Convert an Account Number to an IBAN: How to Build Your IBAN

The old account number in grandpa’s savings book, the bank code from a letter dated 2010, a ten-digit string on an old invoice: any of these can be turned into today’s valid IBAN in just a few steps. The formula sits inside every German IBAN: two letters for the country, two digits for the checksum, eight digits for the bank code, ten digits for the account number. This article shows exactly how the conversion works and how the MOD 97-10 check digits are produced.

Why an IBAN instead of an account number?

Until 2014, an account number and a bank code (BLZ) were all you needed for a transfer in Germany. The SEPA migration (Single Euro Payments Area) switched the entire European payment system to one common format. Since 1 February 2014 for companies, and since 1 February 2016 for private individuals, the IBAN has been the only account format used for transfers and direct debits in the SEPA area.

The IBAN does not replace the old account number, it contains it. A German IBAN is a combination of the country code, the check digits, the bank code and the account number. If you still have old account details without an IBAN, you can convert them in a few steps.

How a German IBAN is built

A German IBAN always has exactly 22 characters and is structured like this:

Position	Length	Content	Example
1 to 2	2 letters	Country code	DE
3 to 4	2 digits	Check digits	89
5 to 12	8 digits	Bank code (BLZ)	37040044
13 to 22	10 digits	Account number	0532013000

So the formula is:

IBAN = DE + check digits + BLZ (8 digits) + account number (10 digits)

Your old account number and bank code sit directly inside the IBAN, with the country code “DE” and a two-digit check value added in front to catch typos. You can read more about the IBAN format on our IBAN Germany page.

Step by step: convert an account number and bank code to an IBAN

Suppose you have the following old account details:

• BLZ: 37040044 (Commerzbank Cologne)
• Account number: 532013000

Step 1: Pad the account number to 10 digits

The account number inside the IBAN is always 10 digits long. If yours is shorter, it is padded on the left with zeros:

532013000 -> 0532013000

Step 2: Assemble the BBAN

The BBAN (Basic Bank Account Number) is the combination of the bank code and the padded account number:

37040044 + 0532013000 = 370400440532013000

Step 3: Calculate the check digits

The check digits are calculated with the MOD 97-10 method (ISO 7064). In simple terms it works like this:

1. Append the country code as numbers plus “00” to the BBAN: 370400440532013000131400
   • D = 13, E = 14 (A=10, B=11, … Z=35)
2. Divide this number by 97 and take the remainder
3. Check digits = 98 minus remainder

For our example:

• 370400440532013000131400 mod 97 = 9
• 98 - 9 = 89

The check digits are 89.

Step 4: Put the IBAN together

DE + 89 + 37040044 + 0532013000 = DE89370400440532013000

Written in the usual groups of four: DE89 3704 0044 0532 0130 00

Step 5: Verify the IBAN

To make sure no mistake crept into the calculation, validate the IBAN with our IBAN validator. The tool checks the check digits and the format, and shows you the matching bank name and BIC.

What if the account number is shorter?

Many German account numbers have fewer than 10 digits. The rule is simple: pad on the left with zeros until you reach 10 digits.

Original account number	Padded (10 digits)
532013000	0532013000
12345678	0012345678
1234567	0001234567
123456	0000123456

Note: At a small number of banks (in particular certain savings banks and cooperative banks), a different account-number mapping applies. In those special cases the account number is not simply padded on the left but embedded into the BBAN according to bank-specific rules. These special cases are documented in the so-called IBAN rules file maintained by the Deutsche Bundesbank. When in doubt, check your IBAN with your bank or use our IBAN validator.

Understanding the check digits

The check digits are the heart of the IBAN’s safety net. They catch 99 percent of all typos, including:

• Transposed digits (for example 37040044 instead of 37004044)
• Single wrong digits (for example 37040044 instead of 37040045)
• Missing or extra digits

The MOD 97-10 method is mathematically reliable: for a correct IBAN, dividing the rearranged number by 97 always leaves a remainder of 1. Any deviation, whether from a typo or a wrong bank code, produces a different result and is detected.

Since October 2025, banks in the SEPA area additionally check whether the IBAN and the recipient name match (Verification of Payee). A correct IBAN on its own is therefore no longer enough, the name has to be right too.

Finding the bank code for the conversion

If you no longer remember your bank code, there are several ways to find it:

• Read it from an old IBAN: positions 5 to 12
• In online banking under your account details
• On a bank statement or your debit card
• In the BIC calculator: enter the bank name and get the bank code plus BIC
• In the bank directory: search all German banks by bank code

Austria and Switzerland: different formats

Converting an account number to an IBAN works on the same principle in Austria and Switzerland, but with a different layout.

Austria (AT): 20 characters

Position	Length	Content
1 to 2	2	Country code: AT
3 to 4	2	Check digits
5 to 9	5	Bank code (5 digits)
10 to 20	11	Account number (11 digits)

The Austrian bank code has only 5 digits (instead of 8 in Germany) and the account number has 11 digits (instead of 10). The check-digit method is identical (MOD 97). There is more on this on the IBAN Austria page.

Switzerland (CH): 21 characters

Position	Length	Content
1 to 2	2	Country code: CH
3 to 4	2	Check digits
5 to 9	5	BC number (bank clearing)
10 to 21	12	Account number (12 digits)

Instead of a bank code, Switzerland uses a BC number (Bank Clearing Number) with 5 digits. The account number is 12 characters long. You will find the details on the IBAN Switzerland page.

Can I keep my old account number?

Your old account number still exists, it is contained in your IBAN. Some banks show both the IBAN and the old account number in online banking. For payments, however, only the IBAN matters. If someone gives you an old account number and bank code, you can convert them into an IBAN as described above.

Tip: convert the IBAN and then check it with the IBAN validator before you initiate a transfer. That way you avoid returned direct debits and delays.

Frequently asked questions

Can I calculate my IBAN myself?

In principle, yes. You need your bank code (8 digits) and your account number (padded to 10 digits). You calculate the check digits with the MOD 97-10 method. In practice, though, it is safer to have your bank confirm the IBAN or to check it with the IBAN validator, especially because of the special cases at some banks mentioned above.

What happens if I enter a wrong IBAN?

In most cases the bank rejects an incorrect IBAN, because the check digits do not add up. If the IBAN is formally correct but belongs to a different account, the transfer goes to that account. Since Verification of Payee was introduced (October 2025), banks also check the recipient name, which further reduces misdirected transfers.

Does the conversion also work for business accounts?

Yes. The IBAN conversion applies to all German account types: current accounts, business accounts, savings accounts and instant-access accounts. The principle is always the same: DE + check digits + bank code + account number. What matters is that the correct bank code and account number are used.

The key points

To turn an old account number into an IBAN you need three things: the country code (DE), the eight-digit bank code and the account number padded to ten digits. The correct check digits are worked out with the MOD 97-10 method and cannot be guessed mathematically, which is why the cleanest approach is to enter the assembled data into the IBAN validator and let it handle the check digits and the validation. If the account number belongs to a bank with special rules (savings banks, some cooperative banks), it is safest to have the bank itself confirm the IBAN.

Sources

• Deutsche Bundesbank – IBAN und BIC
• Deutsche Kreditwirtschaft – IBAN-Regeln
• ISO 7064 – MOD 97-10 check method
• Regulation (EU) No 260/2012 – SEPA

My bank has merged, is my old bank code still valid?

After a bank merger, a new bank code is usually assigned and the old one is kept as a forwarding number. Transfers to the old IBAN often still work for a transition period, but you should ask your bank for the new IBAN as soon as possible. Check your current IBAN in online banking or use the bank directory to find your bank’s current bank code.