What is the format of a Spanish IBAN?

A Spanish IBAN is always 24 characters: the country code ES, two IBAN check digits, then 20 digits that encode the CCC (Codigo Cuenta Corriente). The CCC breaks down as 4 digits bank code, 4 digits branch code, 1 internal check digit, 1 internal check digit, and 10 digits account number. Example structure: ES91 2100 0418 4502 0005 1332.

What is the ISO 7064 MOD-97 checksum and why does it matter?

The MOD-97 check (defined in ISO 13616) applies to every IBAN worldwide. The algorithm rearranges the IBAN by moving the first four characters to the end, replaces each letter with two digits (A=10 ... Z=35), interprets the result as a large integer, and computes its remainder when divided by 97. A valid IBAN must give a remainder of exactly 1. Because the number can be hundreds of digits long, the calculation is folded progressively to avoid integer overflow: each digit is incorporated one at a time using the identity (r * 10 + d) mod 97.

What are the two internal check digits in a Spanish CCC?

Spain adds two extra check digits inside the BBAN itself, beyond the standard IBAN checksum. The first covers the 8-digit bank-and-branch combination; the second covers the 10-digit account number. Both are computed using Algoritmo D: multiply each digit by a weight from the repeating sequence 1-2-4-8-5-10-9-7-3-6, sum the products, take the sum modulo 11, then map the remainder to a check digit via the table 0->1, 1->0, 2->9, 3->8, 4->7, 5->6, 6->5, 7->4, 8->3, 9->2, 10->1.

Why does a valid MOD-97 result not guarantee the account exists?

MOD-97 and Algoritmo D are structural checks only. They confirm that the digits are internally consistent — i.e., the number was formed by the correct algorithm — but they cannot query the Banco de Espana register or any bank system to verify that an account with that number was ever opened or is currently active. Only your bank or payment service can confirm existence.

Can I use this tool to process customer banking data under GDPR?

Yes. The validator runs entirely inside your browser tab using JavaScript. No IBAN, partial result, or metadata is transmitted to any server, stored in a cookie, or captured by analytics. You can disconnect from the internet and the tool continues to function identically. This makes it safe even when handling personal financial data regulated by GDPR or the Spanish LOPD-GDD.

What is a safe test IBAN for Spain?

ES9121000418450200051332 is a structurally valid Spanish IBAN widely used in documentation (bank 2100 = CaixaBank, branch 0418, check digits 45, account 0200051332). It passes MOD-97 and both CCC check digits. Use obviously-constructed or publicly-documented test values rather than any real customer account number in test suites or public examples.

Spain IBAN Validator

The Spain IBAN Validator performs a three-layer structural check on any 24-character Spanish IBAN. Most online tools run only the international MOD-97 checksum; this validator goes further and also verifies the two internal CCC check digits that Spain uses on top of the IBAN standard — the same checks your bank’s systems apply when processing a domestic transfer.

The tool is aimed at developers integrating Spanish bank account fields, finance teams reconciling payment files, and anyone who wants to understand why a Spanish IBAN passes or fails, not just that it does.

How a Spanish IBAN is structured

A Spanish IBAN is always exactly 24 characters long. The characters break down left-to-right like this:

Position	Length	What it is
1–2	2	Country code: always `ES`
3–4	2	IBAN check digits (ISO 7064 MOD-97)
5–8	4	Bank code (entidad)
9–12	4	Branch code (oficina)
13	1	First internal CCC check digit
14	1	Second internal CCC check digit
15–24	10	Account number (numero de cuenta)

The last 20 characters (positions 5–24) form the BBAN (Basic Bank Account Number) and also correspond to the traditional Spanish CCC (Codigo Cuenta Corriente) that predates IBAN adoption. All 20 BBAN characters must be digits.

The three validation layers

Layer 1 — ISO 7064 MOD-97-10 (the IBAN standard checksum)

This algorithm is defined in ISO 13616 and applies to every IBAN in the world. The steps are:

Move the first four characters (ES + two check digits) to the end of the string.
Replace every letter with its two-digit numeric equivalent: A becomes 10, B becomes 11, through to Z which becomes 35. For a Spanish IBAN only E and S need substituting (E = 14, S = 28).
Interpret the entire digit string as one large integer and compute its remainder when divided by 97.
The IBAN is valid under this layer only if the remainder equals exactly 1.

Because the digit string can be very long, the remainder is computed progressively — one digit at a time using (remainder * 10 + digit) mod 97 — so no arbitrary-precision arithmetic is needed.

Layer 2 — First internal CCC check digit (bank + branch)

Spain’s Algoritmo D applies a weighted sum to the 8 digits formed by concatenating bank code and branch code, prepended with a 0 to give a 9-character input. The weights cycle as 1-2-4-8-5-10-9-7-3-6. After summing the products, the total is reduced modulo 11, and the result is mapped through the table {0:1, 1:0, 2:9, 3:8, 4:7, 5:6, 6:5, 7:4, 8:3, 9:2, 10:1} to produce the expected check digit. The digit at position 13 of the IBAN must match.

Layer 3 — Second internal CCC check digit (account number)

The same Algoritmo D is applied to the 10-digit account number directly (no leading zero prepended). The digit at position 14 of the IBAN must match the computed value.

Worked example

Take the test IBAN ES91 2100 0418 4502 0005 1332 (all spaces removed: ES9121000418450200051332):

Country: ES
IBAN check digits: 91
Bank code: 2100 (CaixaBank)
Branch code: 0418
Internal check digit 1: 4
Internal check digit 2: 5
Account number: 0200051332

For the MOD-97 check, rearranging gives 210004184502000513 32ES91. Expanding E = 14, S = 28 gives a long numeric string. Computing that string mod 97 yields 1 — pass.

For check digit 1: prepend 0 to 2100 + 0418 = 021000418, apply Algoritmo D weights 1-2-4-8-5-10-9-7-3-6 to each digit, sum, mod 11, map through the table — the result is 4, matching position 13.

For check digit 2: apply the same weights to 0200051332 (10 digits, weights cycle from the start), sum, mod 11, map — the result is 5, matching position 14.

All three layers pass, so the IBAN is valid.

Privacy and offline use

All validation logic runs entirely in your browser tab. No IBAN, partial string, or timing signal is transmitted to any server. The tool contains no analytics hooks that capture form input. You can open this page, disconnect from the internet, and it will continue to work exactly the same way — making it suitable for validating sensitive financial data under GDPR or the Spanish LOPD-GDD.