The account number is a unique number used to identify each bank account. With the account number, payment transactions can be processed quickly and securely.
Account numbers are not assigned randomly, but usually contain a check digit, i.e. another digit is calculated from a given number with the help of an algorithm, which together with the given number forms the account number. The check digit is usually placed at the end of the account number. In the case of the International Bank Account Number (IBAN), the checksum has two digits and comes immediately after the country code, e.g. DE78.
Within a credit institution, additional organizational order criteria are sometimes mapped in the account number. Examples of this are the number of the branch managing the account, which is often mapped in the first digits of the account number, or a sub-account number (e.g. two digits at the end of the account number), which indicates whether current accounts, savings accounts, loan accounts, fixed-term deposits or escrow accounts are involved and which enables payments to be forwarded to the correct subsequent systems during further electronic processing.
In Germany, payment transactions are standardised by the DFÜ agreement; external account numbers consist of up to ten decimal digits. If necessary, filling zeros can be used for low account numbers. Together with the bank code, the account number enables unique identification of a German bank account.
In the internal systems, large banks in particular sometimes use longer – e.g. twelve-digit – account numbers which contain the number of a main branch and with which uniqueness can be achieved within the bank without storing the associated bank sort code.
The check digit calculation method is not uniform among German banks; there are currently 143 different methods specified for the individual bank sort codes.
The check digit calculation methods change at irregular intervals. The check digit calculation methods B8, C6 and D4 were changed on the validity date 6 June 2011. In addition, the new check digit calculation methods D7 and D8 became valid. Software that validates bank details must take the current check digit calculation methods into account.
In Austria, up to eleven-digit account numbers and five-digit bank sort codes are used. As in Germany, there are institution-specific check digit calculation methods.
Account numbers in Switzerland have up to 16 digits. The length and any check digit calculation methods are not uniform. Postal current accounts play a special role for payment transactions.
In Switzerland, banks changed the specification of account numbers to the IBAN format with 21 digits since 1 January 2006. During the transition period, the old account numbers continued to be accepted – since 1 January 2010, the use of the IBAN of the beneficiary has been mandatory. Banks are therefore allowed to reject remitters without IBAN, but many banks do not make use of this. For domestic transfers, routing continues via the BC number, which is coded in the IBAN. For online transfers, it is common to automatically fill in the field with the BC number when entering the IBAN.
Belgian account numbers have twelve digits and consist of three blocks (e.g. 091-0122401-16).
- Three-digit block: Protocol number, which provides information about the bank.
- Seven-digit block: Actual account number
- Two-digit block: check digit
The check digit is the remainder of the division of the first two blocks by 97 (mod 97). If the remainder is 0, the check digit is 97. If the check digit is less than ten, a 0 is prefixed.
French account numbers have 11 digits and may contain letters. On cheques and bank statements, they are normally given together with a code for the bank (Code banque = 5 digits), the counter code (Code guichet = 5 digits), then the actual account number (Numéro de compte = 11 digits) and a checksum (Clé RIB = 2 digits) as a 23-digit RIB (“relevé d’identité bancaire”). The checksum at the end of the RIB can be calculated according to the uniform algorithm (mod 97).
In Spain, the account number (Código cuenta cliente) not only identifies the account itself, but also contains the identification data of the account-holding institution, comparable to the German bank code. Its structure is uniform throughout Spain. Spanish account numbers consist only of digits and are 20 digits long, whereby they are divided into 4 sections: First a group of four, which describes the bank, then another group of four to identify the branch, followed by 2 check digits and finally the actual 10-digit account number. Here, the first check digit checks the first 8 digits, the second checks the last 10 digits.
Spanish account numbers are formatted as follows:
2100 0418 45 0200051332
Unlike Germany, the check digit calculation method is uniform across the country and uses a weighted modulus-11 check algorithm.
Calculation example for check digit 1:
1. add leading zeros if necessary: 0, 0, 2, 1, 0, 0, 4, 1, 8 Weighting table: 1, 2, 4, 8, 5,10, 9, 7, 3, 6 2. Add up weighted: 0, 0, 8, 8, 0, 0, 0.28, 3.48 = 95 = 1*0+2*0+4*2+8*1+5*0+10*0+9*0+7*4+3*1+6*8 = 8+8+28+3+48 3. Determine residual modulus 11: 95 % 11 = 7 4. Subtract result from 11: 11-7 = 4 5. If result equals 10, set result to 1 or if result equals 11, set result to 0. Total score: 4
The second check digit is calculated according to the same scheme.
- Description of the check-digit calculation methods at the Deutsche Bundesbank (updated quarterly)
- Check account number for check digit calculation for German IBAN (with test data)
- free PHP library for calculating check digits for german account numbers (with test data)
- another free library C-library with ports for PHP and Perl, incl. bank search by various criteria
- IBAN check digit calculation (PDF; 75 kB)
- SEPA Account Converter Windows program for converting old account numbers into SEPA account numbers
- Deutsche Bundesbank: Check digit calculation methods for checking the accuracy of account numbers (as at June 2018,  PDF).
- European Committee for Banking Standards: TR201 – Register of European Account Numbers(cnb.cz(Memento of 4 June 2008 in the Internet Archive))
- IBAN number: Not yet mandatory. 29. November 2009.
- bankzweiplus.ch (PDF) “For domestic bank payments, the IBAN or account number must be entered. Should you use the IBAN, the BC number will be entered automatically, as this is included in the IBAN.”