A Bulgarian IBAN is always 22 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.
Bulgaria is one of the few countries whose IBAN encodes the account type as a separate two-digit field, sitting between the branch code and the account number itself. The bank is identified by the first four letters of its BIC.
The two digits straight after BG 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 22 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 22, always. That is BG, two check digits, and 18 characters of account detail. Anything longer or shorter is not a valid BG IBAN, whatever else is right about it.
It is the 4 characters directly after the check digits. In the example BG80 BNBG 9661 1020 3456 78, that is BNBG.
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.