GST Invoice Rules: Numbering, Mandatory Fields and E-Invoicing
A GST invoice with a missing field or a broken number series can cost your customer their input tax credit. This checklist covers numbering, mandatory fields, HSN digits, deadlines and when e-invoicing applies.
By BizToolsIndia Editorial Team · Published · 4 min read
Key takeaways
- Invoice numbers must be consecutive, unique for the financial year and at most 16 characters (letters, numbers, - and /).
- HSN codes need at least 4 digits if your turnover is up to ₹5 crore and 6 digits above that.
- Service invoices are generally due within 30 days of supply; goods invoices at or before removal or delivery.
- E-invoicing is mandatory once aggregate turnover crossed ₹5 crore in any year since 2017-18.
- If your turnover is ₹10 crore or more, e-invoices must be reported to the IRP within 30 days of the invoice date.
Invoice numbering: the 16-character rule
Rule 46 of the CGST Rules requires a consecutive serial number, unique for the financial year, of not more than 16 characters. It may contain letters, numbers, hyphens (-) and slashes (/). A number series must not repeat within the year.
| Format | OK? | Why |
|---|---|---|
| INV/26-27/0001 | Yes | 14 characters, year-wise series |
| MUM-2627-00045 | Yes | Branch prefix, separate series per branch is allowed |
| INVOICE_2026_27_0001 | No | Underscore not allowed and longer than 16 characters |
| 0001 (restarted every month) | No | Numbers repeat within the financial year |
Start a new series on 1 April each year, keep separate series for tax invoices, credit notes and branches if you like, and never reuse a cancelled number — record it as cancelled instead.
Mandatory fields checklist
- Supplier's name, address and GSTIN
- Invoice number and date
- Recipient's name and address, and GSTIN if registered
- Place of supply with the state name, for inter-state supplies
- Delivery address, if different from the place of supply
- Description of goods or services, with HSN or SAC code
- Quantity and unit, for goods
- Total value, discounts and taxable value
- GST rate and amount, split into CGST + SGST/UTGST or IGST
- Whether tax is payable on reverse charge
- Signature or digital signature of the supplier or authorised person
The GST Invoice Generator includes every field above and works out CGST/SGST or IGST from the place of supply. For a step-by-step walkthrough, see how to make a GST invoice.
How many HSN digits do you need?
| Aggregate turnover in the previous year | HSN/SAC digits on invoices |
|---|---|
| Up to ₹5 crore | 4 digits (mandatory for B2B invoices) |
| Above ₹5 crore | 6 digits |
When to issue the invoice
- Goods: on or before removal of goods for supply, or delivery where no movement is involved.
- Services: generally within 30 days of the date of supply (45 days for certain banking and financial services).
- Continuous supplies: on or before the due date of payment, or as set out in the contract.
E-invoicing: does it apply to you?
E-invoicing means reporting B2B invoices to an Invoice Registration Portal (IRP), which returns an Invoice Reference Number (IRN) and a QR code. It has applied since 1 August 2023 to businesses whose aggregate turnover exceeded ₹5 crore in any financial year from 2017-18 onwards. Once you cross the threshold, the requirement continues even if turnover later falls.
From 1 April 2025, businesses with aggregate turnover of ₹10 crore or more cannot report an invoice, credit note or debit note to the IRP more than 30 days after its date. Plan your billing cycle so documents are reported promptly.
Note
Tax invoice, bill of supply or receipt?
| Document | When to use it |
|---|---|
| Tax invoice | A registered supplier making a taxable supply |
| Bill of supply | Composition taxpayers, or exempt supplies — no GST charged |
| Payment receipt | Acknowledging money received; not a substitute for an invoice. Use the Receipt Generator |
| Quotation / proforma | Before the sale, to offer a price. Use the Quotation Generator |
Frequently asked questions
What is the maximum length of a GST invoice number?
16 characters, using letters, numbers, hyphens and slashes. It must be consecutive and unique for the financial year.
Can I have different invoice series for different branches?
Yes. Multiple series are allowed, as long as each is consecutive and no number repeats within the financial year.
Is e-invoicing mandatory below ₹5 crore turnover?
No. It is mandatory only if aggregate turnover exceeded ₹5 crore in any financial year from 2017-18. Smaller businesses can issue regular tax invoices.
What is the 30-day e-invoice rule?
From 1 April 2025, taxpayers with aggregate turnover of ₹10 crore or more cannot report documents older than 30 days to the IRP.