A Slovenian IBAN is always 19 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.
Every Slovenian IBAN begins SI56 — like Portugal, the check digits are in practice fixed, so the country code and the two digits after it are always the same four characters. At 19 characters it is one of the shorter IBANs in use.
The two digits straight after SI 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 19 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 19, always. That is SI, two check digits, and 15 characters of account detail. Anything longer or shorter is not a valid SI IBAN, whatever else is right about it.
It is the 5 characters directly after the check digits. In the example SI56 2633 0001 2039 086, that is 26330.
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.