A Polish IBAN is always 28 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.
At 28 characters Poland has one of the longest IBANs in Europe. The part after PL and the check digits is the domestic NRB, unchanged — which is why Polish account numbers quoted locally look almost identical to the IBAN with the first four characters removed.
The two digits straight after PL 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 28 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 28, always. That is PL, two check digits, and 24 characters of account detail. Anything longer or shorter is not a valid PL IBAN, whatever else is right about it.
It is the 8 characters directly after the check digits. In the example PL61 1090 1014 0000 0712 1981 2874, that is 10901014.
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.