Belgium runs on 230 V at 50 Hz, with sockets following Types C and E. At 230 V the supply matches what UK and European appliances expect, so no converter is needed.
You will find identical sockets in Czechia, France, Monaco and Poland. Nearby but different: Cyprus, the United Kingdom and the Isle of Man – an adapter for Belgium will not fit there. Across Europe this setup is fairly rare: 6 of 50 countries.
Protective earth
Grounded and ungrounded sockets may occur
Belgium has sockets with a protective earth (Type E) and without (Type C). A plug that fits mechanically does not guarantee earthing.