Market cap explained: what it is and why it matters

What is market capitalisation?

Market capitalisation, or market cap, is calculated by multiplying a company's current share price by the total number of shares it has in issue. If a company has 500 million shares outstanding and each share trades at 200p (£2), its market cap is £1 billion. The number changes continuously as the share price moves during trading hours.

Market cap is the standard measure of company size because it reflects market consensus about value, not just the company's accounting records. Two companies with the same book value can have very different market caps, depending on how the market assesses their future growth prospects, competitive position and earnings power. You can buy shares in companies across all market cap tiers through our share dealing account or stocks and shares ISA.

UK market capitalisations (March 2026, FTSE Russell)

How market cap is calculated

Market cap = Share price x Total shares outstanding

Example: Share price 250p x 400 million shares = £1 billion market cap

Note that 'shares outstanding' means all shares in existence, including restricted shares held by founders, employees and institutional investors, not just the freely tradeable 'free float'. Index providers like FTSE Russell use free-float adjusted market cap for their weightings, which is the portion of shares actually available to public investors. This distinction matters: a company might have a large total market cap but a smaller free-float market cap, reducing its index weighting.

Large-cap, mid-cap and small-cap: what do they mean?

Companies are grouped into size tiers based on their market cap. The precise boundaries vary by provider and region, but the UK conventions broadly map to the FTSE index structure (Openbook Analytics, June 2026):

Source: Openbook Analytics (June 2026) and InvestingGuide.co.uk. Note that FTSE 100 entry requires a minimum market cap of approximately £6bn, though this threshold is not fixed. Dividend yield typically correlates with market cap tier: large-cap companies are more likely to pay consistent dividends, while small and micro-caps typically reinvest earnings or are pre-profitability.

Why does market cap matter for investors?

Index inclusion and fund exposure

A company's market cap determines which index it belongs to. Qualifying for the FTSE 100 requires being among the 100 largest companies on the LSE by free-float market cap. Index inclusion triggers automatic buying from passive funds tracking that index, providing a potential short-term price boost. Conversely, being ejected from an index forces passive funds to sell, creating downward pressure. The largest UK companies by market cap highlights how FTSE 100 composition has shifted significantly in recent years, with HSBC overtaking AstraZeneca as the largest constituent in 2025.

Risk and return profile

Market cap is a proxy for risk. Large-cap companies tend to have lower volatility because they have more diversified revenue streams, stronger balance sheets and greater analyst coverage that prices new information quickly. Small-cap and micro-cap companies can move dramatically on single news items, and their lower liquidity makes large trades more impactful on price.

Portfolio diversification

Deliberately diversifying across market cap tiers provides exposure to different risk-return profiles. An investor holding only large-caps captures the stability of FTSE 100 names but may miss faster growth from emerging mid-cap companies. Over-concentration in one company, regardless of its market cap, creates single-stock risk. The UnitedHealth collapse case study illustrates how even mega-cap stocks can fall dramatically on specific adverse events, reinforcing why diversification across companies, sectors and market cap tiers matters.

Valuation context

Market cap is the starting point for many valuation metrics. It feeds into the price-to-earnings ratio (via earnings per share), the enterprise value (market cap plus net debt), and the price-to-book ratio. Understanding whether a company is valued generously or modestly relative to peers of similar size is a key part of fundamental equity analysis. The broader question of why the UK stock market has been losing its global financial dominance is itself a market cap question: UK-listed companies have seen their valuations compress relative to US peers, reflected in lower aggregate market caps for equivalent businesses.

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