Croatia OIB validator

OIB (Osobni identifikacijski broj, Personal Identification Number) is the 11-digit number the Porezna uprava (Croatian Tax Administration) assigns to every citizen, resident, and registered business in Croatia. It replaced the older Yugoslav-era JMBG for tax and administrative purposes when it was introduced in 2009 — a deliberate break from JMBG’s design, since an OIB carries no encoded birth date or region. The first 10 digits are simply an assigned sequence, and the 11th is a calculated check digit.

How this validator works

OIB uses a different checksum than the JMBG-family numbers used elsewhere in the Balkans: the ISO 7064 MOD 11,10 algorithm, which works as a running check value rather than a one-shot weighted sum. Starting from 10, the tool folds in each of the first 10 digits one at a time — add the digit to the running value, take the result modulo 10 (treating an exact 0 as 10), double that, and reduce modulo 11 to get the next running value. After all 10 digits, the check digit is 11 minus the final running value, reduced modulo 10 — computed locally in your browser and compared to the 11th digit you entered. Because the running calculation is sensitive to every digit’s position and value, a single typo or transposed pair almost always produces a mismatch.

What a pass doesn’t tell you

A matching check digit confirms the 11 digits are mathematically consistent — not that the Porezna uprava has actually issued that OIB to a real person or business. Since OIB doesn’t encode a birth date or location the way JMBG does, there’s also nothing to decode: the sequence itself carries no personal information beyond the number.

Scope: this page covers OIB format and checksum validation only — not registry lookups. Nothing you enter is sent to a server or stored.

Content last reviewed 2026-08-09.