The Belgian national register number: structure and masking
Eleven digits that reveal a date of birth and a sex, and link a person across databases. How the number is built, where it hides in a file, and how to mask it.
Updated on 29 September 2026
Eleven digits appear throughout Belgian case files: in writs of summons and petitions, payslips, social security documents, copies of identity cards. They look like an administrative detail. They are in fact a direct identifier: the national register number reveals a date of birth and a sex, and it is the key that links one person across public databases.
How the number is built
Every person is given an identification number at their first registration in the National Register, organised by the law of 8 August 1983. Its composition is set by the Royal Decree of 3 April 1984 and described in the documentation of the Directorate-General Identity and Citizens' Affairs (IBZ). It has eleven digits in three groups, usually written YY.MM.DD-SSS.CC.
The example used in this guide is fictitious: 85.03.15-123.69.
| Digits | Meaning | In the example |
|---|---|---|
| 1 to 6 | Date of birth: year, month, day | 85.03.15: 15 March 1985 |
| 7 to 9 | Sequence number among persons born on the same day: odd for men (001 to 997), even for women (002 to 998) | 123: odd |
| 10 and 11 | Check number | 69 |
How the check number works
The check number is 97 minus the remainder of the division by 97 of the nine-digit number formed by the date of birth and the sequence number:
- Take the first nine digits:
850315123. - Divide by 97: the remainder is 28.
- Subtract it from 97: 97 − 28 = 69. The number is
85.03.15-123.69.
For persons born from 2000 onwards, the digit 2 is placed in front of the nine digits before dividing. Take 04.06.08-042.67, again fictitious: 2040608042 divided by 97 leaves 30, and 97 − 30 = 67. Without the leading 2, the check number would have been 38. The check number therefore also tells the century: 04.06.08-042.38 would describe a woman born on 8 June 1904.
A mistyped number almost always fails this test. That is what allows software to recognise a real national register number with near certainty, and to ignore other series of eleven digits.
Bis numbers and the social security number
A person who is not registered in the National Register but has a link with Belgian social security (a cross-border worker, for example) receives a bis number from the Crossroads Bank for Social Security. It has the same structure, but the birth month is increased by 40 if the sex is known when the number is created, or by 20 if it is not. A month of 43 or 23 therefore signals a bis number.
The social security identification number (INSZ/NISS) is either the national register number or the bis number (Crossroads Bank for Social Security).
Why the number should be masked
- It speaks for itself. Without any name,
85.03.15-123.69says: a man born on 15 March 1985. - It is a unique key. The number links the same person across registers and files. The GDPR allows Member States to set specific conditions for processing a national identification number, which may then be used only under appropriate safeguards (art. 87 GDPR).
- Its use is restricted by Belgian law. The national register number may not be used without authorisation, nor for purposes other than those for which authorisation was granted, subject to the cases the law itself provides for (law of 8 August 1983, art. 8). An AI tool, a law review or a training audience has no need for it.
- Partial masking leaks.
85.03.**-***.**still gives the year and month of birth. That may be acceptable inside the firm, not in a publication.
Where it hides in a legal file
- Writs of summons and petitions. The Judicial Code requires the claimant's or petitioner's national register number, bis number or enterprise number, where applicable, on pain of nullity (art. 702, 1° and art. 1034ter, 2°). A summons in the file will therefore often carry one.
- Employment and social security documents: payslips, certificates, correspondence with social security institutions.
- Copies of identity cards. The number is printed on the Belgian identity card. In a scanned copy it is part of an image, not text.
- Places people forget: headers and footers, file names, e-mail subjects, tables in annexes.
- In any notation:
85.03.15-123.69,850315-123-69,850315 123 69or85031512369.
How to mask it with Localex
- Open the tool and load the document, or paste the text.
- In What to mask, the National register number item is ticked by default. It covers bis numbers and the usual notations, with or without separators. The identity card number is a separate item, also ticked.
- In Findings, a national register number whose check number is valid is marked Checksum verified.
- Choose the replacement for this item: a numbered token for an AI round trip (the key restores it), a black bar for a publication, or a partial mask only for internal use.
- If a number in the document is mistyped, it will fail the checksum and may not be recognised as a national register number. Add it under Always mask, or select it in the text and choose Mask every occurrence.
- Tick I have read the result and export. The leak scan searches the output file for the original values before the download.
Honest limits
- The checksum makes detection of well-formed numbers very reliable; it cannot recognise a number with a typing error. The review catches those.
- A number inside an image (a scanned identity card, a photographed document) is only read after text recognition (OCR), one of the AI features, free after e-mail verification.
- A number split across two lines or two pages, or broken by a page footer, may escape the rules. Read the review carefully around page breaks.
- Masking the number does not make the document anonymous: the name, the address and the date of birth usually sit right next to it. See pseudonymisation vs anonymisation.