Banks in many countries still issue account details as a bank code plus a local account number. If you need to send money internationally, you have to convert those into an IBAN first, and most online forms do not do it for you.
The parts of an IBAN
An IBAN is built from three pieces: the two-letter country code, a bank code (BLZ in Germany), and the account number. The country determines how those pieces fit together and how many digits each must have. Short account numbers are padded with leading zeros to fill the required positions.
How the check digits are calculated
Once the country code, bank code, and account number are assembled, two check digits are worked out and inserted after the country code. The process: move the first four characters to the end, replace each letter with a two-digit number, then run a modulo-97 calculation on the resulting string. The check digits are chosen so the result is 1. This is the same algorithm banks use to validate IBANs.
Country lengths vary
Each country sets its own IBAN format and total length. German IBANs are 22 characters; French IBANs are 27; some are as short as 15, others as long as 34. The calculator applies the correct format for the selected country automatically.
Confirm before you pay
The IBAN this tool produces will have a correct format and valid check digits, which means it passes the same mathematical check any bank would run. But a correct format is not the same as a correct account. A bank code might be out of date, or the account number in your records might have an error. Before using a calculated IBAN for an actual payment, confirm the details with your bank or the account holder directly.
TL;DR: Enter the country, bank code, and account number in the IBAN calculator and it assembles the full IBAN with correct check digits.