How IBAN Check Digits Work

Every IBAN includes two check digits that catch 99.9% of typing errors. They use a mathematical formula called MOD-97, first published in ISO 7064. Here's how it works.

The MOD-97 Algorithm

The check digits (positions 3-4 in every IBAN) are computed using the MOD-97 algorithm. Here is the step-by-step process:

Step 1: Rearrange the IBAN

Move the first four characters (country code + check digits) to the end of the string.

Example for GB29 NWBK 6016 1331 9268 19:

Original:  GB29 NWBK 6016 1331 9268 19
Rearrange: NWBK 6016 1331 9268 19 GB29

Step 2: Convert Letters to Numbers

Replace each letter with its numeric value: A=10, B=11, ..., Z=35.

A=10, B=11, C=12, ..., Z=35
N → 23, W → 32, B → 11, K → 20, G → 16, B → 11

Step 3: Compute MOD-97

Interpret the resulting string as a huge integer and divide by 97. A valid IBAN always gives remainder 1.

NWBK60161331926819GB29 → (huge integer) mod 97 = 1 ✓

How Check Digits Are Generated

When a bank creates an IBAN, it:

1. Starts with the country code + "00" + BBAN (domestic account details)
2. Moves the first 4 chars to the end
3. Converts letters to numbers
4. Computes remainder = big_number mod 97
5. Check digits = 98 − remainder (zero-padded to 2 digits)

Why 97?

97 was chosen because:

• It's the largest two-digit prime number
• It catches 99.94% of single-digit typos and transposition errors
• The math works efficiently for numbers of any length

Computing MOD-97 in JavaScript

Since IBAN numeric strings can be 30+ digits (too large for JavaScript's Number type), we process in chunks:

function mod97(numStr) {
  let remainder = numStr;
  while (remainder.length > 2) {
    let chunk = remainder.slice(0, 9); // 9 digits safely < MAX_SAFE_INTEGER
    remainder = (parseInt(chunk, 10) % 97).toString() + remainder.slice(chunk.length);
  }
  return parseInt(remainder, 10) % 97;
}

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