Czech bank accounts live a double life. Domestically they are written as prefix-number/bankcode, for example a 6-digit prefix, a 10-digit account number, a slash and a 4-digit bank code. Internationally, exactly the same digits are repackaged into a 24-character CZ IBAN: CZ, two mod-97 check digits, the bank code first, then the zero-padded prefix and account number. A test IBAN for Czechia therefore has to be consistent in both representations if your product ever converts between them.
Modulo 11, twice
The Czech National Bank’s standard requires the prefix and the account number to each pass a weighted modulo-11 check using the weights 6, 3, 7, 9, 10, 5, 8, 4, 2, 1. That is two national checksums layered under the ISO mod-97 check. Random digits fixed up only at the mod-97 layer will fail any validator that implements the CNB rules, which includes Czech banks’ own onboarding forms and several open-source libraries. The generator on this page produces prefixes and account numbers that satisfy the modulo-11 rules, so the resulting IBANs are coherent all the way down.
Test-suite ergonomics
The domestic-to-IBAN conversion is a gift for unit testing: one generated value yields two fixtures, the IBAN string and its prefix-number/bankcode domestic form, and a round-trip test between them catches padding and ordering bugs in one assertion. For browser-level testing, a Playwright script filling a Czech payout form with a generated CZ IBAN exercises the real validation path instead of the error branch, and sandbox environments for CZK payouts accept the same values at the syntax gate.
Generation is fully client-side: nothing leaves your browser, nothing is stored, and every output carries a TEST label.
The line that matters
Structural validity means the string obeys the rules, not that an account exists. None of these IBANs map to a real account at any Czech bank. Use them in tests, fixtures, demos and documentation only, never for actual payments or to present as genuine bank details; payee-verification checks in EU payment rails will flag fabricated recipients in any case.