Also known as: Goods and Services Tax Identification Number · GST number
15-character alphanumeric identification number assigned to every GST-registered business in India, encoding state, PAN, and registration entity.
GSTIN (Goods and Services Tax Identification Number) is the 15-character alphanumeric ID assigned by GSTN to every business or person registered under the GST regime. The format is precise — the first two digits are the state code (07 = Delhi, 27 = Maharashtra, 29 = Karnataka, 33 = Tamil Nadu, and so on), the next ten characters are the entity's PAN, the 13th character is the entity code (1 for the first registration in a state, 2 for the second, and so on for additional places of business), the 14th is a fixed "Z", and the 15th is a check-sum digit.
GSTIN is required on every invoice you raise (your own GSTIN) and to claim ITC on every invoice you receive (your supplier's GSTIN). A wrong or expired GSTIN on a purchase invoice means you cannot claim the ITC — and depending on the buyer's registration status, it can also mean the invoice is treated as B2C in your GSTR-1 with downstream complications.
ReadyBooks.ai validates every GSTIN at the point of entry — when you add a customer, when you record a vendor, when you upload an invoice. The validation checks the check-sum, the state code consistency, and (via the GSTN API where enabled) the live registration status. Expired or cancelled GSTINs are flagged so you can refuse to record the invoice or treat it under RCM if applicable.