The Weight You Cannot Buy: Free Float and the Limits of UK Small-Cap Index Exposure

Index weights carry an air of precision. A stock appears in a FTSE index at a given percentage, and investors naturally treat that number as a measure of importance, liquidity and ownership opportunity. For UK small caps, that assumption deserves closer scrutiny. The weight shown is rarely a simple reflection of market capitalisation. It is usually adjusted for free float—the portion of shares that index providers believe is available to ordinary outside investors. In smaller companies, that estimate can flatter the real investability of the stock.

Why free float matters more than headline market cap

Free float exists because not all shares are equal for index purposes. A founder may control a large block, a strategic investor may hold another, and directors may own shares subject to lock-ups or informal constraints. Index providers exclude some of these holdings when calculating weights, on the view that they are not freely tradable in the ordinary course. The logic is sound. The difficulty lies in estimation.

For large, heavily traded UK companies, the free-float adjustment is often a second-order issue. Ownership is dispersed, turnover is high and the difference between total market capitalisation and investable market capitalisation is usually well understood. In the small-cap segment of the London market, the gap can be far more consequential. A company with a modest market value may have a free float of only a fraction of its issued share capital. The index weight may look meaningful relative to its peer group, but the shares available to buy and sell on any given day may be considerably narrower.

Where the estimate can flatter

The first source of distortion is concentrated founder or family ownership. A long-standing controlling stake can be entirely rational for the company and entirely problematic for index users. It reduces the shares in circulation, concentrates voting power and can make the stock less responsive to ordinary market flows. If the index provider treats part of that stake as unavailable, the remaining free float may be small. If the treatment is too generous, the index weight overstates the stock’s capacity to absorb passive money.

Strategic holdings create a similar problem. A peer company, a private equity backer or a long-term institutional holder may own a block that is not formally locked up but is unlikely to trade. The line between a strategic stake and a tradable one is not always clear. Index methodologies rely on public disclosures, classifications and provider judgement. They cannot observe the intention of every large holder. In small caps, where a handful of investors can dominate the register, that gap between legal availability and practical availability matters.

Lock-ups add a further layer. After an IPO or a placing, directors and early investors may be subject to contractual restrictions. The shares exist, and the company’s market capitalisation includes them, but they cannot yet be sold. Once a lock-up expires, the free float may rise—or the market may discover that the newly available shares are more than it can absorb. Either way, the index weight at a point in time may not reflect the investability of the stock a few months later.

Thin trading is the final and perhaps most underappreciated distortion. Free float measures the shares that could in theory be available. It does not measure how many actually change hands. A small cap can have a respectable free-float percentage and still trade in modest daily volumes, with wide spreads and limited depth. For an investor trying to build or exit a position, the index weight is a poor guide to execution risk. For a passive fund tracking the index, the same issue appears as tracking cost: the fund may need to trade more of a stock than the market can comfortably handle.

The passive-flow illusion

This is where the free-float estimate becomes more than an academic detail. Index inclusion can attract passive and quasi-passive flows. A higher weight implies a larger share of those flows. If the free float is overstated, the stock may receive more index-driven demand than its real liquidity can support. The effect can be self-reinforcing in the short term: inclusion brings buying, buying supports the price, and the weight appears justified. But the underlying constraint has not disappeared. When flows reverse, or when a large holder decides to sell, the market may find that the apparent weight was never fully investable.

For UK small caps, this is not a theoretical concern. The segment already carries a liquidity premium and a research discount. An index weight that looks larger than the tradable reality can compound both. It can also mislead investors who use index weights as a proxy for portfolio importance. A stock at 1 per cent of an index is not necessarily 1 per cent of the investable opportunity in any practical sense. It may be 1 per cent of a methodology, and something rather different in the order book.

A practical checklist for reading UK index weights

Investors should start by separating headline market capitalisation from the index weight. The difference is the free-float adjustment. Ask what percentage of shares the provider treats as freely available, and compare that with the largest disclosed holdings. A small free float alongside a dominant founder stake is a signal to look more closely at liquidity.

Second, review the register. Public disclosures, annual reports and major holding notifications can reveal concentrated ownership. Look for strategic stakes, private equity positions and family holdings that may not trade regularly. The question is not only whether the shares are legally free to trade, but whether the holder is likely to do so.

Third, check lock-ups and placing arrangements. Recent IPO and placing documentation often sets out when restrictions expire. A free-float estimate can change around those dates. Investors should consider whether the market has already priced the prospect of additional shares becoming available.

Fourth, examine trading volumes rather than relying on free float alone. Average daily volume, bid-offer spreads and market depth give a better sense of what can be bought or sold without moving the price. A stock with a small free float and thin volumes may be unsuitable for larger positions, regardless of its index weight.

Finally, treat index weights as a starting point, not a conclusion. They are a useful summary of how a provider sees the market, but they are not a guarantee of investability. For UK small caps, the practical question is always the same: how much stock can actually change hands, at what cost, and in what size? The index weight may answer a different question.

Caird Capital’s view is that free float deserves the same scrutiny as market capitalisation. In the large-cap part of the UK market, the adjustment is often a detail. In the small-cap part, it can be the story. Investors who read the weight without reading the float may find themselves owning an exposure that is larger, or smaller, than they intended.