German account numbers contain a hidden check digit, usually the last one. It is calculated from the remaining digits using a method chosen by each bank. The idea is that any single-digit typo changes the check digit and the mismatch gets caught before a payment goes through. The problem is there are dozens of different methods, and each bank picks its own.
How the check works
To verify a Kontonummer you need both the account number and the eight-digit bank code (BLZ). The BLZ tells the tool which check-digit method that bank uses. The account number is then tested against that method. If the check digit is consistent with the rest of the number, the account number passes.
Enter both into the German account number checker: paste the BLZ, paste the account number, and the tool reports pass or fail along with the method it used and the bank name.
When a method is not covered
The Bundesbank publishes dozens of check-digit procedures and not all are in common use. For less common methods the tool says so rather than guess. A “method not covered” result is not a failure. It means the tool cannot verify that particular bank’s format. Contact the bank directly if you need certainty.
What a passing result means
A pass means the check digit is internally consistent. It does not mean the account is open, that the name on the account matches, or that a payment would succeed. Use this check as a first filter to catch obvious typos before you enter details into your banking software, not as a guarantee the account is real.
TL;DR: Enter the BLZ and account number in the account number checker. A pass means no typo; it does not confirm the account is active.