• Each Invoice uploaded by the taxpayer will get a unique number called an Invoice Reference Number (IRN).
• IRN is 64 Characters in length.
• This IRN is a unique number in the GST system, irrespective of the taxpayer, financial year and document type.
• The e-invoice system generates IRN once the taxpayer uploads the invoice details.
• IRN is generated using the computer algorithm (RSA256) based on the combination of the Supplier GSTIN, Document Type, Document Number and year of the Invoice (Financial Year).
• IRN is the hash of Supplier GSTIN + Fin. Year + Doc Type + Doc Number.
• Financial Year is calculated based on the date of the Invoice. The financial year starts on 1st April and ends on 31st March and is represented as YYYY-YY. If the date is 03.01.2020, the financial year is 2019-20.
• Document types are INV for Invoice, CRN for Credit Note, DBN for Debit Note.
• Document Number is a number printed on the Document being issued to the purchaser, and it is to be of a maximum of 16 alphanumeric and / -.
• Document number should not be starting with 0, / and -. Also, alphabets in document numbers should not have alphabets in lower cases. If so, then the request is rejected.
• For example, a taxpayer with 01AAAAA9999A19N has issued the Invoice with Number ABC01234 on 12.12.2019 to his supplier, the string used for hashing is

01AAAAA9999A19N2019-20INVABC01234.