German BLZ check — a Bankleitzahl against the Bundesbank register

Every German IBAN carries its Bankleitzahl in plain sight: the eight digits at positions 5–12, right after DE and the two check digits. Reading them is arithmetic. Knowing whether those eight digits belong to an institution — and to which one — needs a register, and Germany publishes an unusually good one: the Deutsche Bundesbank Bankleitzahlendatei, republished monthly, the same file German payment traffic routes by.

That is the whole difference between a format check and a BLZ check. The mod-97 checksum catches typos; it knows nothing about whether a bank code was ever allocated.

One call

curl -X POST https://api.ibanforge.com/v1/iban/validate \
  -H "Content-Type: application/json" \
  -d '{"iban": "DE89370400440532013000"}'

{
  "valid": true,
  "bic": {
    "code": "COBADEFF",
    "bank_name": "COMMERZBANK Aktiengesellschaft",
    "city": "Frankfurt am Main"
  },
  "bank_code_check": {
    "value": "37040044",
    "status": "verified",
    "match": "register",
    "register": "Deutsche Bundesbank Bankleitzahlendatei",
    "authoritative": true,
    "as_of": "2026-08"
  }
}

The BLZ 37040044 was extracted from the IBAN, checked against the Bundesbank's own file, and resolved to Commerzbank. authoritative: true says the answer came from the allocation authority itself — not from a BIC directory that happens to mention German banks. as_of says which monthly edition answered.

What the Bankleitzahlendatei publishes

For an allocated BLZ, what the register holds about the institution is served in bank_code_check.institution:

Field	In the German register	Note
`name`	yes	the institution's name as the Bundesbank prints it
`post_code`	yes	seat postcode
`town`	yes	seat town
`street`	no	always `null` — the Bankleitzahlendatei has no street column at all. The field is honestly empty rather than filled in from another source
`lei`	no	the German file carries none (Austria's register does — see Austrian bank codes)

A null here is a fact about the register, not a gap in the lookup. Filling that street from a BIC directory would produce an address the Bundesbank never published.

Retirements — the part only Germany tells you

The Bankleitzahlendatei is the only one of our national registers that publishes the end of a code as clearly as its beginning. When a BLZ is marked for deletion — usually after a merger — the answer carries two extra fields:

Field	Meaning
`retired: true`	the register marks this code for deletion. The code was allocated, so the status stays `verified` — this is a warning, not a denial
`superseded_by`	the successor BLZ that takes over, when the register names one

That distinction is what makes it useful at payout time. A retired BLZ still validates and still names a real institution, so nothing in a format check will flag it — yet beneficiary details built on it have a shelf life, and the register tells you what replaces them.

What `not_in_register` means here

Because the Bundesbank is the allocation authority for German bank codes, an absence is a fact about Germany, not about our coverage: the eight digits are allocated to nobody. That is what status: "not_in_register" with authoritative: true means, and it is a strong reason to stop a payment before it is submitted. The IBAN will keep answering valid: true next to it — structure and allocation are two different questions, and only the second one needs a register.

The honest limits

We check the bank code, never the account or its holder. A verified BLZ says the institution exists and names it; it says nothing about the ten digits that follow in the IBAN.
Institution, not branch. The BLZ identifies the institution. Branch-level detail is not derivable from a German IBAN, by us or anyone else.
Monthly freshness. The Bundesbank republishes monthly and we follow that cadence, so a code allocated mid-month appears in the next edition. Every answer stamps the edition it read in as_of.
No street, by design. See the table above: the register does not publish one.