How long is a Czech IBAN?

A Czech IBAN is always exactly 24 characters: the country code CZ, two IBAN check digits, a 4-digit bank code, a 6-digit account prefix, and a 10-digit account number. The whole BBAN is numeric.

What is the předčíslí (account prefix)?

Czech account numbers historically had an optional 6-digit prefix in front of the main 10-digit number. In the IBAN this prefix is always zero-padded to six digits, even when a customer's printed account number omits it.

What are the Czech modulo-11 checks?

The Czech National Bank requires that both the account prefix and the main account number pass a weighted modulo-11 test. Each digit is multiplied by a weight from the sequence 1, 2, 4, 8, 5, 10, 9, 7, 3, 6 (powers of two modulo 11, applied right to left), and the weighted sum must be divisible by 11.

How does the MOD-97-10 checksum work?

The first four characters (CZ plus check digits) are moved to the end, letters are converted to numbers (C=12, Z=35), and the resulting integer is reduced modulo 97. A structurally valid IBAN gives a remainder of exactly 1.

Does a valid result mean the account exists?

No. A pass only means the IBAN is mathematically and structurally well-formed. It cannot confirm the account is open, funded, or owned by a particular person — only the holding bank can verify that.

Czech IBAN Validator

The Czech IBAN Validator confirms that a Czech bank account number in IBAN format is structurally correct. It verifies the CZ country code, the 24-character length, the 4-digit bank code, the full ISO 7064 MOD-97-10 checksum, and both Czech National Bank weighted modulo-11 checks — all without sending a single character to a server.

Czech domestic payments settle through the CERTIS system, and a correctly formed IBAN is required for SEPA and cross-border transfers. Because the Czech account number embeds its own modulo-11 self-check, a validator can catch many typos that the IBAN checksum alone would miss.

How it works

A Czech IBAN follows a fixed structure from the SWIFT IBAN Registry:

CZ — the country code for the Czech Republic
kk — 2 IBAN check digits (ISO 7064 MOD-97-10)
bbbb — 4-digit bank code
pppppp — 6-digit account prefix (předčíslí), zero-padded
10 digits — the main account number

Total: 24 characters, all digits in the BBAN.

Validation runs three independent checks:

IBAN MOD-97-10 — move CZ plus the check digits to the end, expand letters to numbers, and confirm the result modulo 97 equals 1.
Prefix modulo-11 — multiply each of the six prefix digits by the weights 1, 2, 4, 8, 5, 10 (powers of two modulo 11, applied right to left); the weighted sum must be divisible by 11.
Account modulo-11 — the same weighting extended across the 10-digit account number using 1, 2, 4, 8, 5, 10, 9, 7, 3, 6; the sum must again be divisible by 11.

Worked example

Take CZ65 0800 0000 1920 0014 5399:

Country code: CZ
IBAN check digits: 65
Bank code: 0800 (Česká spořitelna)
Account prefix: 000019
Account number: 2000145399

For the prefix 000019, weighting 9 and 1 right-to-left gives a sum divisible by 11, so it passes. The 10-digit account number passes the same way, and the rearranged string reduces to a MOD-97 remainder of 1. All three checks pass, so the IBAN is valid.

Because the Czech account number carries its own modulo-11 self-check, a single mistyped digit usually breaks at least one of the three tests. Every character is processed locally — nothing is uploaded, logged, or transmitted.