How to Validate an IBAN Number: Step-by-Step Guide

Validating an IBAN is a critical step in any payment processing system to ensure accuracy and prevent costly transfer errors. IBAN validation involves checking the format, verifying the country code, confirming the correct length, and most importantly, validating the check digits using the MOD-97 algorithm defined in ISO 13616. Proper validation catches typing mistakes, formatting errors, and invalid account numbers before they enter your payment pipeline, saving time, money, and customer frustration.

The first step in IBAN validation is format verification. Every valid IBAN must start with a two-letter country code followed by two check digits (0-9), then the Basic Bank Account Number (BBAN). Remove all spaces and convert the entire string to uppercase before validation, as IBANs are case-insensitive but should be normalized for processing. Reject any input that contains special characters, starts with numbers, or does not follow the country-code-plus-digits pattern. This basic format check eliminates obvious errors immediately.

Next, verify that the country code is recognized and supported by your system. Valid IBAN country codes include DE (Germany), FR (France), GB (United Kingdom), ES (Spain), IT (Italy), and over 75 other countries. Each country has a specific IBAN length: German IBANs are always 22 characters, French IBANs are 27 characters, UK IBANs are 22 characters, and so on. If the input length does not match the specification for the given country code, the IBAN is invalid. Maintaining an up-to-date reference table of country codes and their corresponding lengths is essential for this validation step.

The most important validation step is verifying the check digits using the MOD-97 algorithm. This mathematical process ensures the IBAN's structural integrity and catches over 99% of transcription errors. Here is how it works: First, move the first four characters (country code and check digits) from the beginning to the end of the IBAN. For example, "DE89370400440532013000" becomes "370400440532013000DE89". Next, replace each letter with its corresponding numeric value, where A=10, B=11, C=12, and so on up to Z=35. In our example, "D" becomes 13 and "E" becomes 14, giving us "3704004405320130001314 89".

After converting all letters to numbers, you will have a large numeric string. Calculate the remainder when dividing this number by 97 (modulo 97 operation). If the remainder equals 1, the IBAN passes the check digit validation and is structurally valid. If the remainder is any other number, the check digits are incorrect, indicating a typo or invalid IBAN. Most programming languages provide big integer libraries to handle these large numbers accurately. For example, in JavaScript you can use BigInt, in Python the standard int type handles it automatically, and in Java you can use BigInteger.

Implementing IBAN validation in your codebase is straightforward once you understand the algorithm. Here is a simplified validation function in JavaScript: First, normalize the input by removing spaces and converting to uppercase. Check that the format matches the country-code-digits pattern using a regular expression. Verify the length against your country specification table. Then rearrange the IBAN by moving the first four characters to the end, replace all letters with their numeric equivalents (A=10 through Z=35), and calculate the modulo 97 of the resulting number. Return true if the result is 1, false otherwise.

For production systems, consider implementing validation at multiple points in your data flow. Perform client-side validation in the browser to give users immediate feedback as they type, reducing form abandonment and support requests. Then repeat the same validation on your backend server to guard against malicious input or client-side bypasses. Store only validated IBANs in your database, and log validation failures for monitoring and fraud detection. You can test your validation logic using our <a href="/" class="text-primary-600 hover:text-primary-700">IBAN generator</a> to create known-valid test cases, and verify your implementation with our <a href="/validator" class="text-primary-600 hover:text-primary-700">online IBAN validator</a>.

Common validation mistakes to avoid include forgetting to normalize the input (removing spaces and converting to uppercase), using the wrong character-to-number mapping (remember A=10, not A=1), attempting to validate IBANs as regular integers instead of big integers (leading to overflow errors), and failing to update your country length specifications when regulations change. Always validate against the most current ISO 13616 specifications, and consider subscribing to banking standards updates to catch changes early. By implementing robust IBAN validation following these best practices, you will dramatically reduce payment errors and improve the reliability of your international banking integrations.