Quarterly Social Security Declaration (DmfA)
Quarterly multifunctional declaration that Belgian employers send to the ONSS/RSZ to report pay and working time and trigger the contributions due.
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Who this applies to
- Belgian employers with staff on payroll
- Companies paying a director as an employee
What to file
The DmfA, reporting remuneration and working-time data for every employee. The ONSS calculates contributions from it. The employee share is 13.07% and the employer share about 27%, with the basic employer contribution capped at €85,000 per employee per quarter from 2026.
How to file
Through the ONSS/RSZ systems, usually by a payroll provider or social secretariat. New hires are notified through DIMONA at least one day before they start. AccountsOS does not submit to the ONSS.
Payment due
Contributions are paid quarterly after the DmfA calculation, although some employers make advance monthly payments.
Penalties for missing this deadline
Late declaration or payment is subject to consequences set by the ONSS. Confirm them with the ONSS.
Filing checklist
- Confirm every employee and their hours for the quarter
- Check any new hires were notified through DIMONA
- File the DmfA
- Pay the contributions the ONSS calculates
- Keep the declaration receipt
Documents you'll need
- Payroll records for the quarter
- Working-time records
- DIMONA notifications
Common mistakes to avoid
- Forgetting the DIMONA notification before a new hire starts
- Applying the €85,000 cap to the additional contributions, which are not capped
- Treating the seeded reminder date as final without checking it with the ONSS
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