A Greek 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 Greek IBAN gives three digits to the bank and four to the branch, leaving sixteen characters for the account. Greek banks issue account numbers that are shorter than that, so the account portion is usually zero-padded at the front.
The two digits straight after GR 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 GR, two check digits, and 23 characters of account detail. Anything longer or shorter is not a valid GR IBAN, whatever else is right about it.
It is the 3 characters directly after the check digits. In the example GR16 0110 1250 0000 0001 2300 695, that is 011.
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.