Invoice number format generator

What it calculates

A good invoice number is unique, sequential and short. Under GST rules in India an invoice number may use at most 16 characters made of letters, digits, hyphen and slash. Many businesses restart the series every financial year (April to March).

The formula

• Number = prefix + separator + optional year or financial year + separator + zero-padded sequence
• Financial year label: dates from April to December use that year, January to March use the previous year (2025-26)
• Valid when length is at most 16 and only letters, digits, / and - are used

Worked example

Prefix KN, financial year, 4 digit sequence

1. Date 1 June 2025 falls in FY 2025-26
2. Sequence 7 padded to 4 digits is 0007
3. Number = KN/2025-26/0007 (15 characters, valid)
4. A longer scheme such as INV-2025-26-000000001 is 21 characters and fails the check

Questions