Belgium ONSS / RSZ Number Validator
Validate Belgian ONSS / RSZ employer social security numbers online with Modulo 97 verification.
About Belgium's ONSS / RSZ Employer Number
The ONSS (National Office of Social Security) or RSZ number is issued to employers in Belgium to track payroll taxes, pension contributions, and mandatory social insurance.
It is required for quarterly multi-functional returns (DmfA), immediate employment notifications (Dimona), and employer social security contributions.
Anatomy of the Belgian ONSS / RSZ Employer Number
Structure of the employer identifier issued by the Belgian ONSS/RSZ:
| Position | Field | Length | Official Meaning | Example |
|---|---|---|---|---|
| 1 to 3 | Scheme Prefix (Kengetal) | 3 digits (optional) | Social security regime indicator (000 for standard private sector employers). | 000 |
| 4 to 10 | Registration Sequential Number | 7 digits | Unique employer account number assigned by the ONSS/RSZ. | 1234567 |
| 11 & 12 | Check Digits | 2 digits | Control digits computed via Modulo 97 multiplied by 100 with 96 complement. | 49 |
Calculation Algorithm (Modulo 97 Weighted by 100)
Official mathematical rule applied to the sequential registration number:
Remainder = (Base × 100) mod 97; Check = 96 - Remainder; If Check == 0 then Check = 97
- 1. Weighting: Multiply the 7-digit registration number by 100.
- 2. Modulo 97 Division: Compute the integer remainder when dividing the weighted number by 97.
- 3. Complement of 96: Subtract the remainder from 96 to obtain the 2-digit check number.
- 4. Zero Adjustment: If the result is 0, assign 97 as the check digit.