Validate MOD-97 checksum

Test a numeric string against the ISO 7064 MOD 97-10 check used internationally for IBAN and similar identifiers.

Algorithm

After removing spaces and non-digits:

1. Treat the entire string as a big integer (or process iteratively in chunks for long values).
2. Compute remainder mod 97.
3. The string is valid when the remainder equals 1.

This is equivalent to moving the check digits to the end, converting letters to numbers (A→10 … Z→35), and requiring divisibility by 97 — the same rule applied to IBAN validation.

When to use it

Validate custom reference numbers that adopt MOD-97, cross-check IBAN implementations you are building, or explain why European bank accounts embed a mathematical checksum.

Limitations

The check confirms format integrity, not that a bank account is open or reachable. Very long inputs are handled as strings of digits; leading zeros matter. Letter conversion follows the IBAN scheme — arbitrary alphabetic payloads without that mapping are outside scope.

Example

For example, enter the default sample values on the form and compare the output with a known reference; adjust one input at a time to see how the result changes.