You can use this IBAN calculator for determining Greek IBANs based on account numbers. However, you should be careful with this because not all Greek banks calculate the IBAN in the same way; as a consequence, you need to know what exactly to enter into the account number field in order to still get the right results.
Because these inconsistencies increase the risk of errors, Greek IBANs are not covered by our correctness guarantee.
Even for Greek IBANs, this website might still be useful for you:
Greek IBANs have this structure:
pp is a 2-digit checksum,
ttt is the 3-digit bank code,
kkkk is the 4-digit branch code,
and xxxxxxxxxxxxxxxx is the account number. Unfortunately, not all banks write into this field what they call "Αρ.Λογ.". Sometimes they include the branch code again in the account number, sometimes they don't. Maybe there are still more variants.
Here is an example for different IBAN structures: Novabank and Alphabank.
Example 1: Novabank does not repeat the branch code.
Αρ.Λογ.: 115 / 1208 017
IBAN: GR73 0380 1150 0000 0000 1208 017
Here Novabank writes only "1208017" in the digits that are reserved for the account number. The branch code 115 appears only once.
Example 2: Alpha Bank repeats the branch code.
Αρ.Λογ.: 294 / 00 232 0000 587
IBAN: GR41 0140 2940 2940 0232 0000 587
Here the branch code 0294 appears twice: after the bank code 014, where one should always expect it, but also as part of the account number.
To get the right result with the IBAN calculator in both cases, you need to enter "1208 017" as account number in the case of Novabank, but "294 00 232 0000 587" for Alpha Bank (although you have already picked branch 294 before entering the account number).