How Does IBAN Validation Work?
IBAN validation is a two-step process: format checking and mathematical verification using the MOD-97 algorithm. Understanding this helps you know exactly why a validation tool might reject an IBAN.
Step 1 — Format Check
The first step checks that the IBAN matches the expected format for its country. Each country has a fixed IBAN length and a specific pattern. For example, a German IBAN is always exactly 22 characters: 2 letters + 2 digits + 8-digit bank code + 10-digit account number. If the length or character types are wrong, the IBAN fails immediately.
Step 2 — MOD-97 Check Digit Verification
The MOD-97 algorithm works as follows:
- Move the first 4 characters (country code + check digits) to the end of the string.
- Replace each letter with its numeric value: A=10, B=11, ..., Z=35.
- Interpret the resulting number and calculate it modulo 97.
- If the result equals 1, the IBAN is mathematically valid.
This check catches the vast majority of typos and transposition errors.
Common IBAN Validation Errors
The most common reasons an IBAN fails validation include: incorrect country code, wrong total length, invalid check digits (most common with manual transcription), letters in positions that should be numbers, and spaces or special characters in the wrong places.
Validate Any IBAN Instantly
Our free IBAN validation tool performs both checks instantly and also identifies the issuing bank where possible. No registration required, and we never store the IBANs you submit.