A Czech IBAN is always 24 characters long. Here is what each part of it means, with a real example broken down field by field — and a checker that runs on your machine.
Czech account numbers have an optional prefix in front of the main number, and both are carried inside the IBAN. When the prefix is unused it appears as zeros, which is why so many Czech IBANs contain a long run of them.
The two digits straight after CZ are not part of the account — they are a checksum over everything else. Move the first four characters to the end, turn every letter into a number (A=10 … Z=35), divide the result by 97, and a genuine IBAN always leaves a remainder of exactly 1. That is what the box above is doing, on your machine, with nothing sent anywhere.
If the number is on a scanned PDF you cannot select, retyping 24 characters is where mistakes get made. Snip2Field sits on your Windows desktop: press a hotkey, drag a box over the IBAN, and it reads the field, runs this exact checksum and copies it — ready to paste into your books or a transfer form. Fully offline.
See Snip2FieldExactly 24, always. That is CZ, two check digits, and 20 characters of account detail. Anything longer or shorter is not a valid CZ IBAN, whatever else is right about it.
It is the 4 characters directly after the check digits. In the example CZ65 0800 0000 1920 0014 5399, that is 0800.
You can tell whether it was written down correctly: the check digits catch a single wrong character or two swapped ones. You cannot tell whether the account exists — only the receiving bank knows that, which is what the recipient-name check does when a transfer is made.