In short: Social security contributions are compulsory payments from income into funds for pensions, sickness and maternity, unemployment, accidents at work and health, which give you the right to benefits when your income stops.
Simply put: A tax gives you no specific right; a contribution does. Part is paid by the employee (deducted from gross), part by the employer on top. Your pension, sick pay, maternity pay and unemployment benefit depend on what was paid in — so it is worth checking your contributions.
Example: Two people earn the same, but one is insured on a lower amount. When ill or retired, that person receives less.
The EU rule: rates are national; EU law coordinates systems so that a person is covered by one country's legislation at a time (Regulation (EC) No 883/2004, Article 11(1)).
Example: Bulgaria — born after 1959, on an employment contract: the employee pays 13.78 %, the employer 18.52 % plus 0.4–1.1 % for accidents at work (Social Security Code, Articles 6 and 157; NRA). As at 19.10.2026.
Check in your country — The rates, any minimum and maximum base, and your online insurance record at the social security institute.
Source: Regulation (EC) No 883/2004: https://eur-lex.europa.eu/eli/reg/2004/883/oj · NSSI (Bulgaria): https://www.noi.bg/dohod01082026/