The short answer
The RSZ number (NSSO number in English, ONSS number in French) is the number under which a company is registered as an employer with Belgium's National Social Security Office (RSZ). A company only gets one when it hires staff. So it is not the same as the enterprise number, which every company in the KBO has. You look up a company's RSZ number in the Employer Directory (Werkgeversrepertorium) on socialsecurity.be. If you only want to know whether a company is an employer, the KBO is enough: it lists "Werkgever RSZ" (RSZ employer) under the company's capacities.
The RSZ number identifies an employer at the RSZ
The RSZ collects social security contributions from employers and employees. To link each declaration to the right employer, every employer gets its own registration number. You will also see it called the employer number, RSZ registration number or NSSO identification number. It is the same number in each case.
The number comes with employer status. Anyone who hires one or more employees for the first time must report this to the RSZ. That applies to a natural person, a company, a non-profit association or a public body. A company without employees has no RSZ number.
A full RSZ number has 12 digits
According to the explanation of Belgian identification numbers on the social security portal, the RSZ number has three parts:
a 3-digit category code;
the actual 7-digit registration number;
a 2-digit check number.
In practice you often see a shorter form. The Dimona online service asks for a definitive RSZ number of 8 digits, without dots or hyphens. So you type 12345678, not 123-456.78. A provisional RSZ number starts with 51 and has 10 digits. Which form you see depends on the document or the application.
A company gets its RSZ number when it first hires
A new employer registers with the RSZ through WIDE, the online employer identification service on socialsecurity.be. The company must already have an enterprise number in the KBO. This is how it works:
When applying in WIDE, the employer receives a provisional number in the format 51.xxx.xxx-xx. It uses that number for Dimona, the declaration of when employees start and leave.
Once the application is processed, the RSZ sends a confirmation with the definitive RSZ number, the NACE code, the assigned employer category and a few other codes. The confirmation arrives in the company's e-Box Enterprise or by post.
The RSZ adds the employer to the Employer Directory and also records the employer status in the KBO.
Only an employer listed in the Employer Directory can file a Dimona or a DmfA. The DmfA is the quarterly declaration of employees' wages and working time.
Without staff, the RSZ number can lapse
An employer with no staff for a full quarter must tell the RSZ. A correct Dimona OUT for the last employee is enough. If the employer stops completely, or expects to have no staff for at least two full quarters, it requests deregistration through WIDE. The RSZ number is then closed, and so is the employer status in the KBO. The RSZ itself contacts employers that file no declaration for four quarters in a row.
For anyone reading the KBO, this means "Werkgever RSZ" tells you a company is registered as an employer. It does not tell you how many people work there.
The RSZ number is not the same as the enterprise number
Government bodies identify employers by their KBO enterprise number. The RSZ number exists alongside it and is linked to it. Four numbers often get mixed up:
Number | Who has one | Format | Where to find it |
|---|---|---|---|
RSZ number (NSSO number) | Companies with employees | 12 digits in full form; Dimona asks for 8 digits, a provisional number has 10 digits and starts with 51 | Employer Directory, RSZ confirmation, your payroll provider |
Enterprise number | Every company in the KBO | 10 digits, starting with 0 or (since September 2023) 1 | KBO Public Search |
VAT number | Companies with a VAT-liable activity | BE followed by the enterprise number | Invoices, company website, VIES |
National register number | Individuals living in Belgium | 11 digits: date of birth, sequence number, check number | Electronic identity card |
The national register number belongs to a person, not a company. Within social security, the personal number is called the INSZ. For Belgian residents, the INSZ is the same as the national register number. Read more about the enterprise number in Belgian enterprise number lookup, and about VAT numbers in checking an EU VAT number.
How to look up a company's RSZ number
Step 1: find the enterprise number
If you only know the name, first look up the company in KBO Public Search. There you find the enterprise number, the registered office and the establishment units. Searching the Belgian company register explains how.
Step 2: check in the KBO whether the company is an employer
On the KBO record, scroll to the "Hoedanigheden" (capacities) block. If it shows "Werkgever RSZ" with a start date, the company is registered as an employer. Further down you also see "RSZ-activiteiten", the NACE code the RSZ assigned to the company. The RSZ number itself is not on the KBO record. If there is no "Werkgever RSZ", there is most likely no RSZ number to find.
Step 3: search the Employer Directory
Open the Employer Directory on socialsecurity.be and choose "Consult". You can search by RSZ number, enterprise number, name or address. The public version shows the latest public data for the employer. The secured version shows more, but only the employer and its authorised representative can use it.
Step 4: for your own number, check your own documents
As an employer, you find the definitive RSZ number in the RSZ confirmation in your e-Box Enterprise. Your payroll provider (sociaal secretariaat) has it too. As an employee, ask HR or your employer's payroll provider. What a payslip must show differs per joint committee, so the number is not always in the same place.
For sales and procurement, "Werkgever RSZ" is a useful signal
If you work with lists of companies, RSZ status tells you whether a company has staff. That separates the one-person business from the SME with a team. For size, look at the social balance sheet in the annual accounts. The joint committee tells you what kind of staff work there: blue-collar, white-collar or both. In Bizzy you see each company's joint committee and the staff size from its filed accounts.
Frequently asked questions
Is the RSZ number public?
Yes, to a limited extent. The public Employer Directory on socialsecurity.be shows the public data of every employer. You can search by name, address, enterprise number or RSZ number.
Does every company have an RSZ number?
No. Only companies registered as an employer with the RSZ have one. Every company does have an enterprise number in the KBO.
Is the RSZ number shown in the KBO?
The status is, the number is not. KBO Public Search shows "Werkgever RSZ" under capacities, with the date it started. For the number itself, use the Employer Directory.
What is a provisional RSZ number?
It is the number WIDE assigns when a company applies to register as an employer. It starts with 51 and has 10 digits. The employer uses it for Dimona until the RSZ assigns the definitive number.
What is the difference between the RSZ number and the INSZ?
The RSZ number identifies an employer. The INSZ identifies a person within social security. For Belgian residents, the INSZ is the national register number. For non-residents, it is their number in the BIS register.
What happens to the RSZ number when a company no longer has staff?
An employer with no staff for a quarter reports it through a Dimona OUT for the last employee. If it stops, or expects at least two quarters without staff, it requests deregistration through WIDE. The RSZ number then lapses and "Werkgever RSZ" disappears from the KBO.