How to use the iban structure checker and builder
- Choose Check IBANs to test numbers, or Build an IBAN to calculate check digits.
- To check, paste IBANs one per line. Spaces are ignored and letters can be in either case.
- To build, enter the two letter country code and the account part (the BBAN) that follows the check digits.
- Read whether each IBAN passes, the remainder, the length rule and the check digits it would need.
Formula
move the first four characters to the end, then replace A to Z by 10 to 35
remainder = (that number) mod 97, computed piece by piece
valid when remainder = 1; check digits = 98 - ((country + 00 + account) remainder)
The check digits are chosen so that the rearranged number leaves a remainder of 1 when divided by 97. The remainder is calculated a few digits at a time, so no huge integers are needed.
Checking GB82 WEST 1234 5698 7654 32
Moving GB82 to the end and turning letters into numbers gives 3214282912345698765432161182. Dividing by 97 leaves 1, so the IBAN passes. Its length of 22 characters also matches the rule for Great Britain.
Changing the last digit to 3 leaves a remainder of 28, so it fails, and the page reports that the check digits would need to be 55. Building from country GB and account part WEST12345698765432 returns GB82 WEST 1234 5698 7654 32.
What a passing IBAN means
The mod 97 test catches nearly every single mistyped digit and most swaps. It cannot show that the account exists. A pass means the characters are consistent, so confirm the details with the payee.
The length column compares the number of characters with a short list of common countries. Countries that are not in the list still get the mod 97 test but no length check.
Building check digits
Build mode puts 00 where the check digits go, finds the remainder and takes 98 minus it. It adds nothing about banks or branches, so you must supply a real account part.
The result is only as good as the account part you enter. Do not invent account numbers for real payments.
Assumptions and limits
- The length list covers a set of common countries, not every country in the official registry.
- Only the mod 97 test and the length are checked. Bank codes inside the account part are not verified.
- Spaces are removed and letters are made upper case before the test.
- Nothing you type is stored or sent anywhere.
- Up to 100 lines are checked at once.
Frequently asked questions
Why is the remainder 97 based?
The standard uses 97, the largest prime below 100, so the check digits fit in two digits. A prime modulus makes single digit errors and most swaps change the remainder.
Can I trust an IBAN that passes?
It is consistent, not confirmed. Always verify the account name and number with the person you are paying, especially before a large transfer.
Why is there a length rule?
Each country fixes the total length of its IBANs. A number with the wrong length cannot be valid even if the mod 97 test happens to pass, so the page shows both.
Why are some countries missing from the list?
The page includes a short list of widely used countries. For others, the mod 97 test still runs but the length is marked as not in the list.