This article is educational content explaining how stock market indices generally construct and review their membership. It is not investment advice, and it does not describe any specific current event, company, or security.
Every three months, a committee meets, runs a set of numbers, and quietly redraws the boundary of one of the world’s most-watched stock indices, without a press conference, a vote by shareholders, or even much public notice. Companies can move from obscurity to inclusion, or from the top tier down into a lesser one, based on a single ranking exercise. Yet most people who follow “the market” through an index like the FTSE 100 have never seen the rulebook that decides who gets in. How does a company actually earn, or lose, its place on that list?
Size Is the Starting Point, Not the Whole Story
The core criterion behind most major indices, including the FTSE 100, is market capitalisation: the total value of a company’s shares in issue, multiplied by their price. Index providers rank all eligible companies listed on a given exchange by this measure and draw a line at a fixed number, in this case the top 100 by size on the London Stock Exchange. This sounds mechanical, and largely it is, but a few adjustments matter before a raw ranking becomes a final list.
First, indices typically use “free float” market capitalisation rather than total market capitalisation. Free float excludes shares that are not readily available for trading, such as large blocks held by founding families, governments, or other strategic holders. Two companies with identical total valuations can rank differently in an index if one has far more of its shares locked up and not circulating in the open market. Second, most indices apply nationality and eligibility rules, generally requiring a company to have its primary listing on the relevant exchange and to meet minimum standards around liquidity (how easily its shares can be bought and sold) and free-float percentage, often a minimum of around 10 to 25 percent depending on the index.
The Committee, the Buffer Zone, and the Quarterly Clock
Index composition is not usually decided by a single algorithm running unsupervised. Most major benchmarks, including the FTSE UK Index Series, are overseen by an independent committee of market practitioners who apply the published rule book and adjudicate edge cases, such as corporate actions, mergers, or newly eligible listings that fall near the cutoff. This committee structure exists precisely so that index changes are rules-based and transparent rather than subject to the judgment of any single institution acting alone.
To prevent companies from bouncing in and out of an index every quarter because of ordinary share-price volatility, most index providers build in a buffer zone around the cutoff line. A company already in the index might only be removed if it falls below, say, rank 110, while a company outside the index might only be added if it rises above rank 90. This kind of hysteresis, a gap between the entry and exit thresholds, reduces unnecessary turnover and the trading costs that come with it for funds tracking the index.
The review itself happens on a fixed, published calendar. The FTSE 100 is formally reviewed quarterly, based on closing market values on a specific date, with changes typically taking effect a few weeks later. This predictability is deliberate: fund managers running index-tracking products need advance notice of additions and deletions so they can adjust their holdings in an orderly way rather than scrambling.
Why the Mechanism Matters Beyond the List Itself
Understanding this process explains a broader feature of index investing: passive funds that track the FTSE 100 or similar benchmarks are not making judgment calls about individual companies. They are following a rules-based process that was designed, refereed, and scheduled well in advance. When a company enters or exits a major index, it is not a verdict on that company’s underlying business quality; it is a reflection of relative market size, eligibility criteria, and a periodic, mechanical recalculation. The quarterly rhythm, the free-float adjustment, and the buffer zone are the unglamorous plumbing that keeps a benchmark both representative of the market it tracks and stable enough for millions of investors, directly or through funds, to rely on it.
