN or SAC and the rate from your catalogue, decides CGST and SGST or IGST from the place of supply, and numbers the invoice inside the 16-character limit in an unbroken series. You see the card, confirm it, and the PDF goes to the customer on WhatsApp.
Stop typing invoice fields
Say the sale and AawazAI fills the GSTIN, HSN, tax split and serial number for you, then sends the bill on WhatsApp. Free to start.
Frequently asked questions
What is the maximum length of a GST invoice number?
Sixteen characters, using letters, numerals, hyphen and slash only. It must be unique within the financial year and part of a consecutive series. Long formats that include the full business name usually break this rule.
Can I issue one invoice for both goods and services?
Yes. Each line carries its own HSN or SAC code and its own rate, and the tax is computed line by line. What you cannot do is apply a single blended rate to a mixed bill.
Do I need to sign a GST invoice?
The rule asks for the signature or digital signature of the supplier or an authorised person. An invoice issued electronically and sent as a PDF, or one carrying a valid digital signature, satisfies this in practice.
What if I have to change an invoice after issuing it?
Issue a credit note if the value goes down or the goods come back, and a debit note if the value goes up. Do not edit or delete the original: the number stays in your series and both documents are reported in GSTR-1.
Is a tax invoice needed for every small sale?
For B2C sales below ₹200, a registered dealer can issue a consolidated tax invoice at the end of the day, unless the customer asks for one. Every B2B sale needs its own invoice.