What is the Croatian OIB?

The OIB (Osobni identifikacijski broj, meaning "personal identification number") is an 11-digit identifier issued by the Croatian Tax Administration (Porezna uprava) to every Croatian citizen, resident foreigner, and legal entity. It was introduced in 2009 to replace a fragmented set of sector-specific numbers and is now used uniformly for tax, healthcare, social security, and government services.

Which algorithm does the OIB use?

The eleventh digit is a check digit computed with ISO 7064 MOD 11,10. The algorithm starts with a running remainder of 10 and processes each of the first ten digits in order: t = (remainder + digit) mod 10 (treating 0 as 10), then remainder = (t times 2) mod 11. After all ten digits the check digit equals (11 minus remainder) mod 10. The step-by-step trace in this tool shows every intermediate value so you can verify the calculation by hand.

Does an OIB encode a date of birth or gender?

No. Unlike the older JMBG used in former Yugoslav countries or the Finnish HETU, the Croatian OIB is a randomly assigned number with no embedded personal attributes. The only structural information it carries is the check digit in position 11. This means validating an OIB offline is limited to confirming the check digit; it cannot reveal anything about the holder.

What does a valid check digit actually prove?

It proves the number is mathematically well-formed and that single-digit transcription errors have been caught. ISO 7064 MOD 11,10 detects all single-digit substitutions and all adjacent transpositions (swapping two neighbouring digits). It does not prove the OIB was ever issued, belongs to a real person, or is currently active — only the Croatian Tax Administration can confirm that.

How many digits is a Croatian OIB?

Always exactly 11 digits. The first ten are the randomly assigned base number and the eleventh is the check digit. Leading zeros are significant — a number like 00123456789 is a valid 11-digit OIB if its check digit is correct, not a 9-digit number.

Is my OIB uploaded to any server?

No. Validation runs entirely in JavaScript inside your browser. No data is transmitted, logged, or stored anywhere. You can disconnect from the internet before entering the number and the tool will still work correctly.

Croatia OIB Validator

The OIB (Osobni identifikacijski broj — “personal identification number”) is Croatia’s universal 11-digit identifier, introduced in 2009 by the Croatian Tax Administration (Porezna uprava) to consolidate a patchwork of sector-specific IDs. It is assigned to every Croatian citizen, permanent resident, and legal entity, and appears on tax returns, healthcare cards, government forms, employment contracts, and corporate filings. Unlike the older JMBG it replaced, the OIB encodes no personal attributes — there is no birth date, gender, or region hidden in the digits. The only structural property you can verify offline is the check digit in position 11.

This validator implements the real algorithm — ISO 7064 MOD 11,10 — and shows you exactly why a number passes or fails, with a full step-by-step trace of every intermediate value. It is useful for developers integrating with Croatian government APIs, accountants processing cross-border tax documents, HR systems importing employee records, or anyone who just received an OIB and wants to rule out a transcription error before submitting a form.

How the ISO 7064 MOD 11,10 algorithm works

The OIB check digit is computed by the ISO 7064 MOD 11,10 recursive algorithm — the same standard used by the Croatian national bank IBAN check digit, among others. The “MOD 11,10” name comes from two moduli applied alternately in each step.

Starting with a running remainder rem = 10, the algorithm loops over each of the first ten digits d[0] through d[9]:

Compute t = (rem + d[i]) mod 10. If t equals 0, set t = 10.
Compute the new remainder: rem = (t * 2) mod 11.

After all ten digits, the check digit is (11 - rem) mod 10.

The validator compares this computed value against the actual 11th digit of the OIB you entered. If they match, the number is well-formed. If not, it tells you what the correct check digit should be.

Why this algorithm is strong

ISO 7064 MOD 11,10 detects:

Every single-digit substitution (e.g. typing a 7 instead of a 3 in any position).
Every transposition of two adjacent digits (e.g. swapping positions 4 and 5).

This makes it significantly more robust than a simple weighted-sum checksum, and is the reason Croatian authorities chose it for a number that appears frequently on official forms where transcription errors are common.

Example

For the base number 1234567890 (digits 1–10), running the algorithm yields a final remainder of 8, so the check digit is (11 - 8) mod 10 = 3. The complete, well-formed OIB is therefore 12345678903 — an obviously fake sample used here purely to illustrate the format.

Entering 12345678904 would fail: the algorithm still expects 3, and the validator would show “FAIL — digit should be 3” along with the full trace.

Position	1	2	3	4	5	6	7	8	9	10	11
Digit	1	2	3	4	5	6	7	8	9	0	3
Role	base	base	base	base	base	base	base	base	base	base	check

Paste any 11-digit number into the tool above and it will recompute the check digit instantly — all inside your browser, with nothing uploaded.