Is the invoice number format prescribed under GST?

GST law requires invoice numbers to be: consecutive (sequential), unique within a financial year, and not exceeding 16 characters. Special characters and alphabets are allowed. Common formats: INV-2026-27-001, GST/26-27/001. You can maintain separate series for different business verticals. The series resets on 1 April each year.

What happens if a mandatory field is missing from a GST invoice?

An invoice missing mandatory fields is not a valid GST tax invoice under the law. The buyer cannot claim Input Tax Credit (ITC) on such an invoice — GSTN may flag it during GSTR-2B reconciliation. Additionally, GST officers can deny ITC during assessments if invoices are non-compliant. Always ensure all mandatory fields are present before sharing invoices with clients.

Compliance Guide

Mandatory Fields in a GST Invoice — Rule 46 Complete Reference

Q: Is buyer GSTIN mandatory on a GST invoice?

Buyer GSTIN is mandatory for B2B (business-to-business) invoices where the recipient is registered under GST. For B2C (business-to-consumer) invoices where the buyer is unregistered, GSTIN is not required. However, for B2C invoices exceeding ₹2.5 lakh (inter-state) or ₹2 lakh (intra-state in some cases), the buyer's name and address are required.

Q: Is HSN code mandatory on all GST invoices?

HSN/SAC codes are mandatory based on the taxpayer's annual turnover. Businesses with turnover up to ₹5 crore must mention 4-digit HSN codes. Businesses with turnover above ₹5 crore must mention 6-digit HSN codes. For service providers, SAC codes are always 6 digits. HSN is not required for businesses with turnover below ₹5 crore for B2C invoices (as per CBIC notification).

Every GST tax invoice must contain the fields specified in Rule 46 of the CGST Rules 2017. Missing even one mandatory field can result in ITC denial for your buyer. Here is the complete checklist.

Rule 46 compliant 16 mandatory fields B2B & B2C rules ITC protection

All 16 Mandatory Fields — Rule 46, CGST Rules 2017

#	Mandatory Field	Details & Notes
1	Name, address, and GSTIN of supplier	Your registered legal name, business address, and 15-digit GSTIN. Must match your GST registration certificate.
2	Nature of the document	"Tax Invoice" for registered suppliers. "Bill of Supply" for exempt goods/services or composition dealers.
3	Consecutive serial number	Unique number not exceeding 16 characters. Can include letters, numbers, slashes, or dashes. Must be consecutive within a financial year.
4	Date of issue	The date the invoice is created. For goods: same as or before delivery date. For services: within 30 days of supply.
5	Name, address, and GSTIN of recipient	GSTIN required for registered buyers (B2B). For unregistered buyers (B2C): name and address. For B2C above ₹2.5L: name, address, and state name.
6	Place of supply	2-digit state code of the place of supply. Determines whether CGST+SGST or IGST applies. Example: 27 for Maharashtra, 29 for Karnataka.
7	HSN or SAC code	HSN for goods, SAC for services. 4 digits if annual turnover ≤ ₹5 crore; 6 digits if > ₹5 crore. Not required for B2C if turnover ≤ ₹5 crore (relaxation per CBIC).
8	Description of goods/services	Clear description of each item or service supplied. Should be specific enough to identify the commodity/service.
9	Quantity and unit of measurement	Quantity in applicable units (kgs, metres, numbers, hours). UOM must match GST unit codes (UQC). For services, quantity is often "1 job/service".
10	Total value of supply	Aggregate value of all items/services before any discounts or taxes.
11	Taxable value with discounts	Value after deducting any trade discounts. This is the amount on which GST is calculated.
12	Applicable tax rate	GST rate for each line item (0%, 5%, 12%, 18%, or 28%). Different items can have different rates.
13	Amount of tax (CGST/SGST/IGST/Cess)	Tax amount calculated on taxable value. CGST + SGST for intra-state; IGST for inter-state. Plus cess where applicable (tobacco, luxury cars, coal).
14	Whether reverse charge applies	"Yes" or "No" — indicates if the buyer must pay GST under RCM. Mandatory field even if answer is "No".
15	Signature or DSC of authorised signatory	Physical signature, printed name, or digital signature certificate (DSC). For e-invoices, IRN + QR code replaces this.
16	Total value of supply in words	"Rupees [amount] Only" — mandatory to write the total invoice value in words.

B2B vs B2C — Different Field Requirements

Field	B2B Invoice	B2C Invoice
Buyer GSTIN	Mandatory	Not required
Buyer Name & Address	Mandatory	Required above ₹2.5L (inter-state)
HSN/SAC Code	Mandatory (4 or 6 digit)	Relaxation for turnover ≤ ₹5 crore
Place of Supply	Mandatory	Mandatory
Invoice Serial Number	Mandatory	Mandatory
Tax Breakup (CGST/SGST/IGST)	Mandatory	Mandatory
Reverse Charge Declaration	Mandatory	Mandatory (usually "No")
Amount in Words	Mandatory	Mandatory

Frequently Asked Questions

How many mandatory fields are there in a GST invoice?

Rule 46 of the CGST Rules 2017 prescribes 16 mandatory particulars for a GST tax invoice, covering supplier details, buyer details, line item specifics, tax breakdown, and the authorised signature.

Is buyer GSTIN mandatory on a GST invoice?

Mandatory for B2B invoices where the buyer is GST-registered. For unregistered B2C buyers, GSTIN is not required — but name and address are required for invoices above ₹2.5 lakh.

Is HSN code mandatory on all GST invoices?

Yes, but the number of digits varies: 4 digits for businesses with turnover ≤ ₹5 crore; 6 digits for turnover above ₹5 crore. Businesses with turnover ≤ ₹5 crore get a relaxation for B2C invoices.

What is the invoice number format for GST?

Sequential, unique, maximum 16 characters. Alphabets, numbers, slashes, and dashes are allowed. Must be consecutive within a financial year. Series resets on 1 April.

What happens if a mandatory field is missing?

The invoice is not a valid GST tax invoice. The buyer cannot claim ITC on it, and GST officers can deny ITC during assessment. Always verify all 16 fields before sharing invoices.

Create a Rule 46-Compliant GST Invoice Instantly

Our free generator includes all 16 mandatory fields. Fill in your details and download a compliant PDF — no login required.

Create Invoice Free View Invoice Examples