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Spain IBAN Check Digits: Calculation & Verification

The check digits of a Spain IBAN are the two digits at positions 3 and 4 (after the country code). In the example ES9121000418450200051332, they are 91. They detect keying errors using the MOD 97 algorithm.

What Are IBAN Check Digits?

Check digits (positions 3–4 of the IBAN) validate the integrity of the account number. They are computed per ISO 13616 using the modulo 97-10 algorithm. Any single-character error or adjacent-character transposition will be detected.

ES
CC
91
Check
21000418450200051332
BBAN

MOD 97 Algorithm: Steps

  1. 1Move the first 4 characters (country code + check digits) to the end of the IBAN.
  2. 2Replace each letter with its numeric value (A=10, B=11, … Z=35).
  3. 3Compute the remainder of dividing this large number by 97.
  4. 4The result must equal 1 for a valid IBAN.

Step-by-Step Example

Example IBAN:ES9121000418450200051332
Step 1 — Rearrange
Moving "ES91" to the end:
21000418450200051332ES91
Step 2 — Letter substitution
Replacing letters (country code and alphabetic BBAN chars):
21000418450200051332142891
E=14S=28
Step 3 — Modulo 97
Computing the modulo:
21000418450200051332… mod 97 = 1
Result
✓ Valid IBAN (remainder = 1)

Errors Detected by MOD 97

Detected
  • ✓Any single digit/letter error
  • ✓Transposition of two adjacent characters
Not detected (generally)
  • ~Some twin errors

Why It Matters

The MOD 97 algorithm detects ~99.96% of simple typing errors. For a 24-character Spain IBAN, the probability of incorrectly accepting an invalid IBAN is 1/97 ≈ 1%.

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