How to find your supplier's NSSO number: a guide for clients

Before paying an invoice from a contractor or supplier, you must check under Article 30bis whether any social security or tax debts exist. That check starts with the company number, but in practice you also often come across the NSSO number. Where do you find it? And what do you do if the supplier cannot provide it right away?

In this article we walk through four reliable ways to find your supplier's NSSO number. We finish with a faster route: Checkplicht automatically links the company number and the NSSO, so you no longer have to puzzle it out.

Why the NSSO number matters

The NSSO number is the same as the company number of an employer. It is used to:

• register social security contributions;
• attach debts to the NSSO;
• carry out a withholding obligation check.

For self-employed people without staff, the INASTI/RSVZ number plays a similar role. In most cases the company number is enough, but the NSSO number helps you be sure you are checking the right entity.

Method 1: on the company extract

The company extract is the most reliable source. Every business receives a ten-digit company number in the format 0123.456.789. You can find it:

• on the supplier's invoice, usually at the top or next to the VAT number;
• in your accounting supplier file;
• via the official Crossroads Bank for Enterprises.

The company number is the key to the NSSO check. On our page check withholding obligation you can see what a full check looks like.

Note: a branch unit may have a different address, but the legal entity and company number remain the same. Always check the entity shown on the invoice.

Method 2: on invoices and quotes

Suppliers usually list their company number on:

• invoices;
• quotes;
• order confirmations;
• general terms and conditions.

If you are specifically looking for the NSSO number, it often appears on payslips or on documents about social security contributions. If in doubt, ask for a recent NSSO attestation or written confirmation of the correct number.

Method 3: through official portals

The Belgian authorities offer several free lookups:

• socialsecurity.be – information on employers and their NSSO status;
• financien.belgium.be – FAQ and guidance from FPS Finance on the withholding obligation;
• rsvz.be – specific to self-employed people and their social status.

These portals give you insight into the rules, but for a concrete debt check you must search and record each number separately. That is manageable for one or two suppliers, but not for a larger file.

Method 4: ask the supplier directly

Cannot find the number? Send a short email with this request:

"For our withholding obligation check under Article 30bis we need your company number and, where applicable, your NSSO number. Could you provide a recent NSSO attestation?"

Keep that email. If a supplier cannot or will not provide the number, that is a risk signal. Consider suspending payment until you are certain.

What if the number is wrong?

A check often fails because the number is too short. Excel and some accounting packages swallow the leading zero of a company number, so 0843.212.345 becomes 843.212.345. We do not add digits, but our tool warns you that the number is too short. You correct it yourself, so the data remains legally sound.

Read more about fixing a check in our step-by-step guide checking social security debts.

Work faster with Checkplicht

Checkplicht combines the steps above into one check:

1. You enter the company number or import your suppliers via Excel.
2. The platform automatically looks up the correct NSSO, FPS Finance and RSVZ status.
3. You get a clear red or green result per supplier.
4. If withholding is required, Checkplicht shows the exact percentages and directs you to the correct official payment instrument.

Discover our pricing or start with a free trial.

Frequently asked questions

Is the company number the same as the NSSO number?

For employers, the company number is usually the same number the NSSO uses. For self-employed people without staff, the INASTI/RSVZ number is relevant.

Can I check without an NSSO attestation?

Yes, an attestation is not a mandatory condition. You check based on the company number through the official sources. An attestation is useful, however, as proof that the supplier is cooperating.

What if the supplier refuses to give the number?

Document the request. Do not pay before you have the number, or withhold the legal amount until you are sure. This protects you against joint liability.

Why is my check reported as "too short"?

Belgian company numbers always have ten digits. Check whether Excel or an import removed the leading zero.

Where do I request the official NSSO attestation?

You request the NSSO attestation through the NSSO portal or via your social secretariat. More background in our article article 30bis explained.