---
url: /gst-manager-acellemail/guides/gstin-validation.md
description: >-
  How customer GSTINs are captured on the AcelleMail contact page, how GST
  Manager validates the 15-character format, and how a valid GSTIN switches an
  invoice between B2B and B2C.
---

# GSTIN validation

A customer's **GSTIN** — their 15-character GST registration number — is what tells GST Manager to
treat them as a business (**B2B**) rather than a consumer (**B2C**). This guide shows where that
number is captured, how it's validated, and what it changes on the invoice.

::: info B2B vs B2C
A customer **with** a valid GSTIN is **B2B**; a customer **without** one is **B2C**. The distinction
drives classification and appears as a separate section in your GSTR-1 report.
:::

## Where the GSTIN is captured

Your customer enters their GSTIN on the **AcelleMail contact page**, in their **Tax ID** field. The
field carries the hint **"Enter your 15-character GSTIN to receive GST tax invoices."** so customers
know why it's worth filling in, plus a format hint: **"Format: 22AAAAA0000A1Z5 — state code + PAN +
entity + Z + check digit."**

![The Tax ID field on the AcelleMail contact page](/gst-manager-acellemail/img/gst-gstin-validation-taxid.png)
*AcelleMail → Contact → Tax ID*

## How the number is validated

GST Manager checks the GSTIN as it's entered. A valid number is **15 characters** and breaks down as
`22AAAAA0000A1Z5`:

* **State code** — the leading two digits identify the state.
* **PAN** — the embedded 10-character PAN of the registered entity.
* **Entity** and **Z** — the entity code followed by a fixed `Z`.
* **Check digit** — the final character, verified against the rest.

When the format, state code, embedded PAN and check digit all pass, you see
**"GSTIN format is valid."** If any part fails, you see **"GSTIN is invalid."** — correct the number
before relying on it.

## What a valid GSTIN changes

Once a customer has a valid GSTIN on file, their invoices are classified **B2B**; without one, they
stay **B2C**. Combined with the customer's state and your home state, this decides whether tax
splits into **CGST + SGST** or a single **IGST** line — see
[GST classification](/gst-manager-acellemail/features/gst-classification) for the full logic. Make
sure your own **Company GSTIN** and home state are set first, in
[Configure GST](/gst-manager-acellemail/guides/configure-gst).

## Related pages

* [GST classification](/gst-manager-acellemail/features/gst-classification)
* [Configure GST](/gst-manager-acellemail/guides/configure-gst)