Understanding IBAN Check Digits and the MOD-97 Algorithm
The two check digits in every IBAN protect against typos using a modular arithmetic algorithm. Here is exactly how it works.
What Are Check Digits?
Positions 3 and 4 of every IBAN are check digits — two numbers calculated from the rest of the IBAN that allow anyone to verify the number is internally consistent. They do not identify the bank or account; they are purely a validation mechanism.
How the MOD-97 Algorithm Works
The validation process follows these steps:
- Move the first 4 characters (country code + check digits) to the end
- Replace each letter with its numeric equivalent: A=10, B=11, ..., Z=35
- Divide the resulting number by 97
- If the remainder is 1, the IBAN is valid
For a deeper explanation with worked examples, see our complete guide to IBAN checksums.
What Can Check Digits Catch?
MOD-97 detects all single-digit errors and most transpositions of adjacent digits. It cannot guarantee the account exists or that the bank code is valid — only that the IBAN passes the mathematical check.
Try It Yourself
Enter any IBAN into our decoder to instantly see the checksum validation result, along with the decoded bank and account components.