In general, the index consists of a minimum of 10 and a maximum of 20 companies traded at the Brussels Stock Exchange.

The composition of the BEL 20 index is reviewed annually based on closing prices on the last Friday in February.

In addition to meeting a set of criteria demanding a company be "representative of the Belgian equity market", at least 15% of its shares must be considered free float in order to qualify for the index.

[2] In addition, a candidate for inclusion must possess a free float market capitalisation (in Euros) of at least 300000 times the price of the index on the last trading day of December.

[2] At each annual review, the weights of companies in the index are capped at 15%,[2] but range freely with share price subsequently.