What is ONSS / RSZ (Belgium)?
The ONSS (Office National de Sécurité Sociale) or RSZ (Rijksdienst voor Sociale Zekerheid) is the National Social Security Office. It collects employee and employer social security contributions in Belgium every quarter.
Finn does your books in Belgium
AccountsOS runs full accounting for Belgium businesses, tax, deadlines and invoicing, in plain English. Ask Finn about yours, no signup needed.
Current Rate (Quarterly declarations and payments)
Employee 13.07% of gross salary (no ceiling). Employer approximately 27% for white-collar staff in the private sector (no ceiling).
Example
An employee on €3,000 gross a month has €392.10 of ONSS deducted (13.07%). The employer pays roughly €810 of contributions on top (about 27%).
How ONSS / RSZ (Belgium) works in Belgium
Employee contributions are 13.07% of gross salary with no upper ceiling, and are deducted before income tax is calculated. Employer contributions are about 27% on top of gross salary for white-collar employees. This is made up of a basic contribution of around 25% plus about 3% of additional contributions. For manual workers, contributions are calculated on 108% of gross wages.
From 2026, a reform under the Programme Act of 18 July 2025 caps the basic employer contribution (about 24.92%) at €85,000 of pay per employee per quarter. The additional contributions and the employee 13.07% still apply to the full salary. The threshold is expected to fall to €67,500 per quarter from 2027.
AccountsOS does not submit to the ONSS. Finn estimates the contributions and tracks the dates.
Related terms
The DmfA (multifunctional declaration) is the quarterly return Belgian employers send to the ONSS/RSZ, reporting pay and working time for every employee. The ONSS uses it to calculate the contributions due.
Payroll withholding tax (précompte professionnel / bedrijfsvoorheffing) is the income tax an employer deducts from wages each month and pays to FPS Finance. Returns are filed monthly through the Tax For All (TFA) portal.
The SME reduced rate taxes the first €100,000 of profit at 20% instead of 25% for qualifying small companies. The maximum saving is €5,000 a year. From tax year 2026, a director must receive at least €50,000 gross annual remuneration.
Confused by Belgium accounting jargon?
AccountsOS explains Belgium terms in plain English and applies the right rules to your books automatically.
Try Free