A Swiss IBAN is always 21 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.
Switzerland is not in the EU but is in SEPA, so Swiss IBANs behave like any other for euro transfers. The first five digits are the Bankclearing-Nummer, the number Swiss banks have used domestically for decades. Liechtenstein uses the same 21-character layout under the country code LI.
The two digits straight after CH 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 21 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 21, always. That is CH, two check digits, and 17 characters of account detail. Anything longer or shorter is not a valid CH IBAN, whatever else is right about it.
It is the 5 characters directly after the check digits. In the example CH93 0076 2011 6238 5295 7, that is 00762.
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.