IBAN Validator: mod-97 Checksum + Country Length Check

Real-world use cases

• Verify a supplier IBAN before sending an invoice payment

  A vendor emails new bank details and asks you to update your records. Before the finance team wires four or five figures, paste the IBAN here. If the country prefix, length and mod-97 checksum all pass, you know the number is at least well formed and free of a typo introduced by copy-paste. Then confirm the name and bank with the vendor over a known channel, because format validity alone never proves the account is theirs.

• Catch data-entry errors in a payments or payroll system

  You are building or testing a system that stores customer or employee IBANs. Run a sample through this tool to see exactly why a record fails: wrong length for the country, a character that is not A to Z or 0 to 9, an unknown country prefix, or a mod-97 mismatch from a single mistyped digit. The exported pure functions mirror what a server-side validator should do, so you can reproduce the verdict in your own code.

• Sanity-check an IBAN you typed from a paper statement

  Bank statements print the IBAN in groups of four, and it is easy to drop or swap a digit while typing it into a transfer form. Paste your version here, let the tool regroup it cleanly, and check the verdict. A green result means no single-digit typo slipped through; a red one tells you which check failed so you know where to look on the original statement.

• Teach or learn how the IBAN checksum works

  The mod-97 scheme is a clean, real-world example of a check-digit algorithm built on modular arithmetic. Students and developers can enter a known-good IBAN, flip one digit, and watch the verdict turn from valid to a checksum failure. The expected-length table for more than 60 countries doubles as a quick reference for how IBAN structures differ around the world.

Common pitfalls

• Treating a valid checksum as proof the account exists. The mod-97 check only confirms the IBAN is well formed and typo-free, for example that nobody dropped a digit. It cannot tell you the account is open or belongs to your recipient. A randomly built string can pass mod-97 by luck, so always confirm the name and bank separately.

• Expecting US, CA or CN account numbers to validate. The United States, Canada and China do not use IBAN, so their prefixes are reported as an unknown country. Domestic US transfers use routing numbers, China uses CNAPS codes, and cross-border transfers rely on SWIFT/BIC plus the account number, not an IBAN.

• Forgetting that each country has its own fixed length. A German IBAN is 22 characters and a French one is 27, so copying a German length onto a French number guarantees a length failure even if every character looks plausible. Check the expected-length field, not just the checksum.

Privacy