Guide
How IBAN Checksum Validation Works
Learn the MOD97-10 idea behind IBAN checksum validation in plain English, including normalization, rearrangement, letter conversion, and chunked calculation.
Who this guide is useful for
Developers, QA testers, finance operations teams, and curious users who want to understand IBAN validation results.
The checksum idea
The IBAN checksum is designed to catch many typing mistakes. It does this by converting the IBAN into a numeric sequence and checking whether the MOD97-10 remainder has the expected result.
The plain-English steps
A validator normalizes the input, checks the country length, moves the first four characters to the end, converts letters to numbers, and calculates the remainder.
- Remove spaces and hyphens, then uppercase letters.
- Check that the country code and length are supported.
- Move the country code and check digits to the end.
- Convert A to 10, B to 11, through Z to 35.
- Run MOD97-10 and expect a remainder of 1.
Why chunked calculation is used
The converted numeric sequence can be too long for safe full-number conversion. A chunked MOD97 calculation processes smaller pieces so the full IBAN is never converted into one large Number.
Practical checksum example
For the example DE89 3704 0044 0532 0130 00, the checker uses DE as the country code, 89 as the check digits, and the remaining BBAN characters for the MOD97-10 process.
- Normalize to: DE89370400440532013000
- Move first four characters to the end before numeric conversion.
- Convert letters such as D and E into numeric values.
- A valid IBAN checksum produces MOD97 remainder 1.
Important limitations
BankCodeKit explains payment-code formats and browser-local checks. It does not provide financial advice and does not verify real account ownership or payment outcomes.
- Checksum validation does not catch every possible mistake.
- A valid checksum does not confirm account existence.
- A valid checksum does not confirm account ownership.
- A valid checksum does not guarantee a successful payment.
FAQ
What does MOD97 mean?
It means the validator checks the remainder after dividing the converted numeric sequence by 97.
Why should the remainder be 1?
The IBAN checksum standard expects a valid rearranged and converted IBAN to produce remainder 1.
Why not use Number for the full converted IBAN?
The converted sequence can be too large for safe full-number handling, so chunked calculation is safer.
Does checksum validation prove the recipient is correct?
No. It only checks the structure and checksum.