GST
GSTIN validation: the complete checklist for businesses
Every GSTIN has 15 characters and one hidden check digit. Here's how to validate GSTINs quickly — and what a mismatch actually means for your invoices.
15 Jun 2026 · 5 min read · ToolBharat Team
Every GSTIN follows a rigid format: state code, PAN, entity number, Z, and a checksum. Invalid GSTINs on invoices can cause input-tax-credit denials and audit flags.
The format
Positions 1–2: state code. Positions 3–12: PAN of the taxpayer. Position 13: entity code within the state. Position 14: literal Z. Position 15: checksum.
Common mistakes
- Copying with trailing spaces or O/0 confusion.
- Using an old GSTIN after a change in constitution.
- Assuming the state code matches billing address (it doesn't have to).
Automating the check
Run bulk GSTINs through the GSTIN Validator. Rejected rows are highlighted with the specific reason (format, checksum, or state code).