IBAN Validation and SWIFT/BIC Codes: Structure and Verification

International Bank Account Numbers (IBAN) standardize cross-border bank payments across more than 80 countries. Validating IBANs client-side prevents costly wire transfer rejections.

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The Structure of an IBAN

1. Country Code (2 letters): ISO 3166-1 alpha-2 (e.g. GB, DE, FR, TR).
2. Check Digits (2 digits): Calculated using the MOD-97 algorithm.
3. Basic Bank Account Number (BBAN): Up to 30 alphanumeric characters containing bank code, branch code, and account number.

MOD-97 Validation Steps (ISO 7064)

1. Move the first 4 characters (country code and check digits) to the end of the string.
2. Replace each letter with two digits ($A = 10, B = 11, \dots, Z = 35$).
3. Interpret the resulting string as a decimal integer and compute modulo 97.
4. The IBAN is valid if and only if: $\text{Remainder} = 1$.

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