By weighting companies based on their market capitalisation, the index ensures that companies with larger market capitalisations have a greater influence on the index's performance.

The index is maintained by FTSE Russell, a subsidiary of the London Stock Exchange Group.

It aims to represent at least 98% of the full capital value of all UK companies that qualify as eligible for inclusion.

[4] The index consists of 11 ICB sectors, five of which had a market capitalisation exceeding £250 billion as of 31 December 2024[update].

[5] Each calendar quarter, the FTSE All-Share's constituents are reviewed and some companies exit or enter the index, resulting in irregular trading volume and price changes as market participants rebalance their portfolios.