A French IBAN is always 27 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.
A French IBAN contains the whole of the old RIB: bank code, branch code, account number and the two-digit clé RIB that French banks used as their own checksum long before the IBAN existed. The account portion can contain letters, so a French IBAN is not purely numeric.
The two digits straight after FR 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 27 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 27, always. That is FR, two check digits, and 23 characters of account detail. Anything longer or shorter is not a valid FR IBAN, whatever else is right about it.
It is the 5 characters directly after the check digits. In the example FR14 2004 1010 0505 0001 3M02 606, that is 20041.
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.